aapm protocol for 40–300 kv x-ray beam dosimetry in ... protocol for 40–300 kv x-ray beam...

AAPM protocol for 40–300 kV x-ray beam dosimetryin radiotherapy and radiobiology

C.-M. Ma, Chaira)

Radiation Oncology Dept., Stanford University School of Medicine, Stanford, California 94305-5304and Ionizing Radiation Standards, National Research Council of Canada, Ottawa K1A 0R6, Canada

C. W. Coffeyb)

Department of Radiation Oncology, Vanderbilt Medical Center, B 902 Vanderbilt Clinic, Nashville,Tennessee 37232-5671

L. A. DeWerdc)

University of Wisconsin, 1530 MSC Medical Physics, 1300 University Avenue, Madison, Wisconsin 53706

C. Liud)

Department of Radiation Oncology, University of Florida, Gainesville, Florida 32610-385

R. Nathe)

Department of Therapeutic Radiology, Yale School of Medicine, 333 Cedar Street, New Haven,Connecticut 06510

S. M. Seltzerf)

Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

J. P. Seuntjensg)

Medical Physics Unit, McGill University, Montreal General Hospital, 1650 Avenue Cedar,Montreal H3G 1A4, Canada and Ionizing Radiation Standards, National Research Council of Canada,Ottawa K1A 0R6, Canada

~Received 5 March 2001; accepted for publication 12 March 2001!

The American Association of Physicists in Medicine~AAPM! presents a new protocol, developedby the Radiation Therapy Committee Task Group 61, for reference dosimetry of low- and medium-energy x rays for radiotherapy and radiobiology (40 kV<tube potential<300 kV). It is based onionization chambers calibrated in air in terms of air kerma. If the point of interest is at or close tothe surface, one unified approach over the entire energy range shall be used to determine absorbeddose to water at the surface of a water phantom based on an in-air measurement~the ‘‘in-air’’method!. If the point of interest is at a depth, an in-water measurement at a depth of 2 cm shall beused for tube potentials>100 kV ~the ‘‘in-phantom’’ method!. The in-phantom method is notrecommended for tube potentials,100 kV. Guidelines are provided to determine the dose at otherpoints in water and the dose at the surface of other biological materials of interest. The protocol isbased on an up-to-date data set of basic dosimetry parameters, which produce consistent dosevalues for the two methods recommended. Estimates of uncertainties on the final dose values arealso presented. ©2001 American Association of Physicists in Medicine.@DOI: 10.1118/1.1374247#

Key words: low- and medium-energy x rays, dosimetry protocol, calibration, ionization chambers,reference dosimetry, relative dosimetry

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TABLE OF CONTENTS

I. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . 869A. Historical review. . . . . . . . . . . . . . . . . . . . . . . . . 869B. Scope of this document. . . . . . . . . . . . . . . . . . . . 870C. List of nomenclature, symbols, and units. . . . . . 870

II. RADIATION QUALITY SPECIFICATIONAND DETERMINATION. . . . . . . . . . . . . . . . . . . . 871A. Energy ranges considered. . . . . . . . . . . . . . . . . . 871B. Beam quality specifier. . . . . . . . . . . . . . . . . . . . . 871C. Determination of HVL. . . . . . . . . . . . . . . . . . . . . 872

III. EQUIPMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872A. Phantoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87B. Dosimeters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872C. Electrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 873

868 Med. Phys. 28 „6…, June 2001 0094-2405 Õ2001Õ28„6

D. Quality assurance of the dosimetry equipmentand x-ray tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8731. Ionization chamber. . . . . . . .. . . . . . . . . . . . . 8732. Electrometer. . . . . . . . . . . . . . . . . . . . . . . . . . 8733. Tube potential of x-ray generator. . . . . . . . . 874

IV. AIR-KERMA CALIBRATION PROCEDURES.. 874

V. FORMALISM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874A. The in-air method: Absorbed dose to water at

the surface for low- and medium-energy xrays (40 kV<tube potential<300 kV). . . . . . . . . 875

B. The in-phantom method: Absorbed dose towater at 2 cm depth in water formedium-energy x rays(100 kV,tube potential<300 kV). . . . . . . . . . . . 875

868…Õ868Õ26Õ$18.00 © 2001 Am. Assoc. Phys. Med.

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869 Ma et al. : AAPM’s TG-61 protocol for kilovoltage x-ray beam dosimetry 869

C. Other considerations in the calibrationmeasurement. .. . . . . . . . . . . . . . . . . . . . . . . . . . . 8751. Ion collection efficiencyPion. . . . . . . . . . . . . 8752. Polarity correctionPpol. . . . . . . . . . . . . . . . . 8763. End effectdt. . . . . . . . . . . . . . . . . . . . . . . . . 8764. Electrometer correction (Pelec). . . . . . . . . . . 8765. Temperature–pressure correctionPTP. . . . . . 8766. Inverse-square consideration for in-air

calibration with close-ended cones. . . . . . . . 8767. Method for determination ofPstem,air. . . . . . . 8768. Method for in-phantom calibration of

chambers not listed in this protocol. . . . . . . 877D. Consistency between the in-air method and

the in-phantom method. .. . . . . . . . . . . . . . . . . . 877VI. GUIDELINES FOR DOSIMETRY IN OTHER

PHANTOM MATERIALS. . . . . . . . . . . . . . . . . . . 878VII. GUIDELINES FOR RELATIVE DOSIMETRY

AT OTHER POINTS IN WATER. . . . . . . . . . . . 878A. Characteristics of clinical beams. . . . . . . . . . . . . 878B. Recommendations for relative dose

measurements in water. . . . . . . . . . . . . . . . . . . . . 878C. Electron contamination of clinical beams. .. . . . 879

VIII. EVALUATION OF UNCERTAINTIES. . . . . . . 880IX. FUTURE CONSIDERATIONS. . . . . . . . . . . . . . . 880ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . 881APPENDIX A. THEORETICAL BASIS FOR ACODE BASED ON AIR-KERMA CALIBRATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

A.1. Low-energy x rays(40 kV<tube potential<100 kV). . . . . . . . . . . 881

A.2. Medium-energy x rays(100 kV,tube potential<300 kV). . . . . . . . . . 881

APPENDIX B. DETAILS ON CONVERSION ANDCORRECTION FACTORS. . . . . . . . . . . . . . . . . . . . . 883

B.1. The in-air method for low- andmedium-energy x rays. . . . . . . . . . . . . . . . . . . 883B.1.1. In-air mass energy-absorption

coefficient ratio@(men/r)airw #air. . . . . . . . 883

B.1.2. Backscatter factorBw. . . . . . . . . . . . . . 883B.1.3. Chamber stem correction factor

Pstem,air. . . . . . . . . . . . . . . . . . . . . . . . . . 883B.2. The in-phantom calibration method for

medium-energy x rays. . . . . . . . . . . . . . . . . . . 886B.2.1. In-phantom mass energy-absorption

coefficient ratio@(men/r)airw #water. . . . . . 886

B.2.2. Ion-chamber correction factorPQ,cham. . . . . . . . . . . . . . . . . . . . . . . . . . 887

B.2.3. Sleeve correction factorPsheath. . . . . . . 887B.3. Conversion factors to calculate dose in

other biological materials. . . . . . . . . . . . . . . . 887APPENDIX C. SUMMARY OFRECOMMENDATIONS AND WORKSHEETS. . . . 888

C.1. TG-61 Worksheet: Calculating dose towater on the phantom surface. . . . . . . . . . . . . 890

C.2. TG-61 Worksheet: Calculating dose towater at 2 cm depth in water. . . . . . . . . . . . . 891

Medical Physics, Vol. 28, No. 6, June 2001

I. INTRODUCTION

A. Historical review

Kilovoltage~40–300 kV! x-ray beams continue to be usein radiation therapy and radiobiology. According to a survconducted in 1995 by American Association of physicistsMedicine ~AAPM! Radiation Therapy Committee TasGroup 61,1,2 there is renewed interest in radiotherapy trement with superficial and orthovoltage x rays, with mox-ray machines being ordered and installed in North Amerduring the last few years.

For the dosimetry procedures, several dosimetry protocare available for kilovoltage x-ray beam therapy. In 1973International Commission for Radiation Units and Measuments ~ICRU! Report No. 233 recommended ‘‘the in-airmethod’’ for low-energy photons~tube potential: 40–150kV! with the backscatter factors taken from the 1961 BritJournal of Radiology~BJR! Supplement 10,4 and ‘‘the in-phantom method’’ for medium-energy x rays~tube potential:150–300 kV!, respectively. In 1981, the National Council oRadiation Protection and Measurements~NCRP! Report No.695 gave a formula to calculate dose to a phantom materiaa point in air~with a minimum phantom! for tube potentials10 kV through the medium-energy range~up to 300 kV!. Abackscatter factor was needed to calculate dose on the ptom surface. Two years later, the U.K. Hospital PhysicAssociation~HPA! adopted the same methodology as thused by the ICRU Report No. 23 for low- and mediumenergy x-ray beams.6 For the backscatter factors, the HPprotocol recommended the values from the 1983 BSupplement 17.7 In 1987, the International Atomic EnergAgency ~IAEA ! code of practice8 also recommended twodifferent formalisms for low- and medium-energy photoalthough the beam-quality ranges were slightly different~lowenergy: tube potential 10–100 kV, medium energy: tubetential 100–300 kV!. The backscatter factors were derivefrom Monte Carlo calculations. The values of the chambperturbation factor used by the IAEA have been the sourcsome controversy.9–16 In 1991, the Institute of Physical Sciences in Medicine Working Party~IPSM!17 recommended nochange in the conversion factorF given by HPA but gave anew set of backscatter factors which were derived fromcombination of more recent Monte Carlo calculations aexperimental results. The more recent code of practice ofInstitute of Physics and Engineering in Medicine and Biogy ~IPEMB!18 published in 1996 and the code of practicethe Netherlands Commission on Radiation Dosime~NCS!19 published in 1997 further incorporated the chambcorrection factors that were consistent within 2% with tnew IAEA recommendations issued in the second editionTRS-277.13

In North America, a variety of dosimetry procedures habeen used in practice, with a combination of conversion acorrection factors measured and/or taken from differprotocols.1,2,20For the last few years, there have been a nuber of publications concerning this subject leading toformation of several dosimetry task groups outside NoAmerica and new dosimetry protocols for kilovoltage x ray

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The AAPM Radiation Therapy Committee Task Groupwas set up to evaluate the current situation and to recmend suitable dosimetry procedures for kilovoltage x-beam dosimetry for radiotherapy and radiobiology.

B. Scope of this document

This protocol deals with the dosimetry of kilovoltagerays~tube potential: 40–300 kV! for radiotherapy and radiobiology applications. It is anair-kerma-basedprotocol usinga calibration of an ionization chamber in air at a standalaboratory. This protocol is valid only when the conditionscharged particle equilibrium are satisfied. The scope ofprotocol is fourfold:

~1! calibration methodology~dosimeter requirements anphantom configurations!;

~2! determination of absorbed dose to water at referedepths in water;

~3! determination of absorbed dose to water at other dein water; and

~4! determination of absorbed dose to other biological marials on the surface.

C. List of nomenclature, symbols, and units

The following are the symbols used in this document:Bw: backscatter factor defined, for the reference field sand beam quality, as the ratio of water kerma at the surfof a semi-infinite water phantom to water kerma at that poin the absence of the phantom. It accounts for the effectphantom scatter for kilovoltage x-ray beams when the ‘‘air’’ method is used for the dose determination.Cw

med: a factor to convert dose from water to a mediummed,which is dimensionless.Dmed,z: absorbed dose to a mediummedat a depthz, ex-pressed in Gy.Dw,z: absorbed dose to water at a depthz, expressed in Gyg: fraction of the energy of secondary electrons that is lin radiative processes in the medium, which is dimensiless. For low-Z materials, it is less than 0.1% for photobelow 300 keV.HVL: half-value layer, defined as the thickness of an absoing material~usually Al or Cu! necessary to reduce the aikerma rate to 50% of its original value in an x-ray beam,narrow beam conditions. Unit of this quantity is ‘‘mm Al’for low-energy x rays and ‘‘mm Cu’’ for medium-energyrays.HC: homogeneity coefficient, defined as the ratio of the fihalf-value layer~HVL ! thickness to the second HVL thickness of a medium~usually in Al or Cu!, which is dimension-less.Kair: air kerma, expressed in Gy.Kair

in-med: air kerma in medium med, expressed in Gy.Kw: water kerma, expressed in Gy.Kw

in-med: water kerma in medium med, expressed in Gy.M raw: uncorrected electrometer reading. If no sign is incated, the measurement is made collecting the same chas during calibration. If a sign~1 or 2! is indicated~see Sec.


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V C!, it is the sign of the charge collected. Unit C~Coulomb!or rdg ~electrometer reading!.M : electrometer reading corrected for temperature, prsure, ion recombination, polarity effect and electrometercuracy. Unit C~Coulomb!.(m tr /r)med: the mass energy-transfer coefficient for a mdium med. The unit is m2/kg. The mass energy-transfer coefficient is the average fractional amount of incident phoenergy transferred to kinetic energy of charged particlesresult of the photon interactions with the medium. Whmultiplied by the photon energy fluence (C5F•E), whereF is the photon fluence andE the photon energy, it gives thkerma to the medium. The mass energy-absorption cocient is related to the mass energy-transfer coefficient(men/r)med5(m tr /r)med(12g). As g is generally very smallit is often ignored for low- and medium-energy x rays, athe mass energy transfer coefficient is used for the menergy-absorption coefficient. Thus, the kerma is takencollision kerma, and we do not distinguish collision kermand kerma in this protocol unless it is needed.(men/r)med1

med2: the ratio of the mean mass energy-absorptcoefficient for medium 2~med 2! to medium 1~med 1!,which is dimensionless. Each of the mean values is calated by averaging the monoenergetic mass eneabsorption coefficients over the photon energy fluence sptrum at the point of interest either in air or at a depthwater. In ionization chamber dosimetry, we usually have mdium 15air and medium 25water, in which case we hav(men/r)air

w , which is used to convert air kerma to watkerma, either free in air or at a depth in water.NK: air-kerma calibration factor, for a specified x-ray beaquality. This quantity, when multiplied with the correctechamber reading, yields air kerma under the conditionsthe photon fluence spectrum and angular distributions aresame as that for which the calibration factor has beenrived, expressed in Gy C21.P: air pressure inside ion chamber, in kPa. The referemeasurement pressure isPref5101.33 kPa~or 760 mm Hg!.Pdis: displacement correction factor to account for thefects due to the displacement of water by a stemless cham~i.e., only the air cavity and the chamber wall!, which isdimensionless.PE,u: correction factor to account for the effects on thesponse of a stemless chamber due to the change in phenergy and angular distributions between the calibration~inair! and measurement~in phantom!, which is dimensionlessPpol: ionization chamber polarity effect correction factowhich is dimensionless.PQ,cham: overall correction factor to account for the effecdue to the change in beam quality between calibrationmeasurement and to the perturbation of the photon fluencthe point of measurement by the chamber, and the chamstem, which is dimensionless.Psheath: waterproofing sheath correction factor to accountthe effects of the change in photon attenuation and scattedue to the presence of the waterproofing sheath in a wphantom~if present!, which is dimensionless.Pstem,air: stem correction factor to account for the effects

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the change in photon absorption and scattering betweencalibration ~in air! and the measurement~in air! due to thepresence of the chamber stem, which is dimensionless.Pstem,water: combined stem correction factor to account fthe effects of the change in photon absorption and scattebetween the calibration~in air! and the measurement~inphantom! due to the presence of the chamber stem, whicdimensionless.T: temperature, in °C. For the calibration labs in NorAmerica, the reference temperature isTref522 °C.(W/e)air: average energy expended per unit charge of iization produced in dry air, having the value 33.97 J/C. Nthat the ‘‘exposure-to-dose-to-air’’ conversion coefficient drived from this value is 0.87631022 Gy/R.zref: reference depth in water for dose calibration measument, in cm.zref50 for low-energy~up to 100 kV! x-raybeams.zref can be either 0 or 2 cm for medium-energy~100–300 kV! x-ray beams depending on the point of interest.SSD: source to surface distance, in cm. This is usuallnominal distance because the exact position of the xsource focal spot is not well defined.‘‘In-air method’’: calibration method to obtain absorbedose to water at the surface of a water phantom, based oin-air measurement using an ion chamber calibrated freair.‘ ‘ In-phantom method’’: calibration method to obtain absorbed dose to water at 2 cm depth in water, based onin-water measurement using an ion chamber calibratedin air.Use of the term ‘‘shall’’ and ‘‘should’’: recommendations oreference dosimetry and quality assurance in this protohave been systematically preceded by the term shall. Threcommendations must be followed to insure the accuracthe absorbed dose determination using the formalismsdosimetric data provided in this protocol. This term is nused in the sections headed by the term ‘‘Guidelines’’which multiple alternatives may exist for the same purpo‘‘Should’’ has been used in situations, where a recomended practice may be modified by the user providedthe replacement practice does not compromise the dosimaccuracy.

II. RADIATION QUALITY SPECIFICATION ANDDETERMINATION

A. Energy ranges considered

The energy range (40 kV<tube potential<300 kV) con-sidered in this paper is divided into two regions of clinicand radiobiological relevance:

~i! ‘‘low-energy ~or superficial! x rays’’: x rays generatedat tube potentials lower than or equal to 100 kV an

~ii ! ‘‘medium-energy ~or orthovoltage! x rays’’: x raysgenerated at tube potentials higher than 100 kV.

Since this protocol allows for the use of the in-air meththroughout the entire 40–300 kV energy range, the most


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portant reason for this division is to specify a lower limitthe medium energy range, below which the in-phantmethod shall not be used.

B. Beam quality specifier

Specification of a kilovoltage x-ray beam requires knowedge of the photon fluence spectrum at the point of interThe half value layer~HVL ! solely or in combination with thetube potential is often used to characterize the spectrHVL is specified in terms of ‘‘mm Al’’ for low-energy xrays and ‘‘mm Cu’’ for medium-energy x rays. For convnience, however, ‘‘mm Al’’ may also be used for x-rabeams with tube potentials up to 150 kV~a superficial x-rayunit may have tube potentials from 30 to 150 kV!.

The quality of a beam depends on many factors suchtube potential, target angle, target material, window materand thickness, monitor chamber material and thickness,tration material and thickness, shape of collimation, andsource-chamber distance. A measurement of HVL mayaffected by the details of the experimental setup, the produres and the energy dependence of the dosimeters uSection II C describes the setup for the measuremenHVL.

There are a variety of reports on measured x-ray speessentially from the 1960s and the 1970s,25–28 that apply toclinical as well as calibration and research x-ray setups.well, various programs have been developed for the calction of kilovoltage x-ray spectrum and the HVL value bason the calculated spectrum~see Refs. 29 and 34!. Detailedinformation about the target and the target angle, the mrials in the beam and their thicknesses are required for arate HVL calculations. In general, target material, targangle, filtration material and thickness are given by tmanufacturers while other factors are poorly known and mdiffer from the manufacturer’s specifications.

It is generally considered to be insufficient to use ontube potential or HVL to specify a beam.21 Commonly usedclinical beams have been reported to have a wide rangHVL values corresponding to the same tube potentia1,2

Chamber-related factors, such asNK andPQ,cham, as well asthe detector-independent mass energy-absorption coefficfor water to air and the backscatter factors, can vary for x-beams of the same tube potential but different HVL valuand vice versa.22,23Although dosimetry data are increasingderived as a function of both tube potential and HVL,28 theuse of both tube potential and HVL value may not copletely resolve the specification problem for all the quantitinvolved. Moreover, in the context of a protocol, the additiof a quantity in terms of which the data have to be presenincreases complexity and the probability of clinical erroFor the specification of mass energy-absorption coefficratios for in-phantom dosimetry, a recent investigation hexamined the uniqueness of the ratio of ionization at 2 cmionization at 5 cm24 but more work is required to verify thevalidity of such a beam quality specifier.

In this protocol, we separate the issue of beam quaspecification into two main stages. The first stage deals w

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obtaining the air-kerma calibration factorNK from the stan-dards lab. The chamber shall be calibrated at a beam qusufficiently close to the user’s beam quality in terms ofboththe tube potentialand HVL to ensure the validity of thecalibration factor in the clinical situation~see Sec. III B!.Preferably, the chamber should be calibrated at more tone x-ray quality to ensure that the user’s beam qualityproperly covered. The second stage deals with measuringabsorbed dose in the user beam. At this stage HVLonly isconsidered to be the quality specifier. Section VIII deals westimates of uncertainties, which include estimates forlack of complete beam quality specification by using onHVL to specify the quantity involved. For convenience, wonly use tube potential to denote the x-ray energy rangthis protocol.

C. Determination of HVL

The first HVL of an x-ray beam is defined as the thickneof a specified attenuator that reduces the air-kerma ratenarrow beam to one half its original value. The determintion of HVL involves the measurement of the variation withe attenuator thickness of air kerma at a point in a scafree and narrow beam.30,31 This means that for this measurment, detectors shall be used with sufficient buildup thiness to eliminate the effect of contaminant electrons~seeSec. III B!.

Figure 1 shows the experimental setup for the HVL msurement. The beam diameter defined by the diaphragmbe 4 cm or less. The thickness of the diaphragm mustthick enough to attenuate the primary beam to 0.1%. Tdetector shall be placed at least 50 cm away from the atteating material and the diaphragm. A radiographic checkthe alignment of the source, the diaphragm, and the deteshall be performed. A monitor chamber can be used to crect for variations of air-kerma rate especially when the akerma rate is significantly lowered by the addition of filtrtion in the beam during the HVL measurement. In that cait must be properly placed so that it does not perturbnarrow beam by adding to the scatter component, andresponse is not affected by the thickness of the attenua

FIG. 1. The experimental setup for HVL measurement. Shown in the figare source~target!, HVL attenuator, diaphragm, and ion chamber. The lotion of the monitor chamber for normalization of the ion chamber signaapplicable, is shown. The monitor may already be part of the x-ray setunot, it must be positioned such that its response is not affected by chanthe filter thickness. The ion chamber for the kerma-rate measurementbe sufficiently energy independent so that a change in filter thickness caan insignificant change in energy dependence.


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material~see Fig. 1!. For the air-kerma measurement, smasize detectors are desirable. The beam must cover the stive volume of the detector. The detector response shall hlimited beam-quality dependence~within 5% between 40 and300 kV! for accurate HVL measurements. The attenuashall be made of high-purity~99.9%! material and the thick-ness of the attenuator shall be measured with an accurac0.05 mm.

III. EQUIPMENT

A. Phantoms

When using the in-air method, the measurement is pformed free in air, and no phantom is involved~see Sec. VA!. When using the in-phantom method~see Sec. V B!, wa-ter is the phantom material to perform the measurementthe phantom size shall be 30330330 cm3 or larger. For con-venience, plastic phantoms may be used for in-phantomtine quality assurance. However, they shall not be usedin-phantom reference dosimetry for kilovoltage x rays aschamber correction factors and the conversion factors torive dose at a depth in water for these phantoms are notknown. In addition, the water equivalence of some commcial plastics for kilovoltage x rays remains an area of actinvestigation.33

B. Dosimeters

Air-filled ionization chambers shall be used for referendosimetry in kilovoltage x-ray beams. The effective pointmeasurement for both cylindrical and parallel-plate chabers is the center of the sensitive air cavity of the chambAll measurements shall be corrected for temperature, psure, ion recombination, polarity effect, and electrometercuracy. The fully corrected reading is defined asM5M rawPTPPionPpolPelec, whereM raw is the raw uncorrectedreading~in-air or in-phantom!. Descriptions of the variouscorrection factors can be found in Sec. V C. Cognizantchamber response~from either calibration standards labortories, comparison of known chamber, or manufacturedata!, chamber calibration factors should not vary signicantly between two calibration beam qualities so thatestimated uncertainty in the calibration factor for a clinicbeam quality between the two calibration qualities is lethan or equal to 2%.

For low-energy x rays with tube potentials below 70 kcalibrated soft x-ray parallel-plate chambers with a thin etrance window shall be used. Thin plastic~low-Z, e.g., poly-ethylene or PMMA! foils or plates shall be added to thentrance window, if necessary, to remove electron contanation and provide full buildup. When presented for calibtion, it is the responsibility of the user to provide thebuildup plates or foils as part of their instrument to the Acredited Dosimetry Calibration Laboratories~ADCL!, Na-tional Institute for Standards and Technology~NIST!, or Na-tional Research Council of Canada~NRCC!, since the sameplate or foils are to be used when calibrating the clinicbeam. Table I shows total buildup thickness obtained fr

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the calculated ranges of the most energetic electrons in pethylene based on continuous-slowing-down approxima~CSDA!. The thickness of the needed plate or foils mustdetermined by subtracting the window thickness~for ex-ample, 2.5 mg/cm2! from the total thickness listed in TableFor low energy x rays with tube potential of 70 kV or highecylindrical chambers that satisfy the chamber responsequirements described above can also be used.

Measurements for medium-energy x rays~tube potential100–300 kV! are performed with the effective point of mesurement of the chamber placed either at 2 cm depth in w~in cases where the dose at greater depths is of primaryterest! or free in air ~in cases where the surface dose isprimary interest!. Cylindrical chambers that have a calibrtion factor varying with the beam quality by less than 3between 100 and 300 kV shall be used for reference dosetry. If measurements are performed in water with a nonterproof chamber and a waterproofing sleeve, approprcorrection factors shall be applied depending on the slematerial and thickness~see Appendix B.2.3!. Natural or syn-thetic rubber sleeves shall not be used because their chteristics are unknown for kilovoltage x-ray beams. Care shbe taken that there is no talcum powder involved in watproofing the chamber, since talcum particles enteringcavity through the venting hole might dramatically chanthe chamber response.76 The air gap between chamber ansleeve shall not be larger than 0.2 mm. Cylindrical chambhave adequate thimble thickness~50 mg cm22 or more! andtherefore do not require a buildup cap if measurementsdone in air.66

There have been extensive studies on the correctiontors for the commonly used Farmer chamber types forin-phantom measurement.10–12,14–16,32Although in this pro-tocol correction factors are provided for some ionizatichambers only~see Appendix B.2.2!, other cylindrical cham-bers matching the above mentioned requirement~i.e., nomore than 3% variation of their calibration factor for m

TABLE I. Total wall thickness required to provide full buildup and eliminaeffects of electron contamination during calibration of a low-energy~<100kV! clinical beam using thin-window plane-parallel chambers. The windthickness of the chamber~for example, 2.5 mg/cm2! should be subtractedfrom the values listed in this table so as to arrive at the required foil or pthickness for full buildup. The data are calculated from CSDA rangespolyethylene for the most energetic electrons using ICRU Report Notabulations~Ref. 42!. CSDA ranges in PMMA are about 10% higher. Nothat for in-air calibrations in medium-energy x rays~.100 kV!, cylindricalchambers with walls.50 mg/cm2 and without buildup cap shall be used, atheir wall thickness is sufficient to provide full buildup.

Tube potential~kV!

Total wall thickness~mg cm22!

40 3.050 4.060 5.570 7.380 9.190 11.2

100 13.4


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--teve

ac-ll-e

rs

re

c-e

dium energy x rays! may also be used. However, correctiofactors must then be determined experimentally by comping the chambers with a chamber with known correction ftors ~see Sec. V C.8!.

C. Electrometers

Ionization chambers are read out by the use of a chaor current-measuring device, normally termed an electroeter. This device shall be capable of reading currents onorder of 0.01 nA, with an accumulated charge of 50–100 nIf calibrated separately from the ionization chamber, telectrometer shall be calibrated by an ADCL, NISTNRCC and the correction factor applied as part of derivthe corrected ion chamber readingM. This correction factoris generally close to 1.000 but occasionally can differ frounity by as much as 5%. If the combination of electromeand ionization chamber is calibrated together as one deno separate electrometer correction is needed~i.e., Pelec51!.

D. Quality assurance of the dosimetry equipment andx-ray tube

Quality-assurance procedures shall be performed onequipment used for the calibration. The major itemslisted below:

1. Ionization chamber

A means of monitoring the consistency of the ionizatichamber shall be established. This shall be carried outtwo or more of the following procedures:

~i! Use of a check source, usually Sr-90: This involvestimed exposure accumulating charge or current measment. The temperature and pressure corrected chambering shall remain consistent within62%. Care must be exercised to ensure that the chamber is placed in the sposition each time.

~ii ! Redundant chambers: There shall be consistencywithin 2%, in the measurement by using two or more cabrated chambers.

~iii ! Use of another beam, such as60Co: Establish thebaseline response of the chamber at60Co and verify thechamber response is reproducible to within 0.2%. Accofor the energy dependence of the chamber response, wshall be verified, in the determination of the baseline chaber response at the kV radiation quality of interest. Tchamber calibration for medium-energy x rays shouldconsistent with the60Co calibration to within 2%.

The consistency of the response of the ionization chamshall be checked every time reference dosimetry is accplished. The chamber shall be checked for constancy besubmitting it for calibration to the standards laboratory arechecked after it is received. The ion chamber shall be cbrated when first purchased, when repaired, when the cstancy checks so demand, or once every 2 yr.

2. Electrometer

The electrometer shall be checked along with the ionition chamber using the above procedures. In addition,

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other calibrated electrometer can be used with the sameization chamber and should give the same corrected reato within 0.5%. If the electrometer has a timer featucharge can be collected for a time interval to determine drate. This dose rate shall be the same as that for the xmachine timer setting when any end effect, if presentaccounted for~see below!.

3. Tube potential of x-ray generator

Generally the tube potential will not vary significantlConsistency of the x-ray output shall be checked routinelyit changes by more than 3%, the accuracy of settings oftube potential and filament, including the accuracy andearity and end effect shall be investigated.71 This shall alsobe done as a check on an annual basis.

IV. AIR-KERMA CALIBRATION PROCEDURES

Implementation of this protocol involves the calibratioof the ionization chamber in an appropriate x-ray beamterms of air kerma free in air (Kair) in a standards lab reference beam quality. Suppose thatKair is the air kerma at thereference point in air for a given beam quality andM thereading~corrected for temperature, pressure, recombinatpolarity effect, and electrometer accuracy! of an ionizationchamber to be calibrated with its reference point at the sapoint. The reference point for plane parallel chambers as was cylindrical chambers is at the center of the cavity. Tfield size shall be large enough to provide uniform exposof the chamber sensitive volume. The air-kerma calibratfactor NK for this chamber at the specified beam qualitydefined as:

NK5Kair

M. ~1!

The relation between the air kerma and the frequently uexposure calibration factorNX is given by

NK5NXS W

e Dair

~12g!, ~2!

where (W/e)air has the value 33.97 J/C(50.87631022 Gy/R! for dry air as discussed earlier, (12g)corrects for the effect of radiative losses~mainly due tobremsstrahlung emission! by the secondary charged paticles, andg is less than 0.1% for photons below 300 keVair.

Calibration factorsNK shall be traceable to national stadards, i.e., from an ADCL, NIST or NRCC, preferably fornumber of x-ray beam qualities. Both tube potential aHVL shall be used to specify the air-kerma calibration factTable II~a! shows some of the x-ray beams as providedNIST; some ADCLs provide similar beams. Note that a cabration or interpolation between HVLs might be inadequaFor example, depending on the chamber’s energy depdence, significant errors may occur if one attempts interlation between lightly filtered~L series! beams and mediumfiltered ~M series! beams. Interpolation may only be pe


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ays

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ell

een

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formed within the same series, e.g., only for the L seriesonly for the M series. Table II~b! summarizes beam-qualitranges available at NRCC.

The ADCL, NIST, or NRCC may also provide a determnation of the ion-collection efficiency during calibrationHowever, because of the low dose rates used in standlaboratories, this should be generally unity. Ion-collectiefficiency is a measure of the fraction of charge measuredthe chamber versus the total charge released, and depenthe dose rate and the collecting potential and geometry ofchamber. For the implementation of the protocol, a correcreading~see Sec. V C! shall be used. The recombination corection can be significant for the calibration of low enerx-ray machines at source to surface distance~SSD! of a fewcm where dose rates may be typically on the order ofGy/min at the treatment distance.

V. FORMALISM

For low-energy x rays~tube potential less than or equal100 kV!, reference dosimetry shall be performed free inand a backscatter factor shall be used to account for

TABLE II. ~a! Some x-ray beams provided by NIST and the ADCLs for tL and M series. The number part of the beam code represents thepotential in kV of the beam.~b! Ranges of x-radiation qualities relevant tthis protocol provided by NRCC.

~a! First HVLHomogeneity coeff.

~Al !Beam code ~mm Al! ~mm Cu!

L40 0.50 0.59L50 0.76 0.60L80 1.83 0.57L100 2.77 0.57M20 0.15 0.69M30 0.36 0.65M40 0.73 0.69M50 1.02 0.66M60 1.68 0.66M80 2.97 0.67M100 5.02 0.73M120 6.79 0.77M150 10.2 0.67 0.87M200 14.9 1.69 0.95M250 18.5 3.2 0.98M300 22.0 5.3 1.00

~b! First HVL

Peak tube potential ~mm Al! ~mm Cu!

40 0.09–2.1550 0.09–3.7460 0.09–4.8970 0.10–5.8680 0.10–6.72

100 0.15–6.83120 1.48–8.33 0.09–1.27135 1.72–8.98 0.10–1.50150 0.12–1.74180 0.17–2.18200 0.21–2.45250 0.40–3.49300 0.53–4.57

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effect of the phantom scatter. For medium-energy x r~tube potential higher than 100 kV!, two different but mutu-ally consistent formalisms can be used. If the point of intest is at the phantom surface (zref50), the measuremenshall be performed in air and a backscatter factor shallused to account for the effect of the phantom scatter~the‘‘in-air’’ method!. If the point of interest is at a depth iwater, the measurement shall be performed at the referdepth (zref52 cm) in a water phantom and a chamber depdent correction factor~and a waterproofing sheath correctioif applicable! shall be applied to account for all differencebetween the in-air calibration and the measurement inphantom~the ‘‘in-phantom’’ method!.

A. The in-air method: Absorbed dose to water at thesurface for low- and medium-energy x rays„40 kVÏtube potential Ï300 kV…

To use the in-air calibration method for a low- anmedium-energy x-ray beam~tube potential: 40–300 kV!, thereference depth for the determination of absorbed dosethe phantom surface (zref50). The absorbed dose to waterthe phantom surface shall be determined according to

Dw,z505MNKBwPstem,airF S men

r Dair

w Gair

, ~3!

whereM is the free-in-air chamber reading, with the cenof the sensitive air cavity of the ionization chamber placedthe measurement point (zref50), corrected for temperaturepressure, ion recombination, polarity effect, and electromaccuracy;NK the air-kerma calibration factor for the givebeam’s quality;Bw the backscatter factor which accounts fthe effect of the phantom scatter;Pstem,air the chamber stemcorrection factor accounting for the change in photon scafrom the chamber stem between the calibration and measment ~mainly due to the change in field size!, and@(men/r)air

w #air the ratio for water-to-air of the mean maenergy-absorption coefficients averaged over the incidphoton spectrum. The numerical values of the conversand correction factors in Eq.~3! are discussed in AppendiB.1.

Pstem,air is taken as unity if, for a given beam quality, thsame field size is used in the calibration and the measment. Otherwise, the guidelines in Sec. V C.7 shall belowed to establishPstem,air.

The backscatter factorBw must include the effect of endplates in close ended cones, if used, on the determinatiowater kerma at the phantom surface. We have providetable with multiplicative corrections to the open cone valuin Appendix B.1.2.

It shall be remembered that Eq.~3! yields the absorbeddose at the phantom surface under the conditions of chaparticle equilibrium and in the absence of electron contanation from the primary beam~i.e., assuming dose5kerma,see Appendix A.1!. This applies to open cones as well asclosed cones. For some practical guidelines to deal with etron contamination in clinical beams see Sec. VII C.


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The measurement is performed at the point where dosthe phantom surface is required~e.g., the cone end!. If this isnot possible, the measurement shall be performed at a pas close as possible to the point of interest, and correcteobtain the dose there. To this end, an inverse square cotion can be used~see Sec. V C.6!.

B. The in-phantom method: Absorbed dose to waterat 2 cm depth in water for medium-energy xrays „100 kVËtube potential Ï300 kV…

This method requires placing a calibrated ionizatichamber at a reference depth in a water phantom. Ifreference depth is too small there may not be enough builmaterial in the upstream direction to cover the whole chaber. If the reference depth is much larger than 2 cm,ionization signal in the chamber may be too small. Thefore, this protocol has adopted a reference depth of 2Although the conversion and correction factors needed informalism are only slightly dependent on depth, the data pvided in this protocol assume a reference depth of 2 cm.

The absorbed dose to water at the 2 cm reference d(zref52 cm) in water for a 10310 cm2 field defined at 100cm SSD shall be determined using

Dw,z52 cm5MNKPQ,chamPsheath@~men/r!airw #water, ~4!

whereM is the chamber reading, with the center of thecavity of the chamber placed at the reference depth, crected for temperature, pressure, ion recombination, thelarity effect and electrometer accuracy, andNK the air-kermacalibration factor for the given beam’s quality@see Eq.~1!#.PQ,chamis the overall chamber correction factor that accoufor the change in the chamber response due to the displment of water by the ionization chamber~air cavity pluswall! and the presence of the chamber stem, the changthe energy, and angular distribution of the photon beamthe phantom compared to that used for the calibration inPsheathis the correction for photon absorption and scatterin the waterproofing sleeve~if present! and @(men/r)air

w #water

the ratio for water-to-air of the mean mass energy-absorpcoefficients, averaged over the photon spectrum at the reence point in water in the absence of the chamber. Themerical values of the conversion and correction factors in~4! are discussed in Appendix B.2.

C. Other considerations in the calibrationmeasurement

The following points need to be considered in the calibtion measurement of kilovoltage x-ray beams~consult Ap-pendix C for detailed descriptions of the dose calibratmeasurement!:

1. Ion collection efficiency P ion

To determine accurately the dose absorbed in the aithe ionization chamber cavity, the complete collection of tions formed by the radiation is required. Some of the iorecombine with ions of the opposite charge on their waythe collection electrode and are not collected. Models h

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been developed to estimate the true number of ions formfrom measurements made with two different voltages.52 Thevalue is usually obtained by using the normal collecting voage and half that voltage.53–55 Although, recent literaturesuggests many small problems with this procedure, thecent AAPM TG51 protocol56 as well as this protocol havused the same procedures because the accuracy is expto be better than 0.5% at normal chamber operating voltaof 300 V or less.56 For the procedure, letVH be the normalcollecting voltage for the detector,M raw

H be the raw chambereading with biasVH , andM raw

L the raw chamber reading abiasVL , whereVL /VH<0.5. M raw

L andM rawH are to be mea-

sured once the chamber readings have reached equilibrFor continuous beams, the two-voltage approach yields56

Pion~VH!5

12S VH

VLD 2

M rawH

M rawL 2S VH

VLD 2 . ~5!

GenerallyPion is close to unity but care should be exerciswhen using small SSDs. If an ion chamber exhibits a corrtion factor Pion greater than 1.05, the uncertainty becomunacceptably large and another ion chamber with a smarecombination effect shall be used.56

2. Polarity correction P pol

Polarity effects depend on beam quality and cablerangement and shall be measured and corrected for. ThePpol

factor can be deduced from56

Ppol5UM raw1 2M raw

2

2M rawU, ~6!

whereM raw1 is the reading when positive charge is collecte

M raw2 is the reading when negative charge is collected,

M raw ~one ofM raw1 andM raw

2 ! is the reading corresponding tthe charge collected for the reference dosimetry measments~the same as used for the chamber calibration!. In bothcases, the sign ofM raw must be used and usuallyM raw

1 andM raw

2 have opposite signs unless the background is laAdequate time must be left after changing the sign ofvoltage so that the ion chamber’s reading has reached elibrium.

3. End effect dt

The end effect is defined as the amount of time that isaccounted for by the machine timer mechanism duringx-ray beam delivery. This amount of time usually describthe time difference between when the timer mechanism sand when the desired mA and kVp is achieved, or the finitetime required for the shutter to move from the fully closedthe fully open position. A small end effect~0.5–3 s! mayplay a significant role in the output calibration proceduespecially for the small dose range~3 min or less treatmenduration!. The end effect for an x-ray unit can be measurusing the graphical extrapolation method. The graphicallution of zero exposure on an exposure versus expos


d

-

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ctedes

m.

c-ser

r-

,d

e-

e.eui-

tesrts

do-e-

timer plot yields the end effect. The end effectdt can also bederived using a mathematical equation described by Atti57

dt5M2Dt12M1Dt2

M12M2, ~7!

where M1 and M2 are the chamber readings for exposutime Dt1 and Dt2 , respectively. Coffey20 shows that theabove two methods may give slightly different results asmathematical equation uses only two points, whereasgraphical method uses the whole time range of clinical intest. To ensure the accuracy of the measured end effectgraphical method shall be used during the machine commsioning and annual QA. The mathematical method mayused for the monthly QA measurement.

4. Electrometer correction „Pelec…

The device used to read the signal from the ionizatchamber requires calibration as part of the instrument cbration process. This calibration is performed at the ADCor NIST. At NRCC, electrometer and chamber are usuacalibrated together, as one instrument.Pelec represents thecalibration factor for the reading device only.

5. Temperature –pressure correction P TP

The calibration factor assigned by a standards laborato an ionization chamber is based on the mass of gas~air!present in the volume. This mass varies with temperaturepressure when the chamber is open to the atmosphere. Tfore, correction of the amount of charge collected in tchamber must be made to the reference temperature~Tref is22 °C! and pressure@Pref is 101.33 kPa~760 mm Hg!#. Thecorrection required for the actual temperature and pressu

PTP5Pref

P

~T@°C#1273.2!

~Tref@°C#1273.2!. ~8!

6. Inverse-square consideration for in-aircalibration with close-ended cones

Because of the finite size of an ionization chamber itoften impossible to measure the air kerma directly atcone end. The inverse-square relation can be used to dethe air kerma value at the cone end using the measured vat an extended distance provided the effective source ption is known. The effective source position is generally dferent from the x-ray focal spot due to photon scatteringthe end plate. The effective source position can be demined using measurements made at different distancesthe smallest chamber available, and then extrapolatingzero distance to the cone end.3,50 Note that thePstem,airvaluemay change~because of its field-size dependence! if mea-surements are performed at different distances.

7. Method for determination of P stem,air

Pstem,air accounts for the effect of the change in photscatter from the chamber stem between the calibrationstandards laboratory and the in-air measurement in a us

her

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beam. Pstem,air should be measured by intercomparing tchamber with unknownPstem,airwith a reference chamber fowhich Pstem,airis known. Letf u be the field size for which thebeam needs to be calibrated andf c the field size for whichthe chamber has been calibrated at the standards laboraThe effective point of measurement of the chamber unstudy and that of the reference chamber should be placethe same point in air. The stem correctionPstem,air( f u) isdetermined from the equation:

Pstem,air~ f u!5M ~ f c!

M ~ f u!

M ref~ f u!

M ref~ f c!Pstem,air,ref~ f u!, ~9!

whereM ( f c) andM ( f u) represent corrected meter readinfor the chamber under study;M ref( f c) andM ref( f u) the cor-rected meter readings for the reference chamber. Thesize can be changed either by changing the cone and keethe chambers at the same position in space or by measuat several source–chamber distances using a single conerequirements for the reference chamber are the same asmulated for the dosimeters used for reference dosim~Sec. III B!. However, note that theNK value for the refer-ence chamber used for the measurement ofPstem,airdoes notneed to be known as long as it has been established tharesponse variation satisfies the requirements formulateSec. III B. It is suggested that a Farmer type cylindricchamber with flat response be used as a reference chafor the measurement ofPstem,airof another chamber sincehas been established that its stem effect varies by less1%.11,15It is important that, under all circumstances, the ssitive volumes of the chamber under study as well asreference chamber for the measurement ofPstem,air be wellcovered by the radiation beam: the beam diameter shtypically be 50% larger than the sensitive diameter ofchamber.

8. Method for in-phantom calibration of chambersnot listed in this protocol

For a chamber not listed in Appendix B.2.2 the chamshall be intercompared in-phantom, in the beam of interwith one of the listed chamber types. To this end, bothinvestigated chamber and the reference chamber with knair-kerma calibration factor and correction factors shouldexposed in-phantom. Irradiation of the investigated chamshould be preceded and followed by the irradiation ofreference chamber at each radiation quality for which a cbration factor is needed, so as to ensure machine stabilitypositioning reproducibility of the chambers. The pointmeasurement of both the chamber to be investigated asas the reference chamber should be placed at the sameerence depth. All measurements should be normalizedmonitor chamber reading placed in a position where it dnot affect the reading of the reference chamber or the chber under study~for example, in the phantom downstreamthe corner of the field!. The overall calibration and correctiofactor for the investigated chamber can be calculated us


ry.rat

ldinginghe

or-ry

theinlber

an-e

lde

rt,en

eerei-nd

ellref-

as-

g:

~NKPsheathPQ,cham!u5M ref

Mu~NKPsheathPQ,cham!ref , ~10!

whereMu andM ref represent the in-phantom chamber reaing of the investigated chamber and the reference chamrespectively, both corrected for pressure, temperature,recombination, and electrometer accuracy, and the ochamber dependent quantities (NK ,PQ,cham,Psheath) for thereference chamber are provided by the ADCL, NIST,NRCC, and from this protocol.

D. Consistency between the in-air method and the in-phantom method

Depth-dose curves for kilovoltage x-ray beams are dicult to measure~and therefore less accurate! especially nearthe surface. Thus, the determination of the dose on or cto the surface might be less reliable using the in-phantmethod compared to the in-air method. If the point of interis near the surface, the method of choice for calibrationthe therapeutic use of kilovoltage x rays is the in-air methOn the other hand, in instances where an accurate dosetermination at a depth of 2–3 cm is critical, the pointinterest is at a depth and the in-phantom method shalused. For example, in many animal radiology experimewith large animals such as dogs, pigs, etc., the point ofterest is at a depth of several centimeters beneath theThe in-phantom method in these cases can provide a maccurate assessment of the dose because the depthcurves are more consistent when normalized at 2 cm derather than at the surface.

In any case, because of the overlap in methodology inmedium-energy x-ray range, the consistency of the datamust be ensured. The consistency of using either the in-ain-phantom method for medium-energy x rays using the dsets in this protocol has been investigated.34 The proceduresfor the measurement of the central-axis depth-dose curwhich serve as a link between the dose at the reference dto the dose elsewhere in a phantom, were examined. Dedependent correction factors were calculated usingMonte Carlo method for two types of detectors involvedthe measurement of the relative depth-dose curves. Althothe two selected detectors differed significantly in their eergy responses, after correction, the measured depth-curves for both detectors agreed to within 1.5%.34 Using thecorrected depth-dose curves to relate the dose at depth tdose at the surface, and using the data adopted in this pcol, the consistency between the two methods was withinfor a 100 kV~2.43 mm Al! beam and within 0.5% for a 300kV ~3.67 mm Cu! beam. It was concluded that the accuraof the depth-dose measurement was essential to the cotency study. The response of a detector needs to be knaccurately before it can be used for depth-dose measments.

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VI. GUIDELINES FOR DOSIMETRY IN OTHERPHANTOM MATERIALS

This protocol describes methods to determine dose toter at a 2 cmdepth in water or at the surface of a watphantom according to the preferred calibration proceduHowever, for clinical radiotherapy and radiobiology, thdose to biological tissues on~or near! the irradiated surface isof interest.

The surface dose for other materials~med! can be calcu-lated from

Dmed,z505CwmedDw,z50 ~11!

with the conversion factor from dose-to-water to dose-medium given by

Cwmed5

Bmed

BwF S men

r Dw

medGair

, ~12!

where @(men/r)wmed#air represents the ratio of mass energ

absorption coefficients medium to water averaged overprimary photon spectrum free in air, andBmed/Bw the ratioof kerma based backscatter factors medium to water. Tmeans that multiplication ofDw,z50 using the proceduredescribed in this protocol withCw

meddirectly gives the dose athe surface of a phantom of material med. The numervalues for the factors in Eq.~12! can be found in AppendixB.3.

VII. GUIDELINES FOR RELATIVE DOSIMETRY ATOTHER POINTS IN WATER

A. Characteristics of clinical beams

Prior to the development of this protocol a survey onstatus of clinical kilovoltage x-ray dosimetry was carriout.1,2 In the questionnaire, information was requested ontube potential and HVL for the radiation qualities in clinicuse. Figure 2 shows the relation between tube potentialhalf value layer as reported by the participants. The wrange of HVL values for the same tube potential reflectsdifferences in target material and angle, exit window marial and thickness, monitor chamber material and thickneand the variations with filtration material and thickness.

Further information on the characteristics of clinicbeams can be found in Jennings and Harrison43 for x-rayqualities with HVL less than 0.5 mm Cu, and by Smith aSutherland44 for HVL of 0.5 mm Cu and higher. Also thepapers by Scrimger and Connors,45 Niroomand-Radet al.,46

Gerig et al.,47 Kurup and Glasgow,48 Aukett et al.49 and Liet al.33,50 report on typical characteristics of clinical beamThis material has also been reviewed in BJR Suppment 25.51

B. Recommendations for relative dose measurementsin water

The absorbed dose to water at other points in a wphantom can be derived from the measured dose value


a-

e.

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is

al

e

e

ndee-s,

.-

erat

the reference depthzref and the measured percentage dedose~PDD! curves. The measurement of PDD and dose pfiles is difficult in kilovoltage x-ray beams. Solid detectorattractive because of the small size of their sensitive volu~diode detector, TLD, film!, usually show significant beamquality dependence and/or large experimental uncertainWell-designed cylindrical chambers have nearly constantergy response for tube potentials between 40 and 300 kVare suitable for in-phantom measurements. However,measurement depth is limited to no less than the outer raof the chamber. Parallel-plate chambers have been usemeasurements at smaller depths. Those chambers desfor electron beams usually have a calibration factor varywith beam quality by 20%–40% in kilovoltage x rays. Sinificant corrections with depth may be required for the PDmeasurement with these chambers. Specifically designedwindow chambers for low-energy x rays usually have aenergy response in air but not at a depth in a phantom.instance, variations in chamber response of more than 1have been observed for the Capintec PS-033 chambThus, a depth-dependent correction factor may be requfor these chambers to be used in the PDD measuremFurthermore, the depth dependence of the conversion fa

FIG. 2. Relation between tube potential and reported HVL valufor low- and medium-energy beams as reported by North American clin~see Ref. 1!.

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lsncth

bersneck

iptrn

berdis-allyur-and-ay-

tou-enre-p-

tor

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~from the measured ionization to absorbed dose! may intro-duce additional uncertainties in the measured percendepth-dose curves.22,23

Although the information on suitable detectors for relatix-ray dosimetry is far from comprehensive, some work hbeen performed recently to evaluate specific detector tyfor their suitability to measure depth-dose curves in kilovoage x-ray beams.34 As a general requirement to evaluate tsuitability of a specific detector, the relative response freeair as well as in-phantom should be compared with a wbehaved~Sec. III B! cylindrical chamber at depths where resonable measurements with the cylindrical chamber canperformed. Diamond detectors and the NACP plane parachamber have been found to require relatively small dedependent corrections in medium-energy x-ray beams34,58al-though one should investigate the specific device in termmeeting the requirements for accurate relative measments.33 Diode detectors are not suitable for relative dosietry in this photon energy range.

If a suitable detector for relative dosimetry cannotidentified in the clinic the data from the British JournalRadiology Supplement 2551 shall be used.

C. Electron contamination of clinical beams

This paper deals with the determination of the absoldose at the reference depth (zref) under the conditions of fullcharge particle equilibrium, i.e., the dose value is equivato the kerma value~see Appendix A!. This requires that cali-brations and measurements be made using a chamber henough buildup so that it indeed measures kerma. It isticularly important for the in-air method that the chambsignal is not affected by the contaminating electrons genated in air and on the inside surface of the treatment coThe presence and specific magnitude of this electron ctamination, measured as increased surface dose, depenthe HVL of the x-ray beam, the size of the treatment coand the buildup of the chamber~e.g., the window thicknessof a parallel-plate chamber! used in the dose determinations. 59–62 It has been further shown59,62 that this enhancedsurface dose depends strongly on the material from whthe treatment cone is fabricated with up to a five-foincrease in relative surface dose with lead lined trement cones ~2.0 cm diameter and HVL53.0 mmCu!.Klevenhagen60 reported that the relative surface dose achanges across the radiation field with the greatest enhadose being in the periphery of the treatment field nearedge of the applicator.

In terms of clinical radiation effects, the dose measuredthe actual surface of the skin would have little meaningcause of the insensitivity of the most superficial skin layeHowever, this reported effect may have clinical ramificatiodepending on the depth of the radiosensitive dermal anddermal layers of the overlaying skin tissues. Epidermal thinesses have been reported63 to be 4.7, 6.6 and 40.6 mg/cm2

on the body trunk, the arms and legs, and the fingertrespectively. Thus the enhanced radiation dose from eleccontamination to the epidermis may be clinically releva


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and should not be ignored. Hence, the ionization chamwindow thickness used in surface dose determinationcussed above becomes an important issue radiobiologicas well as dosimetrically. The consideration of the high sface dose may be even more important in intracavitaryintraoperative radiotherapy59,64 in that the irradiated epithelial linings and tissues do not have the insensitive outer lers for protection and any enhanced dose is now givenliving cells. To minimize potential radiation overdose to sperficial tissues within the treatment field it has besuggested59,65 that this increase in surface dose can beduced clinically by:~1! increasing the distance from the aplicator cone to the patient surface,~2! inserting an equilib-rium thickness of low-Z absorber between the applicacone and the patient surface, and~3! covering the lead-linedapplicator walls with low-Z material of sufficient thicknesto achieve equilibrium. Alternatively, the user can measthe extent of the dose enhancement region, if presentperforming measurements with a thin window chamber usplates with thicknesses that provide incomplete buildup, retive to the full buildup situation under which the chambwas calibrated. However, little has been reported in theerature on the factors needed to convert the measured iotion to the dose near the skin surface.

A significant low-energy x-ray dosimetry problem alsexists at the interface between two dissimilar materials, esoft tissue and high-Z materials. For example, in some clcal situations, such as treatment of the lip, buccal mucoand eyelid lesions, internal shielding is useful to protecthealthy structures beyond the target volume. Lead or soother high-Z material may be used to reduce the transmidose to an acceptable level. However, backscattered etrons and photons from the high-Z absorber material wenhance the dose to the surrounding tissues in the immevicinity upstream to the shield.

Spiers66 describes early work on a second, particuladifficult situation in clinical dosimetry presented by sotissue/bone transition zones encountered with low-energrays. The changes in dose which occur at these intertransition zones are difficult to measure and quantify duethe difficulties associated with microscopic distances andavailability of proper dosimetry systems. Saunders aPeters67 reported this dose enhancement effect for 280orthovoltage x rays. They reported a dose enhancementtor of approximately three near a polystyrene/lead interfafor x rays of 1.7 mm Cu HVL. Wingateet al.68 reported a1.5–2.2-fold increase in absorbed dose at a one microntance upstream from a soft tissue/glass interface for supcial and orthovoltage x rays. The dose enhancement fellfactor of 1.2 at a distance of 5mm from the interface boundary. Das69 reported an up to 20-fold localized dose enhanment created by the high-Z interface in kilovoltage~60–240kV! x-ray beams. Daset al.70 reviewed kilovoltage x-ray do-simetry at high-Z interfaces.

As with the consideration of enhanced surface dose duphotoelectron contamination, the increased dose due toondary scattered electrons and backscattered photons ainterfaces between soft tissue and high-Z materials may h

y


Medical Physics, Vo

TABLE III. Estimated combined standard uncertainty~1 s! in Dw at the reference depth in kilovoltage x rabeams using a chamber calibrated in-air in terms of air kerma.

Type of quantity or procedureUncertainty

~%!

In-air method~for low and medium energies!1 NK from standards laboratory or ADCL 0.72 Effect of beam-quality difference between calibration and

measurement2.0

3 Backscatter factorBw 1.54 Pstem,air 1.05 @(men/r)air

water#air 1.56 In-air measurement in the user’s beam 1.5

Combined standard uncertainty forDw,z50 3.57 Conversion to dose to tissue at the phantom surface 1.0

Combined standard uncertainty forD tissue,z50 3.68 Determination of dose at other points in water 3.0

Combined standard uncertainty forDw,z 4.7

In-phantom method~for medium energies only!1 NK from standards laboratory or ADCL 0.72 Effect of beam-quality difference between calibration and

measurement2.0

3 Chamber correction factorPQ,cham 1.54 Chamber waterproofing sheath correction factorPsheath 0.55 @(men/r)air

water#water 1.56 In-water measurement in the user’s beam 2.0

Combined standard uncertainty onDw,z52 cm 3.67 Determination of dose at other points in water 3.0

Combined standard uncertainty onDw,z 4.7

beA

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clinical ramifications depending on the total dose prescriand the radiosensitivity of the surrounding normal tissues.suggested by Khan71 for clinical electron beams, one coullimit the dose from secondary radiation by coating the ustream side of the high-Z absorber with an adequate thness low-Z material, i.e., paraffin or other bolus-like mateor aluminum.

VIII. EVALUATION OF UNCERTAINTIES

The final uncertainty in the absorbed dose, which candelivered to the tumor in a clinical situation, should be betthan 65%.77 This final uncertainty comprises several components. The first part occurs as a result of uncertaintiethe calibration chain linking the calibration of the clinicbeam to the standards laboratory~such asNK factor, uncer-tainties in conversion and correction factors, beam-quaspecification uncertainties!. The second part is associatewith clinical uncertainties in treatment planning dose callation, patient setup, immobilization, and treatment. Thecertainty discussion here deals only with the former comnent of the final uncertainty, i.e., the calibration of tclinical beam in terms of the desired quantity~dose to wateror dose to tissue!. Table III lists the several components cotributing to the final uncertainty including type A and typeuncertainties. For a classification of uncertainties we refeRef. 8. Consistent with the procedures followed in thisport, generally four sources of uncertainties can be conered:

l. 28, No. 6, June 2001

ds

-k-l

er

in

y

---

to-d-

~i! uncertainties in the air-kerma calibration chain,~ii ! uncertainties in determining absorbed dose to wate

the reference depth in water,~iii ! uncertainties in determining absorbed dose at ot

points in water, and~iv! uncertainties associated with the transfer of the d

to other biological tissues.

The difference in the uncertainty between the in-air andin-water measurement~Table III, item 6! is mainly due to theuncertainty in the depth determination. The 1% uncertaiin the air-to-tissue dose conversion for the in-air measument ~Table III, item 7! is mainly due to the uncertainty inthe backscatter factor ratio in Eq.~12!.

IX. FUTURE CONSIDERATIONS

The main task of this protocol is to provide recommendtions for the determination of absorbed dose to water atsurface or at 2 cm depth in a water phantom irradiated40–300 kV x-ray beams under the reference conditioGuidelines are also provided for the determination of asorbed dose to other biological materials on the surfacehuman body and the relative measurement of dose to wat other points in a water phantom for kilovoltage x-rbeams. However, many other clinically related issues, sas those listed below, have not been addressed in this pcol. The following is a brief list of issues, which requirfurther investigation and may be addressed in a futAAPM report:

s

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s;

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ri

tono

armef

inr iga, se

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t

ona

ec

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~a! determination of absorbed dose to biological tissuea depth in a human body;

~b! determination of dose to water using solid phantom~c! dosimetry for endocavitary radiotherapy~the Papillon

technique!;~d! dosimetry for kilovoltage x-ray radiosurgery system~e! biological effect of electron contamination;~f! biological effect of photon and electron backscatter

at tissue/high-Z material interface; and~g! relative biological effectiveness~RBE! of kilovoltage

x-ray beams.

ACKNOWLEDGMENTS

We would like to thank the AAPM Radiation TherapCommittee~RTC!, especially Jatinder Palta and Jeff Wiliamson for support and the RTC reviewers Bruce Gerbi,W. O. Rogers, Bruce Thomadsen, and other members JDawson, Chris Deibel, John Gibbons, Jr., M. Saiful HuEllen Yorke, and Robert Zwicker for valuable discussioand comments.

APPENDIX A. THEORETICAL BASIS FOR A CODEBASED ON AIR-KERMA CALIBRATIONS

For kilovoltage x-ray beams, the absorbed dose to wateusually determined with an ionization chamber calibratedair in terms of air kerma~or exposure!. The commonly usedionization chambers are generally considered to be ‘‘phodetectors’’ as the well-known Bragg–Gray cavity theorylonger applies to this energy range.72

In order to determine the dose to water, we start withair-kerma measurement and then convert it to water keusing the ratio of the mean mass energy-absorption cocients for water to air, evaluated for the fluence spectrumthe position of interest~free in air or at the reference depthwater!. The conversion from water kerma to dose to watefairly straightforward based on the fact that, for this enerrange, the difference between kerma and collision kermnegligible and the range of the charged particles is smallthat the quasi charged particle equilibrium can be assumThis is generally true for kilovoltage x-ray beams.

A.1. Low-energy x rays „40 kVÏtube potentialÏ100 kV…

For low-energy x rays~<100 kV!, the measurement icarried out with an ionization chamber free in air, in tabsence of any phantoms. The chamber is calibrated in teof air kerma at a radiation quality sufficiently close to whatpresent in the user’s beam. The air kerma at the poininterest in a user’s beam is given by:

Kairin2air5MNKPstem,air, ~A.1!

where M represents the corrected, free-in-air, ionizatichamber reading in the user’s beam of the same beam quand field size as those in the calibration,NK the air-kermacalibration factor at the user’s beam quality, andPstem,air acorrection factor accounting for the difference in stem eff


at

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isn

n

na

fi-at

syisod.

ms

of

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t

between the calibration beam and the users beam due todifference in field size between the two beams. This msured air kerma can be converted to water kerma, free inthrough the ratio of mean mass energy-transfer coefficiefor water to air @(m tr /r)air

w #air , evaluated over the photofluence spectrum free in air, in the absence of a phantom.then have

Kwin2air5MNkPstem,air@~m tr /r!air

w #air . ~A.2!

Physically,Kwin–air represents water kerma to a small mass

water, just large enough to provide full electron buildup, bsmall enough not to alter the primary photon fluence. Twater kerma at the surface of a water phantomKw can becalculated using the following relationship:

Kw5Kwin2airBw , ~A.3!

whereBw is the backscatter factor to account for the effectphantom scatter. Equation~A.3! defines the backscatter factor as the ratio of water kerma at the water surface to wakerma free in air. Generally,Bw is field size, beam qualityand SSD dependent.

The absorbed dose to waterDw at the water surface cabe approximated byKw with the assumption of the existencof charged particle equilibrium and the negligible differenbetween kerma and collision kerma~i.e., assuming (m tr /r)air

w

equals (men/r)airw !. For this energy range, these assumptio

are justified.~Strictly speaking, this approximation is onlvalid for depths beyond the range of the contaminant etrons and where quasi charged-particle equilibrium has bestablished.! We then arrive at

Dw5M•NKPstem,airBw@~men/r!airw #air , ~A.4!

where@(men/r)airw #air can be calculated from

F S men

r Dair

w Gair

5

E0

EmaxS men

r~E! D

w

EFEfree–air~E!dE

E0

Emax S men

r~E! D

air

EFEfree2air~E!dE

,

~A.5!

where FEfree–air(E) represents the photon fluence spectru

differential in energyE, of the incident x-ray beam at thpoint of interest. Note that@(men/r)air

w #air is independentoffield size as it is evaluated over the primary beam only.73

A.2. Medium-energy x rays „100 kVËtube potentialÏ300 kV…

For medium-energy x-ray beams~tube potential 100–300kV, HVL: 0.1–4 mm Cu!, two different algorithms havebeen recommended by previous dosimetry protocols.‘‘the in-air method,’’ the air kerma is measured in air anthen converted to dose to water through the ratio of menergy-absorption coefficient for water to air and a backs

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ter factor. For ‘‘the in-phantom method,’’ the air kermathe reference depth~5 5 cm according to ICRU3 and IAEA8!in water is measured under the reference conditions andconverted to the absorbed dose at the depth of the centthe chamber in undisturbed water using the ratio of menergy-absorption coefficient for water to air and othbeam-quality- and chamber-related correction factors.

The reason for the ICRU3 protocol to adopt the ‘‘in-phantom’’ measurement at 5 cm depth for medium-enex-ray beams results from the fact that it is difficult to maaccurate measurements in the regions at or close to theface of a phantom, and the dose distribution here, unlikegreater depths, is considerably affected by the details ofbeam defining system.3 This was considered to be the reasthat the British Journal of Radiology Supplement 1174 gavetwo distinct sets of depth–dose tables for ‘‘close-ended’’plicators and ‘‘open diaphragms,’’ respectively. ICRU Rport 233 suggested that by normalizing the depth–docurves at a depth rather than at the surface the differencthe recommended depth–dose curves would be virtueliminated. The dose values at greater depths were of clinimportance as medium-energy x-ray beams were primaused for treating deep-seated tumors in the 1970s.3

Although most of the dosimetry protocols published sinthe 1970s adopted the in-phantom method for referencesimetry for medium-energy x-ray beams, the backscamethod is the most used method for this energy range inclinical radiotherapy community, especially in NorAmerica.1,2 This may be explained by the fact that orthovoage beams are used mainly for treating tumors close tosurface of the skin. The primary point of interest is the donear the surface rather than at greater depths. Another reis that it is more convenient to do routine calibration freeair than in a water phantom.75

It is realized that the dosimeter response to kilovoltax-ray beams has not been fully investigated, especially wplaced in a phantom near the surface. The uncertainty inpercentage depth dose measurement can be very largethe phantom surface, depending on the dosimeters used~seeSec. III B!. It is therefore clear that if the primary point ointerest is at the phantom surface the in-air method shalused with the reference depth at the phantom surface in oto reduce the uncertainty in the measured dose. On the ohand, if one is more interested in the dose at a depth~tocheck the dose at the critical organs! than at the surface, ‘‘thein-phantom method’’ shall be used with the reference deat 2 cm depth. Better agreement in measured percendepth dose curves at depths of 1 cm and greater canachieved when normalized to the values at 2 cm referedepth than normalized to the surface values. In additionmeasurement at 2 cm depth provides more signal than on5 cm. These are the main reasons why 2 cm has been chhere.

When the in-air method is used, the measurement isformed with the chamber free in air and the dose to watethe phantom surface can be calculated from Eq.~A.4!. Otherreference conditions are the same as described in the pous section.


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For the in-phantom measurement, use is made of a chber, calibrated free in air in terms of air kerma at a radiatquality the same as or sufficiently close to that being useirradiate the phantom. This chamber is placed at the reence depthzref in a full water phantom irradiated with areference field of medium-energy x rays. Due to phantattenuation and scattering, the photon spectral and angdistribution is different from that free in air at which thchamber is calibrated. Therefore, the calibration factorNK ,which applies to the primary radiation, does not necessaapply to the situation in the phantom. A correction factPE,u is introduced to account for the change in calibratifactor caused by the change in photon energyE and angularu distribution. The combined effect of the chamber stefree-in-air and in-phantom is usually accounted for serately using an overall correction factorPstem,water.

11,15

Similarly we introduce the waterproofing sheath corretion factorPsheath, which accounts for the effect of the plastic sheath to protect a nonwaterproof chamber when usewater.23 Furthermore, by inserting the chamber in the phatom, an amount of water is displaced by the air cavity athe chamber walls~the volume is equivalent to the outedimensions of the chamber, excluding the stem sinceeffect of the stem, if present, has been accounted for srately!. An additional correction factorPdis is then requiredto account for the change in air kerma at the point of msurement due to the displacement of water bychamber.11,16

The air kerma at the reference depthzref , Kairin–water, can be

calculated by

Kairin–water5M•NK •PE,u•Pstem,water•PdisPsheath, ~A.6!

whereM is the chamber reading corrected for temperatupressure, polarity effect, and electrometer accuracy, inuser’s beam at depthzref in the water phantom. We nowintroduce the overall correction factorPQ,cham which incor-porates all ‘‘beam-qualityQ and chamber~cham! depen-dent’’ corrections mentioned above, except for the shecorrectionPsheathsince it is not directly related to the individual chamber type.PQ,cham is defined as

PQ,cham5PE,u•Pstem,water•Pdis. ~A.7!

Air kerma in water, measured as described above, is cverted to water kerma, using mass energy-transfer coefficratios water to air,@(m tr /r)air

w #water averaged over the photofluence spectrum at the point of interest in the phantom inabsence of the chamber, i.e.,

Kw5Kairin2waterF S m tr

r Dair

w Gwater

. ~A.8!

The absorbed dose to waterDw at the reference depth inwater can be approximated byKw with the assumption of theexistence of quasicharged particle equilibrium and the negible difference between kerma and collision kerma. Wthen have

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ykV-end

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tis-

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Dw.Kw5M•NK•PQ,cham•Psheath@~men/r!airw #water. ~A.9!

As stated above,PQ,cham carries the complete chamber dpendence in the correction procedure except for the shcorrection, whereas@(men/r)air

w #water is a chamber independent conversion factor. However, both factors are field-sand depth dependent.

APPENDIX B. DETAILS ON CONVERSION ANDCORRECTION FACTORS

This Appendix contains the numerical data and produres necessary to apply the expressions based on imeasurements for low-energy and medium-energy x-raysimetry and for the in-phantom measurements at medenergies.

B.1. The in-air method for low- and medium-energyx rays

B.1.1. In-air mass energy-absorption coefficientratio †„men Õr…air

w‡air

Table IV gives the values of the in-air mass energabsorption coefficient ratios applicable to the low-energyx-ray range as a function of HVL in Al and to the mediumenergy range as a function of HVL in mm Al and Cu. Thvalues given are from a global fit to data from Seuntjeet al.,28 the IPEMB18 code of practice, and from Ma anSeuntjens.35 For simplicity, only HVL is used to specify thebeam quality. One should therefore keep in mind thatuncertainty on this quantity is no better than61.5%. All thesources are based on the interaction data publishedHubbell,36 which are consistent with the more recent dataHubbell and Seltzer36~b! for kilovoltage beams.

B.1.2. Backscatter factor B w

For tube potentials 40–300 kV, the values of the watkerma based backscatter factorBw are given in Tables V~a!and V~b! as a function of SSD, field size, and HVL~mm Alfor low-energy x rays, and mm Cu for medium-energyrays!. The values are from Grosswendt37,38 and have beenindependently checked using the experimental data fKlevenhagen39 and the Monte Carlo data from Knight anNahum.22 Note the large dependence of the backscattertors on field size in the medium-energy x-ray range. Fshort SSD~5 10 cm!, backscatter factors have been givfor beam qualities up to 4 mm Al HVL. Since the backscatfactor is fundamentally a water–kerma ratio, reliable msurements are nontrivial. Therefore, for the applicationthis protocol, backscatter factors should not be measurethe clinic.

The backscatter factors from Table V apply to open-encollimators. Close-ended applicators require slightly higbackscatter factors because of scattering in the end pTable VI shows the multiplicative correction factors toapplied to the open field values for medium-energy x raysclose-ended cones with a PMMA end plate of 3.2 mm.51


th

e

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-

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e

byy

-

m

c-r

r-fin

drte.

r

B.1.3. Chamber stem correction factor P stem,air

Pstem,air accounts for the effect of the change in photscatter from the chamber stem between the calibrationstandards laboratory and the measurement in a user’s bThe effect of photon scattering from the chamber stembeen included in the calibration factorNK for the beam qual-ity and photon field used in the calibration. When the usebeam quality and field size match those used in the calibtion, no correction is required for the chamber stem effeHowever, if the user’s field size is different from that usedthe calibration the stem effect correction may be significaThe stem effect correction is well within 1% for Farmer typcylindrical chambers11,15 if the field size~diameter! differsby less than 50% for field sizes greater than 5 cm diameNo stem corrections are needed for these chambers~i.e.,Pstem,air51! provided the chamber response variation sa

TABLE IV. Ratios of average mass energy-absorption coefficients wateair, free in air, to convert air kerma to water kerma as a function of H~mm Al! or HVL ~mm Cu!. The values given are from a global fit to dafrom Seuntjenset al. ~Ref. 28!, the IPEMB~Ref. 18! code of practice, andfrom Ma and Seuntjens~Ref. 35!.

First HVL

@(men/r)airw #air~mm Al! ~mm Cu!

0.03 1.0470.04 1.0470.05 1.0460.06 1.0460.08 1.0440.10 1.0440.12 1.0430.15 1.0410.2 1.0390.3 1.0350.4 1.0310.5 1.0280.6 1.0260.8 1.0221.0 1.0201.2 1.0181.5 1.0172.0 1.0183.0 1.0214.0 1.0255.0 1.0296.0 1.0348.0 1.045

0.1 1.0200.2 1.0280.3 1.0350.4 1.0430.5 1.0500.6 1.0560.8 1.0681.0 1.0761.5 1.0852.0 1.0893.0 1.1004.0 1.1065.0 1.109

4.0

7 1.0577 1.0984 1.1165 1.1289 1.1329 1.1339 1.133

0 1.0581 1.1100 1.1420 1.1758 1.1950 1.1981 1.199

0 1.0585 1.1130 1.1521 1.1988 1.2405 1.2507 1.252

1 1.0609 1.1187 1.1577 1.2136 1.2719 1.2881 1.292

2 1.0590 1.1189 1.1611 1.2200 1.2968 1.3213 1.328

8.0

.056 1.053119 1.110161 1.152226 1.219314 1.312345 1.348357 1.362

.056 1.053119 1.112167 1.158240 1.236348 1.354401 1.414426 1.444

.057 1.053120 1.110169 1.158242 1.237360 1.367414 1.434


TABLE V. Water kerma based backscatter factorsBw for a water phantom as a function of field diameter~d!, radiation quality~HVL !, and source surfacedistance~SSD! between~a! 1.5 and 10 cm and~b! 10 and 100 cm for open-ended cones. The values are from Grosswendt~Refs. 37 and 38! and have beenindependently checked using the experimental data from Klevenhagen~Ref. 39! and the Monte Carlo data from Knight and Nahum~Ref. 22!.

~a!SSD~cm!

d~cm!

HVL ~mm Al!

0.04 0.05 0.06 0.08 0.1 0.12 0.15 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 2.0 3.0

1.5 1 1.001 1.005 1.009 1.012 1.014 1.016 1.018 1.021 1.027 1.032 1.035 1.038 1.042 1.045 1.047 1.050 1.055 1.052 1.008 1.009 1.011 1.014 1.018 1.020 1.023 1.028 1.037 1.045 1.051 1.056 1.065 1.070 1.074 1.080 1.089 1.093 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.039 1.049 1.055 1.061 1.071 1.079 1.084 1.091 1.103 1.115 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.040 1.050 1.057 1.064 1.075 1.084 1.090 1.099 1.112 1.12

10 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.041 1.051 1.058 1.065 1.076 1.085 1.092 1.100 1.115 1.1215 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.041 1.051 1.058 1.065 1.076 1.085 1.092 1.100 1.115 1.1220 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.041 1.051 1.058 1.065 1.076 1.085 1.092 1.100 1.115 1.12

3 1 1.007 1.008 1.010 1.012 1.014 1.016 1.018 1.021 1.027 1.032 1.035 1.038 1.043 1.046 1.047 1.050 1.055 1.062 1.008 1.009 1.011 1.014 1.018 1.021 1.024 1.029 1.040 1.049 1.055 1.061 1.070 1.078 1.083 1.089 1.101 1.113 1.008 1.009 1.011 1.014 1.018 1.021 1.025 1.031 1.044 1.056 1.063 1.071 1.083 1.093 1.100 1.109 1.125 1.145 1.008 1.009 1.011 1.014 1.018 1.022 1.026 1.033 1.048 1.061 1.069 1.078 1.093 1.106 1.115 1.127 1.147 1.17

10 1.008 1.009 1.011 1.014 1.018 1.022 1.026 1.033 1.048 1.061 1.071 1.081 1.098 1.112 1.122 1.136 1.159 1.1815 1.008 1.009 1.011 1.014 1.018 1.022 1.026 1.033 1.048 1.061 1.071 1.081 1.098 1.113 1.123 1.137 1.160 1.1920 1.008 1.009 1.011 1.014 1.018 1.022 1.026 1.033 1.048 1.061 1.071 1.081 1.098 1.113 1.124 1.138 1.161 1.19

5 1 1.007 1.008 1.009 1.012 1.014 1.016 1.018 1.021 1.027 1.033 1.036 1.039 1.043 1.046 1.048 1.051 1.056 1.062 1.007 1.009 1.011 1.014 1.018 1.022 1.025 1.030 1.041 1.051 1.057 1.063 1.073 1.081 1.086 1.092 1.104 1.113 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.033 1.046 1.058 1.066 1.075 1.088 1.098 1.105 1.115 1.132 1.155 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.035 1.050 1.064 1.074 1.085 1.102 1.116 1.126 1.140 1.163 1.19

10 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.035 1.051 1.066 1.078 1.090 1.111 1.129 1.141 1.158 1.186 1.2215 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.035 1.051 1.066 1.078 1.090 1.112 1.130 1.144 1.161 1.191 1.2320 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.035 1.051 1.066 1.078 1.090 1.112 1.130 1.144 1.162 1.192 1.23

7 1 1.006 1.007 1.008 1.011 1.014 1.016 1.018 1.021 1.027 1.033 1.035 1.038 1.043 1.046 1.048 1.051 1.056 1.062 1.007 1.008 1.010 1.014 1.018 1.022 1.025 1.031 1.042 1.052 1.058 1.065 1.075 1.083 1.088 1.094 1.106 1.113 1.007 1.008 1.010 1.014 1.019 1.023 1.027 1.034 1.048 1.060 1.068 1.076 1.090 1.101 1.109 1.119 1.137 1.155 1.007 1.008 1.010 1.014 1.019 1.023 1.027 1.035 1.051 1.066 1.076 1.087 1.106 1.123 1.134 1.149 1.173 1.20

10 1.007 1.008 1.010 1.014 1.019 1.023 1.028 1.036 1.053 1.069 1.081 1.093 1.116 1.139 1.154 1.173 1.206 1.2515 1.007 1.008 1.010 1.014 1.019 1.023 1.028 1.036 1.053 1.069 1.081 1.094 1.118 1.142 1.157 1.179 1.214 1.2620 1.007 1.008 1.010 1.014 1.019 1.023 1.028 1.036 1.053 1.069 1.081 1.094 1.118 1.142 1.158 1.180 1.215 1.27

10 1 1.006 1.007 1.009 1.012 1.014 1.016 1.018 1.022 1.028 1.034 1.036 1.038 1.043 1.046 1.048 1.051 1.055 1.062 1.007 1.009 1.011 1.014 1.018 1.022 1.025 1.030 1.042 1.052 1.058 1.064 1.075 1.083 1.088 1.094 1.105 1.123 1.007 1.009 1.011 1.015 1.019 1.023 1.027 1.034 1.048 1.060 1.069 1.078 1.092 1.103 1.110 1.120 1.135 1.155 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.036 1.052 1.068 1.079 1.091 1.110 1.126 1.137 1.152 1.177 1.21

10 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.037 1.055 1.072 1.086 1.100 1.125 1.146 1.161 1.182 1.216 1.2715 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.037 1.055 1.072 1.087 1.101 1.128 1.151 1.167 1.189 1.226 1.2820 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.038 1.056 1.073 1.088 1.102 1.129 1.153 1.169 1.191 1.228 1.29

~b!SSD~cm!

d~cm!

HVL ~mm Al!

0.04 0.05 0.06 0.08 0.1 0.12 0.15 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 2.0 3.0 4.0 5.0 6.0

10 1 1.006 1.007 1.009 1.012 1.014 1.016 1.018 1.022 1.028 1.034 1.036 1.038 1.043 1.046 1.048 1.051 1.055 1.062 1.059 1.057 12 1.007 1.009 1.011 1.014 1.018 1.022 1.025 1.030 1.042 1.052 1.058 1.064 1.075 1.083 1.088 1.094 1.105 1.120 1.118 1.118 1.3 1.007 1.009 1.011 1.014 1.018 1.022 1.026 1.033 1.047 1.060 1.069 1.078 1.092 1.103 1.110 1.120 1.135 1.159 1.161 1.161 1.5 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.036 1.052 1.068 1.079 1.091 1.110 1.126 1.137 1.152 1.177 1.211 1.220 1.224 1.

10 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.037 1.055 1.072 1.086 1.100 1.125 1.146 1.161 1.182 1.216 1.270 1.296 1.308 1.15 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.037 1.055 1.072 1.087 1.101 1.128 1.151 1.167 1.189 1.226 1.288 1.321 1.336 1.20 1.007 1.009 1.011 1.015 1.019 1.023 1.028 1.038 1.056 1.073 1.088 1.102 1.129 1.153 1.169 1.191 1.228 1.293 1.328 1.346 1.

20 1 1.006 1.007 1.008 1.011 1.014 1.016 1.018 1.022 1.028 1.034 1.036 1.039 1.043 1.046 1.049 1.052 1.057 1.061 1.059 1.058 12 1.006 1.008 1.010 1.014 1.018 1.022 1.025 1.031 1.043 1.053 1.059 1.065 1.075 1.083 1.089 1.095 1.107 1.116 1.118 1.118 1.3 1.006 1.008 1.010 1.014 1.019 1.024 1.028 1.035 1.049 1.061 1.069 1.077 1.092 1.105 1.112 1.122 1.138 1.158 1.162 1.165 1.5 1.006 1.008 1.010 1.014 1.019 1.024 1.029 1.037 1.054 1.070 1.080 1.091 1.112 1.131 1.143 1.158 1.183 1.215 1.226 1.234 1.

10 1.006 1.008 1.010 1.014 1.019 1.024 1.029 1.039 1.057 1.074 1.088 1.102 1.129 1.155 1.173 1.196 1.235 1.291 1.317 1.334 1.15 1.006 1.008 1.010 1.014 1.019 1.024 1.030 1.039 1.058 1.075 1.090 1.104 1.133 1.162 1.182 1.208 1.252 1.321 1.356 1.380 1.20 1.006 1.008 1.010 1.014 1.019 1.024 1.030 1.039 1.058 1.076 1.091 1.106 1.136 1.165 1.186 1.213 1.258 1.334 1.373 1.402 1.

30 1 1.006 1.007 1.008 1.011 1.015 1.017 1.019 1.022 1.027 1.032 1.035 1.038 1.043 1.047 1.050 1.053 1.058 1.063 1.061 1.059 12 1.006 1.008 1.010 1.014 1.018 1.022 1.025 1.031 1.042 1.052 1.058 1.064 1.074 1.084 1.090 1.096 1.108 1.120 1.122 1.122 1.3 1.006 1.008 1.010 1.014 1.019 1.023 1.027 1.034 1.048 1.061 1.069 1.077 1.093 1.107 1.115 1.125 1.140 1.164 1.167 1.168 1.5 1.006 1.008 1.010 1.014 1.019 1.024 1.029 1.037 1.053 1.069 1.079 1.090 1.111 1.130 1.142 1.157 1.182 1.221 1.237 1.242 1.

10 1.006 1.008 1.010 1.014 1.019 1.024 1.029 1.039 1.057 1.074 1.088 1.102 1.130 1.157 1.175 1.199 1.238 1.298 1.333 1.350 1.15 1.006 1.008 1.010 1.014 1.019 1.024 1.030 1.039 1.058 1.076 1.091 1.106 1.136 1.165 1.185 1.212 1.257 1.332 1.381 1.403 1.


8.0

441 1.472.055 1.052119 1.112170 1.160246 1.242363 1.375428 1.448463 1.493

.056 1.053118 1.112170 1.162244 1.243366 1.381440 1.460480 1.508

1.0171.0381.0541.0821.1261.1461.155

1.0181.0391.0571.0881.1411.1741.194

1.0181.0381.0551.0871.1471.1891.213

1.0181.0401.0571.0891.1521.1951.226

1.0181.0401.0571.0901.1551.2041.237


TABLE V. ~Continued.!

~b!SSD~cm!

d~cm!

HVL ~mm Al!

0.04 0.05 0.06 0.08 0.1 0.12 0.15 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 2.0 3.0 4.0 5.0 6.0

20 1.006 1.008 1.010 1.014 1.019 1.024 1.030 1.039 1.058 1.076 1.091 1.107 1.138 1.169 1.190 1.218 1.265 1.350 1.404 1.428 1.50 1 1.006 1.007 1.008 1.011 1.014 1.016 1.018 1.021 1.027 1.033 1.035 1.038 1.042 1.045 1.047 1.051 1.057 1.065 1.062 1.059 1

2 1.006 1.007 1.009 1.013 1.018 1.022 1.025 1.031 1.043 1.053 1.058 1.064 1.073 1.081 1.087 1.095 1.107 1.121 1.122 1.121 1.3 1.006 1.007 1.009 1.013 1.018 1.023 1.027 1.034 1.049 1.062 1.070 1.078 1.093 1.106 1.114 1.124 1.142 1.163 1.167 1.169 1.5 1.006 1.007 1.009 1.013 1.018 1.023 1.028 1.037 1.054 1.070 1.081 1.093 1.113 1.132 1.143 1.159 1.185 1.226 1.235 1.241 1.

10 1.006 1.007 1.009 1.013 1.018 1.023 1.029 1.038 1.057 1.076 1.091 1.106 1.134 1.159 1.177 1.202 1.244 1.309 1.336 1.352 1.15 1.006 1.007 1.009 1.013 1.018 1.023 1.029 1.039 1.058 1.077 1.093 1.110 1.140 1.169 1.190 1.218 1.265 1.346 1.387 1.411 1.20 1.006 1.007 1.009 1.013 1.018 1.023 1.029 1.039 1.058 1.077 1.094 1.110 1.142 1.173 1.195 1.224 1.273 1.363 1.414 1.443 1.

100 1 1.006 1.007 1.008 1.011 1.014 1.016 1.018 1.022 1.028 1.034 1.036 1.038 1.042 1.044 1.046 1.050 1.056 1.062 1.061 1.059 12 1.006 1.008 1.010 1.014 1.018 1.022 1.025 1.031 1.043 1.053 1.058 1.064 1.072 1.080 1.085 1.094 1.107 1.121 1.122 1.120 1.3 1.006 1.008 1.010 1.014 1.019 1.023 1.027 1.035 1.050 1.063 1.070 1.078 1.092 1.104 1.112 1.123 1.142 1.163 1.168 1.169 1.5 1.006 1.008 1.010 1.014 1.019 1.023 1.028 1.037 1.055 1.071 1.082 1.093 1.113 1.131 1.143 1.160 1.188 1.225 1.234 1.240 1.

10 1.006 1.008 1.010 1.014 1.019 1.023 1.029 1.039 1.058 1.077 1.091 1.106 1.134 1.158 1.177 1.202 1.245 1.311 1.334 1.351 1.15 1.006 1.008 1.010 1.014 1.019 1.023 1.029 1.039 1.059 1.078 1.094 1.110 1.140 1.169 1.190 1.219 1.269 1.354 1.391 1.417 1.20 1.006 1.008 1.010 1.014 1.019 1.023 1.029 1.039 1.059 1.078 1.095 1.111 1.143 1.172 1.195 1.226 1.278 1.375 1.419 1.451 1.

SSD~cm!

d~cm!

HVL ~mm Cu!

0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.5 2.0 3.0 4.0 5.0

10 1 1.062 1.057 1.056 1.054 1.052 1.050 1.046 1.043 1.037 1.033 1.026 1.0212 1.120 1.118 1.119 1.113 1.108 1.106 1.103 1.097 1.081 1.071 1.057 1.0463 1.159 1.161 1.161 1.155 1.150 1.147 1.143 1.135 1.116 1.102 1.081 1.0675 1.210 1.224 1.226 1.221 1.217 1.214 1.209 1.199 1.170 1.151 1.122 1.101

10 1.269 1.306 1.316 1.313 1.311 1.310 1.307 1.294 1.254 1.227 1.186 1.15415 1.287 1.335 1.348 1.348 1.348 1.348 1.347 1.332 1.289 1.260 1.213 1.17820 1.292 1.344 1.361 1.362 1.362 1.363 1.364 1.349 1.303 1.273 1.225 1.188

20 1 1.061 1.058 1.055 1.054 1.053 1.051 1.048 1.045 1.038 1.033 1.024 1.0202 1.116 1.118 1.119 1.114 1.110 1.107 1.102 1.097 1.084 1.074 1.056 1.0463 1.158 1.164 1.168 1.161 1.155 1.152 1.147 1.140 1.122 1.107 1.082 1.0675 1.214 1.232 1.242 1.238 1.233 1.229 1.219 1.209 1.184 1.164 1.127 1.104

10 1.290 1.331 1.352 1.353 1.353 1.349 1.339 1.326 1.291 1.260 1.204 1.16815 1.320 1.377 1.407 1.412 1.415 1.411 1.403 1.389 1.350 1.316 1.251 1.20720 1.333 1.397 1.434 1.441 1.447 1.443 1.436 1.421 1.381 1.345 1.278 1.230

30 1 1.063 1.060 1.056 1.054 1.052 1.050 1.047 1.044 1.038 1.033 1.024 1.0202 1.120 1.122 1.119 1.113 1.108 1.105 1.101 1.096 1.084 1.073 1.056 1.0463 1.164 1.168 1.169 1.161 1.155 1.152 1.146 1.139 1.121 1.107 1.084 1.0685 1.220 1.242 1.242 1.239 1.235 1.231 1.221 1.211 1.184 1.164 1.130 1.106

10 1.297 1.348 1.363 1.366 1.367 1.360 1.347 1.332 1.292 1.263 1.214 1.17715 1.330 1.401 1.417 1.429 1.438 1.433 1.422 1.405 1.360 1.327 1.270 1.22620 1.348 1.426 1.446 1.464 1.478 1.473 1.464 1.446 1.399 1.364 1.302 1.254

50 1 1.065 1.059 1.054 1.053 1.052 1.050 1.047 1.045 1.038 1.034 1.025 1.0202 1.121 1.121 1.118 1.114 1.111 1.108 1.103 1.097 1.084 1.073 1.056 1.0473 1.163 1.169 1.170 1.163 1.157 1.154 1.148 1.140 1.121 1.106 1.084 1.0695 1.225 1.240 1.247 1.244 1.240 1.235 1.226 1.214 1.184 1.163 1.131 1.108

10 1.308 1.350 1.367 1.372 1.376 1.371 1.360 1.344 1.304 1.274 1.222 1.18415 1.345 1.408 1.433 1.443 1.452 1.450 1.446 1.428 1.379 1.346 1.285 1.23720 1.361 1.439 1.471 1.486 1.499 1.498 1.495 1.478 1.428 1.391 1.325 1.272

100 1 1.062 1.059 1.055 1.053 1.052 1.050 1.047 1.045 1.038 1.034 1.025 1.0202 1.121 1.121 1.117 1.114 1.111 1.108 1.104 1.098 1.085 1.074 1.057 1.0473 1.163 1.169 1.170 1.165 1.160 1.156 1.150 1.142 1.122 1.107 1.085 1.0705 1.224 1.239 1.245 1.243 1.241 1.237 1.227 1.217 1.188 1.167 1.132 1.109

10 1.310 1.349 1.370 1.378 1.383 1.378 1.369 1.353 1.311 1.278 1.226 1.18815 1.353 1.413 1.447 1.456 1.463 1.461 1.458 1.441 1.393 1.356 1.291 1.24420 1.373 1.446 1.490 1.502 1.513 1.514 1.516 1.499 1.447 1.406 1.334 1.282

w-a

mbe

ertemfor

fies the requirements formulated in Sec. III B. For loenergy parallel-plate chambers, however, this correction mbe several percent because of their large chamber body.19 Ingeneral, the stem effect corrections for parallel-plate chabers are not well known. Because of this, these cham


y

-rs

should not be used for reference dosimetry~or cone factor!for fields different from that used in the calibration. Furthinvestigations are needed to systematically determine seffect corrections for these chamber types. A proceduremeasuringPstem,air is described in Sec. V C.7.

of

ioate

A se

ed

2

r t

-cm


FIG. 3. Dependence on field surface area for different energies of the ratthe mass energy-absorption coefficient water to air at 2 cm depth in wData are normalized to the reference field size of 10310 cm2 at an SSD of50 cm. Half value layers corresponding to the kV values are: 0.88 mm~50 kV!, 2.65 mm Al ~100 kV!, 0.57 mm Cu~150 kV!, 1.7 mm Cu~200kV!, and 4.3 mm Cu~300 kV! ~see Ref. 35!.

TABLE VI. Multiplicative correction factors to the backscatter factors listin Table V for use with close-ended cones as a function of HVL~mm Cu!and field diameterd. The values are for a 3.2 mm PMMA (r51.19 g/cm3) end plate. The data are derived from BJR Supplement~Ref. 51!.

d ~cm!

HVL ~mm Cu!

0.5 1 2 3

4.5 1.006 1.005 1.004 1.0045.6 1.006 1.006 1.005 1.0046.8 1.007 1.006 1.006 1.0047.9 1.008 1.007 1.006 1.0059.0 1.008 1.008 1.006 1.006

11.3 1.009 1.008 1.007 1.00613.5 1.009 1.009 1.008 1.00716.9 1.010 1.010 1.009 1.00822.6 1.011 1.011 1.009 1.008

TABLE VII. Ratio of average mass energy-absorption coefficients of wateair at 2 cm depth in water, for a 10310 cm2 field size, SSD550 cm, as afunction of first HVL ~in mm Cu or mm Al!. The data are from Ma andSeuntjens~Ref. 35!.

First HVL

@(men/r)airw #water~mm Cu! ~mm Al!

0.1 2.9 1.0260.2 4.8 1.0320.3 6.3 1.0370.4 7.5 1.0410.5 8.5 1.0460.6 9.3 1.0500.8 10.8 1.0551 12.0 1.0601.5 14.2 1.0722 15.8 1.0813 17.9 1.0944 19.3 1.1015 20.3 1.105


B.2. The in-phantom calibration method formedium-energy x rays

B.2.1. In-phantom mass energy-absorptioncoefficient ratio †„menÕr…air

w‡water

Table VII gives values of @(men/r)airw #water for the

medium-energy range at 2 cm depth and a 10 cm310 cmfield size. Figure 3 illustrates the variations of the values@(men/r)air

w #water when the field size differs significantly fromthe reference field size of 10310 cm2. The data are from Maand Seuntjens.35

ofr.

lFIG. 4. Field size dependence ofPQ,cham at the reference depth~2 cm!.Correction factor toPQ,cham to account for field size dependence for fieldsignificantly differing from the standard 10 cm310 cm. These values can bapplied to all the chambers in Table VIII.

5

o

TABLE VIII. Overall chamber correction factorsPQ,chamfor common cylin-drical chambers in medium-energy x-ray beams. The data applies to 2depth in the phantom, and 10310 cm2 field size. The data are fromSeuntjenset al. ~Ref. 40!.

HVL~mm Cu!

Chamber type

NE2571CapintecPR06C

PTWN30001

ExradinA12 NE2581

NE2611or

NE2561

0.10 1.008 0.992 1.004 1.002 0.991 0.9950.15 1.015 1.000 1.013 1.009 1.007 1.0070.20 1.019 1.004 1.017 1.013 1.017 1.0120.30 1.023 1.008 1.021 1.016 1.028 1.0170.40 1.025 1.009 1.023 1.017 1.033 1.0190.50 1.025 1.010 1.023 1.017 1.036 1.0190.60 1.025 1.010 1.023 1.017 1.037 1.0190.80 1.024 1.010 1.022 1.017 1.037 1.0181.0 1.023 1.010 1.021 1.016 1.035 1.0171.5 1.019 1.008 1.018 1.013 1.028 1.0142.0 1.016 1.007 1.015 1.011 1.022 1.0112.5 1.012 1.006 1.012 1.010 1.017 1.0093.0 1.009 1.005 1.010 1.008 1.012 1.0064.0 1.004 1.003 1.006 1.005 1.004 1.003

978984987989992993995996998998999000000


TABLE IX. Water proofing sleeve correction factorsPsheathfor PMMA, polystyrene, and nylon sleeves of thicknesst when using cylindrical chambers for inwater phantom measurements in medium-energy x-ray beams. The data applies to 2 cm depth in the phantom, and 10310 cm2 field size. The data are fromMa and Seuntjens~Ref. 23!.

HVL PMMA ~Lucite! r51.19 g/cm3 Polystyrener51.06 g/cm3 Nylon r51.14 g/cm3

~mm Cu! ~mm Al! t50.5 mm t51 mm t52 mm t53 mm t50.5 mm t51 mm t52 mm t53 mm t50.5 mm t51 mm t52 mm t53 mm

0.1 3.0 0.998 0.995 0.991 0.986 0.995 0.990 0.981 0.972 0.996 0.992 0.985 0.0.2 4.7 0.998 0.996 0.993 0.989 0.996 0.993 0.987 0.980 0.997 0.994 0.989 0.0.3 6.1 0.998 0.997 0.994 0.991 0.997 0.995 0.989 0.984 0.998 0.995 0.991 0.0.4 7.4 0.999 0.997 0.995 0.993 0.998 0.996 0.991 0.987 0.998 0.996 0.993 0.0.5 8.5 0.999 0.998 0.996 0.994 0.999 0.996 0.993 0.990 0.999 0.997 0.994 0.0.6 9.5 0.999 0.998 0.996 0.995 0.999 0.997 0.994 0.992 0.999 0.997 0.995 0.0.8 11.0 0.999 0.998 0.997 0.996 0.999 0.998 0.996 0.994 0.999 0.998 0.996 0.1.0 12.1 1.000 0.999 0.998 0.997 1.000 0.999 0.997 0.996 1.000 0.999 0.997 0.1.5 13.9 1.000 0.999 0.999 0.999 1.000 0.999 0.998 0.998 1.000 0.999 0.998 0.2 15.2 1.000 1.000 1.000 0.999 1.000 0.999 0.999 0.998 1.000 1.000 0.999 0.3 17.6 1.000 1.000 1.000 1.000 1.000 0.999 0.999 0.999 1.000 1.000 0.999 0.4 19.4 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.5 20.9 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.

dur

oth

amu

heo

n0

an

b1d

tfo

leth

Us

io-d.as-XI

o

B.2.2. Ion-chamber correction factor P Q,cham

PQ,chamaccounts for the change in chamber responseto the change in beam characteristics~energy and anguladistribution! between calibration and measurement, andthe change in photon fluence at the reference point incavity compared to that in water in the absence of the chber and the chamber stem. Values are derived from measments and calculations by Seuntjenset al.,11 Ma andNahum,14–16 Seuntjens and Verhaegen,31 and Seuntjenset al.40 Table VIII gives the values ofPQ,chamfor commonlyused cylindrical chamber types as a function of HVL. Tfield-size dependence of this factor at 2 cm depth is 1%smaller. Figure 4 can be used to estimate this correctioPQ,cham if the field size is significantly smaller than 1310 cm2. The uncertainty of thePQ,cham factor was esti-mated to be about 1.5%.

B.2.3. Sleeve correction factor P sheath

Table IX gives the values ofPsheath for sleeves used toinsulate cylindrical chambers when placed in a water phtom, as a function of the sleeve thicknesst and HVL. Thedata are from Monte Carlo calculations and experimentsMa and Seuntjens.23 Field-size and SSD dependence of amm sleeve is less than 0.2% for PMMA, nylon, anpolystyrene.23 The uncertainty of thePsheathfactor was esti-mated to be about 0.5%.

B.3. Conversion factors to calculate dose in otherbiological materials

This protocol provides guidelines to determine doseother biological tissues on the surface of a human bodyclinical radiotherapy and radiobiology. In this section tabare provided to convert the dose to water to dose to omaterials at the surface. The data have been suppliedICRU four-element soft tissue, ICRU striated muscle, ICRcompact bone, ICRP lung, and ICRP skin. The photon ma


e

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-

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or

serfor

s-

TABLE X. Free-in-air ratios of mass energy-absorption coefficients of blogical tissue to water for application in conjunction with the in-air methoThe data are for SSD550 cm. Except for bone, these values can be usedCw

med as defined in Eq.~12!. For bone, the data in this table shall be combined with the ratio of backscatter factor bone-to-water as given in Tableto arrive atCw

med. The data are from Ma and Seuntjens~Ref. 35!.

‘‘Free-in-air’’ mass energy-absorption coefficient ratiof the specified tissues to water

HVL~mm Al! ~mm Cu!

ICRU 4-element

softtissue

ICRUstriatedmuscle

ICRPlung

ICRPskin

ICRUcompact

bone

0.3 0.917 1.016 1.031 0.890 4.2000.4 0.918 1.020 1.035 0.893 4.2890.5 0.919 1.022 1.037 0.895 4.3350.6 0.920 1.024 1.039 0.897 4.3820.8 0.921 1.028 1.043 0.901 4.4751.0 0.923 1.031 1.046 0.904 4.4941.2 0.925 1.031 1.046 0.907 4.4691.5 0.927 1.032 1.047 0.910 4.4272.0 0.930 1.032 1.047 0.915 4.3503.0 0.934 1.032 1.045 0.922 4.1794.0 0.939 1.030 1.042 0.929 3.9755.0 0.943 1.028 1.039 0.935 3.7696.0 0.947 1.026 1.036 0.940 3.5578.0 0.955 1.021 1.030 0.950 3.133

0.1 0.934 1.032 1.045 0.921 4.2090.2 0.942 1.029 1.040 0.934 3.8080.3 0.947 1.026 1.036 0.940 3.5610.4 0.952 1.023 1.032 0.946 3.3140.5 0.956 1.020 1.029 0.952 3.0680.6 0.960 1.018 1.026 0.957 2.8590.8 0.964 1.015 1.022 0.961 2.6571.0 0.967 1.012 1.018 0.965 2.4561.5 0.975 1.006 1.009 0.975 1.9522.0 0.981 1.001 1.003 0.980 1.6373.0 0.986 0.996 0.997 0.985 1.2804.0 0.988 0.994 0.994 0.987 1.1285.0 0.990 0.992 0.992 0.989 1.026


Medical Physics, Vo

TABLE XI. Ratios of the kerma-based backscatter factors, bone to water, for photon beams 50–300 kV~0.05–5mm Cu! with different field sizes at various SSD for Eq.~12!. The data are from Ma and Seuntjens~Ref. 35!.

SSD~cm!

HVL Bbone/Bw

~mm Cu! ~mm Al! 131 cm2 232 cm2 434 cm2 10310 cm2 20320 cm2

10 0.05 1.6 0.958 0.929 0.897 0.861 0.8540.1 2.9 0.976 0.945 0.905 0.853 0.8380.5 8.5 1.019 1.011 0.974 0.910 0.8751 12.0 1.031 1.041 1.026 0.974 0.9432 15.8 1.038 1.065 1.077 1.047 1.0233 17.9 1.037 1.066 1.092 1.086 1.0704 19.3 1.028 1.053 1.082 1.087 1.0755 20.3 1.022 1.043 1.074 1.087 1.078

30 0.05 1.6 0.958 0.926 0.889 0.850 0.8330.1 2.9 0.976 0.940 0.894 0.837 0.8090.5 8.5 1.019 1.011 0.981 0.887 0.8331 12.0 1.031 1.042 1.033 0.959 0.9022 15.8 1.038 1.067 1.083 1.043 0.9893 17.9 1.037 1.067 1.101 1.090 1.0474 19.3 1.029 1.055 1.088 1.091 1.0655 20.3 1.023 1.045 1.077 1.091 1.079

50 0.05 1.6 0.958 0.927 0.891 0.847 0.8270.1 2.9 0.975 0.942 0.897 0.832 0.8000.5 8.5 1.018 1.009 0.977 0.881 0.8251 12.0 1.031 1.040 1.032 0.958 0.8942 15.8 1.038 1.066 1.085 1.047 0.9833 17.9 1.036 1.069 1.100 1.095 1.0484 19.3 1.028 1.057 1.084 1.094 1.0665 20.3 1.022 1.047 1.072 1.094 1.080

omas

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to

mislerb

decmti

te

alair

to,

sam-Sec.le

te

n.dupntsmi-ion

calnc-thepe-

energy-absorption coefficient values were taken frHubbell36 while the composition of the biological tissues wtaken from the ICRU reports.41,42

Table X presents ratios of mass energy-absorption cocients averaged over the photon fluence spectrum free inof several biological tissues of interest relative to water. Tbackscatter factor ratios relative to water for any of thesues~except bone! considered in Table X does not diffefrom unity by more than 1% for commonly used field sizand can therefore be ignored.34 For bone, Table XI shows theratios of the backscatter factors, bone to water, for phobeams 50–300 kV~0.875–20.8 mm Al! with different fieldsizes at various SSD.

APPENDIX C. SUMMARY OF RECOMMENDATIONSAND WORKSHEETS

The following is a summary of the recommendations frothis protocol, which is intended to assist the clinical physicwith the measurements in the clinic necessary to compthe Worksheets and hence determine the correct absodose.

~1! Water is the phantom material for absolute dosetermination when the point of interest is as a depth of 2for beams of tube potential greater than 100 kV. Plasphantoms~other than PMMA! may be used for in-phantomroutine quality assurance for convenience.

l. 28, No. 6, June 2001

fi-aire-

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-

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~2! Ionization chambers of choice~Sec. III B!: 70–300 kVcylindrical ionization chamber, and 40–70 kV parallel pla~soft x ray! ionization chamber.

The effective point of measurement for both cylindricand parallel plate chambers is the center of the sensitivecavity of the chamber.

~3! The appropriate ionization chamber~s! shall be cali-brated for at least two beam qualities sufficiently closeand bracketing the user’s beam qualities~in terms of bothtube potential and HVL!. More than one beam quality irequired to ensure that the energy dependence of the chber response satisfies the requirements enunciated inIII B. Chamber calibration factors shall be directly traceabto national standards~from an ADCL, NIST, or NRCC!.

~4! For parallel plate chambers, buildup of appropriathickness~Table I! and material~polyethylene, PMMA! mustbe present at the time of ADCL, NIST, or NRCC calibratioFurthermore, for parallel-plate chambers the same builmaterials must be present for all ionization measuremeperformed at clinical and/or research sites including deternation of HVL, reference dosimetry, and chamber evaluatmeasurements~Sec. III B!.

~5! Before ionization data measurements, the cliniphysicist shall examine the equipment for appropriate fution. This analysis of equipment performance includesionization chamber, the electrometer, and the x-ray unit. S

onr-

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onthraheto

ethnheubcufefo

fth

tion,

n-c-redbles

rs-upir

ro-ntalluesmp-ned


cifically, the x-ray unit shall be assessed for proper functiing of the kV, the mA, and the timer including timer lineaity, accuracy, and end effect~Sec. III!.

~6! HVL measurements shall be performed with the apropriate chamber~and buildup materials! and in the sug-gested geometry as indicated in Fig. 1~Sec. II C!.

~7! For energies 40–100 kV, routine dose calibratimeasurements are performed in air, for energies greater100 kV, and depending on the point of interest, dose calibtion measurements are performed either in air or at a dept2 cm in water. For measurements taken in air, measuremare performed at the point where the dose at the phansurface is required~e.g., the cone end!. If this is not possible,the measurement should be performed at a point as clospossible to the point of interest, and corrected to obtaindose there. To this end, an inverse square correction caused~see Sec. V C!. For measurements taken in water, tphysicist is cautioned on the use of natural or synthetic rber sleeves for water proofing the chamber because talpowder particles can enter the chamber and strongly afthe response. PMMA, polystyrene, or nylon shall be usedwater protective covering~Sec. III B!. The reference point othe parallel plate as well as cylindrical chamber type is at


-

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asebe

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center of the sensitive air cavity, along the central axis.~8! All ionization chamber measurementsM raw shall be

corrected for temperature and pressure, ion recombinapolarity effects, and electrometer calibration effects~Sec.V C!.

~9! If the clinical physicist’s standard chamber is not icluded in within the list of chambers in the various corretion factor tables, the chamber shall be relatively compaagainst a chamber, which is included in the respective ta~Sec. V C!.

~10! All dose conversion factors and correction factodependent on energy shall be determined by tabular lookas a function of the HVL of the user’s beam. For in-ameasurements, the correction factor ofPstem,air shall be in-cluded in the dose equation~Sec. V A!; for in-water mea-surements the additional correction factors ofPsheathshall beincluded in the dose equation~Sec. V B!. For Bw and othercorrection factors, it is preferred to use the tabular data pvided in this paper rather than user-determined experimevalues. Unless properly analyzed, user-determined vamay contain uncertainties associated with specific assutions, which are greater than the uncertainties contaiwithin the tabular data~Appendix B!.


C.1. TG-61 Worksheet: Calculating dose to water on the phantom surface

Name: Date:

~1! X-ray unit: , Tube potential: kV, HVL: mm ~Al or Cu!

SSD: cm, Field size: cm2

~2! Ion chamber and electrometer calibration. Date of last calibration:

Ion chamber: , Calibration factorNK 5 Gy/C

Electrometer: , Calibration factorPelec 5 C/scale unit

~3! Chamber signal: M raw 5 scale units

~4! Temperature T5 °C, PressureP 5 kPaS5mm Hg•101.33

760.0DTo normalize to 22 °C and 1 atm:

PTP5273.21T@°C#

295.2•

101.33

P@kPa#5

~5! Total radiation time:t5 min, end effect:dt 5 min

~6! Recombination correctionPion5

12S VH

VLD 2

M rawH

M rawL 2S VH

VLD 2 5

~7! Polarity correction Ppol5UM raw1 2M raw

2

2M rawU 5

~8! Corrected chamber reading M5M rawPelecPTPPionPpol 5 C

~9! Backscatter factor~Table V, Table VI!: Bw 5

~10! Mass energy-absorption coefficient ratio water to air~Table IV!:

F S menr D

air

w Gair

5

~11! Stem correction in air~Sec. V C!: Pstem,air 5

~12! Dose to water: Dw5MNKBwPstem,airF S menr D

air

w Gair

5 Gy

~13! Dose rate: Dw5Dw

t1dt5 Gy/min



C.2. TG-61 Worksheet: Calculating dose to water at 2 cm depth in water

Name: Date:

~1! X-ray unit: , Tube potential: kV, HVL: mm ~Al or Cu!

SSD: cm, Field size: cm2

~2! Ion chamber and electrometer calibration. Date of last calibration:

Ion chamber: , Calibration factorNK 5 Gy/C

Electrometer: , Calibration factorPelec 5 C/scale unit

~3! Chamber signal: M raw 5 scale units

~4! Temperature T5 °C, PressureP 5 kPaS5mm Hg•101.33

760.0DTo normalize to 22 °C and 1 atm:

PTP5273.21T@°C#

295.2•

101.33

P@kPa#5

~5! Total radiation time:t5 min, end effect:dt 5 min

~6! Recombination correctionPion5

12S VH

VLD 2

M rawH

M rawL 2S VH

VLD 2 5

~7! Polarity correction Ppol5UM raw1 2M raw

2

2M rawU 5

~8! Corrected chamber reading M5M rawPelecPTPPionPpol 5 C

~9! Chamber correction factor~Table VIII, Fig. 4, Table IX!:

PQ,chamPsheath 5

~10! Conversion factor~TableVII, Fig. 3!:

F S menr D

air

w Gwater

5

~11! Dose to water: Dw5MNKPQchamPsheathF S menr D

air

w Gwater

5 Gy

~12! Dose rate: Dw5Dw

t1dt5 Gy/min


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