supplement to tg -25bruce j. gerbi, ph.d. tg -51 electron beam calibration 3 tg -51 uses the new...

Bruce J. Gerbi, Ph.D.

TG-51 Electron Beam Calibration 1

TGTG--70: Clinical electron beam dosimetry: 70: Clinical electron beam dosimetry: supplement to TGsupplement to TG--2525

Bruce J. Gerbi, Ph.DBruce J. Gerbi, Ph.D..

University of Minnesota, Minneapolis, MNUniversity of Minnesota, Minneapolis, MN

John John AntolakAntolak, Ph.D. , Ph.D. F. Christopher F. Christopher DeibelDeibel, Ph.D. , Ph.D.

David David FollowillFollowill, Ph.D. , Ph.D. Patrick D. Higgins, Ph.D. Patrick D. Higgins, Ph.D.

Michael HermanMichael Herman, , Ph.D. Ph.D. M. M. SaifulSaiful HuqHuq, Ph.D. , Ph.D.

DimitrisDimitris MihailidisMihailidis, Ph.D. , Ph.D. David. W. O. Rogers, Ph.D. David. W. O. Rogers, Ph.D.

Ellen Ellen YorkeYorke, Ph.D, Ph.D..

ConsultantsConsultants::

FaizFaiz Khan, Ph.D.Khan, Ph.D. Kenneth Hogstrom, Ph.D.Kenneth Hogstrom, Ph.D.

TGTG--7070

�� Task Group of the Radiation Therapy Task Group of the Radiation Therapy Committee of the AAPMCommittee of the AAPM

�� Formally charged in April, 2001Formally charged in April, 2001�� Inception date: July 22, 2001Inception date: July 22, 2001�� Sunset date: December, 2005Sunset date: December, 2005

TGTG--70: Goal of the Task Group70: Goal of the Task Group

�� Maintain the original intent of TGMaintain the original intent of TG--2525–– provide a useful set of procedures and provide a useful set of procedures and

processes for the practicing clinical processes for the practicing clinical physicist for the use of clinical electron physicist for the use of clinical electron beams in the energy range from 5beams in the energy range from 5--25 25 MeVMeV

–– Not simply a reNot simply a re--write of TGwrite of TG--2525•• TGTG--25 was very well written and extensive25 was very well written and extensive•• Much of the information is still very usefulMuch of the information is still very useful

TGTG--70 Goals (continued)70 Goals (continued)

�� To define clearly the tasks that a physicist To define clearly the tasks that a physicist needs to perform with regards to highneeds to perform with regards to high--energy electronsenergy electrons

�� To supplement the material of TGTo supplement the material of TG--2525�� To cover topics that are new since To cover topics that are new since

TGTG--25 or that were not fully developed in 25 or that were not fully developed in that reportthat report



TGTG--70: Table of Contents70: Table of Contents

I. INTRODUCTIONI. INTRODUCTIONII. NOTATION AND DEFINITIONSII. NOTATION AND DEFINITIONSIII. DOSE MEASUREMENTSIII. DOSE MEASUREMENTS

A. Calibration protocol, TGA. Calibration protocol, TG--5151B. Electron beam quality specificationB. Electron beam quality specificationC. Dosimetry equipment C. Dosimetry equipment

1. Ionization chambers1. Ionization chambers2. Phantoms2. Phantoms

D. Measurement of central axis percentage depth dose in waterD. Measurement of central axis percentage depth dose in water1. Measurements using cylindrical ionization chambers 1. Measurements using cylindrical ionization chambers 2. Measurements using plane2. Measurements using plane--parallel ionization chambers in waterparallel ionization chambers in water3. Measurements using diodes in water3. Measurements using diodes in water4. Water phantom considerations4. Water phantom considerations

TGTG--70: Table of Contents (2)70: Table of Contents (2)E. Output factors for electron beamsF. Dose determination in small, irregular electron fieldsG. Non-water phantoms: Conversion of relative dose measurements from non-water phantoms to water

1. Measurements using cylindrical ionization chambers in non-water phantoms

2. Measurements using plane-parallel ionization chambers in non-water phantoms

3. Film dosimetry4. Measurement of central axis percentage depth dose using

non-water phantoms

IV. ELECTRON BEAM ALGORITHMS

V. ICRU 71 – Prescribing, Recording, and Reporting electron beam therapy

TGTG--70: Table of Contents (3)70: Table of Contents (3)VI. CLINICAL APPLICATIONS OF ELECTRON BEAMS

A. Heterogeneities in electron treatmentsB. The use of bolus in electron beam treatmentsC. Electron field abutment

VII. LIBRARY OF CLINICAL EXAMPLESA. Intact Breast- Tangent plus Electrons (IMC), Electron BoostB. Chest Wall - Tangent plus Electrons, Electrons only, Conformal BolusC. Electron ArcD. Total ScalpE. ParotidF. NoseG. Eye – Eyelid, Retinoblastoma, OrbitH. Posterior Cervical NodesI. CraniospinalJ. IntraoperativeK. Total Skin Electron Treatment (TSET)L. Total Limb

VIII. REFERENCES

III. A. Calibration Protocol, TGIII. A. Calibration Protocol, TG--5151

�� TGTG--51 accomplished its two main objectives:51 accomplished its two main objectives:1. Incorporate the new absorbed dose to water standard1. Incorporate the new absorbed dose to water standard

–– Absorbed dose is more robust than the Air Kerma Absorbed dose is more robust than the Air Kerma StandardStandard

–– Dose to water is closer to the dose to tissueDose to water is closer to the dose to tissue2. Simplify the calibration formalism (as much as 2. Simplify the calibration formalism (as much as

possible)possible)

�� Defines Defines dosedose at one point, the reference pointat one point, the reference point



TGTG--5151

�� Uses the new absorbed dose standard, Uses the new absorbed dose standard, �� FullFull calibration to be done in water onlycalibration to be done in water only�� Reference depth, Reference depth, ddrefref = 0.6= 0.6RR5050 -- 0.1 cm 0.1 cm

rather than rather than ddmaxmax

�� Protocol uses realistic electron beam data Protocol uses realistic electron beam data for stopping powers of water to airfor stopping powers of water to air

�� Uses new factors, Uses new factors, kk’’RR5050 and and kkecalecal

CowDN

60

,

TGTG--51: Calibration equation for electrons51: Calibration equation for electrons

[ ]GyNkkMPD CowDecalR

Qgr

Qw

60

50 ,'=

energyat factor n calibratiochamber water todoseabsorbed

energyelectron

referenceat doseintoenergy at dosemapst factor tha

factordependent mber energy/cha

correctiongradient

readingmeter corrected

Qquality,beamat water todose

60,

60

'

60

50

CoN

Cok

k

P

M

D

CowD

ecal

R

Qgr

Qw

=

=

=

=

==

III.B. Electron beam quality specificationIII.B. Electron beam quality specification

� Electron beam quality specified by R50 , the 50% depth of dose the 50% depth of dose maximummaximum instead of

� R50 can be obtained:– from the depth ionization curve in terms of I50

•• Correct the raw depth ionization curve for depth by offCorrect the raw depth ionization curve for depth by off--setting setting the depth by 0.5 the depth by 0.5 rrcavcav toward surface for cylindrical chamber, toward surface for cylindrical chamber, no no offset for offset for pp--pp chamber.chamber.

•• Then use the following equations:Then use the following equations:

– R50 = 1.029 I50 – 0.06 (cm) 2 ≤ I50 ≤ 10 cm– R50 = 1.059 I50 – 0.37 (cm) I50 > 10 cm

• The measurement must be made at 100 cm SSD.

–– directly from % Depth Dose curve using diode system directly from % Depth Dose curve using diode system (checked for agreement with ion chamber data)(checked for agreement with ion chamber data)

0E

III.C. Equipment, TGIII.C. Equipment, TG--7070

� Ionization chambers – calibration and relative measurements-- Both cylindrical and planeBoth cylindrical and plane--parallel chambers are parallel chambers are acceptableacceptable

�� Diodes Diodes –– for measurement of %for measurement of %dddd data data (checked v. ion chambers for accuracy)(checked v. ion chambers for accuracy)

�� Phantom materialPhantom material–– Water is preferred whenever possibleWater is preferred whenever possible–– NonNon--water materials are allowed (but not for absolute water materials are allowed (but not for absolute

calibration, as per TGcalibration, as per TG--51)51)



PlanePlane--Parallel ChambersParallel Chambers

�� PlanePlane--parallel chambers are recommended to parallel chambers are recommended to calibrate electrons of energy calibrate electrons of energy RR5050 ≤≤≤≤≤≤≤≤ 2.6 cm 2.6 cm

�� Not many waterproof pNot many waterproof p--p chambersp chambers–– Markus, NACP, Memorial, Markus, NACP, Memorial, ExradinExradin–– pp--pp chambers cannot be waterproofed easilychambers cannot be waterproofed easily

�� Interface problems between pInterface problems between p--p chamber and p chamber and surrounding water medium have not been surrounding water medium have not been addressedaddressed

�� Backscatter from Backscatter from pp--pp chamber materialchamber material

Cylindrical Ion ChambersCylindrical Ion Chambers

� Commonly available� Used routinely: calibration & automated

scanning systems� Gradient correction required� Fluence correction required

Corrected Meter Reading, Corrected Meter Reading, MM

�� Make measurement with chamber at Make measurement with chamber at ddref ref to to obtainobtain MMrawraw

–– Correct Correct MMrawraw for for PPionion,, PPTPTP ,, PPelecelec ,, PPpolpol to get to get MM

–– So, of course, you need these correction So, of course, you need these correction factorsfactors

rawpolelecTPion MPPPPM =



Gradient Correction, Gradient Correction, PPgrgr

�� PPgrgr (remember (remember PPreplrepl = = PPgrgrPPflfl))

–– depends on the userdepends on the user’’s beams beam–– must be measured for each beammust be measured for each beam

( )( )

00.1

5.0

=

+=

refraw

cavrefrawQgr dM

rdMP ((cylindricalcylindrical))

(plane(plane--parallel)parallel)

Electron Beam Dosimetry, Electron Beam Dosimetry, kk´́RR5050

)) cmRqualityelectronatevaluatedcelreplwater

airL

Rqualityelectronarbitraryatevaluatedcelreplwater

airL

RPP

PPk

5.750

50

50 |

|'

=••

••=

ρ

ρ

The energy/chamber dependent factor which relates dose at The energy/chamber dependent factor which relates dose at an arbitrary electron energy, expressed as an arbitrary electron energy, expressed as RR5050, to the , to the reference energy, reference energy, RR5050 = 7.5 cm.= 7.5 cm.

--Uses Uses L/L/ρρρρρρρρ for an electron spectrum representative of for an electron spectrum representative of realisticrealistic electron beamselectron beams

--The values of The values of kk’’RR5050 are appropriate only at are appropriate only at ddrefref

=50

'Rk

Based on TGBased on TG--21 formalism21 formalism

Determination ofDetermination of kk´́RR5050

�� From available figures, or using analytical From available figures, or using analytical fits (to within 0.2%)fits (to within 0.2%)

�� Farmer cylindrical chambers, 2 Farmer cylindrical chambers, 2 ≤≤≤≤≤≤≤≤ RR5050 ≤≤≤≤≤≤≤≤ 9 cm:9 cm:

�� PlanePlane--parallel chambers, 2 parallel chambers, 2 ≤≤≤≤≤≤≤≤ RR5050 ≤≤≤≤≤≤≤≤ 20 cm:20 cm:

( ) ( )67.350

500710.09905.0' R

ecylkR

−

+=

( ) ( ) 214.050

' 145.02239.150

RppkR −=

Electron Beam Dosimetry, Electron Beam Dosimetry, kkecalecal

)) energycobaltatevaluatedcelwallreplwater

airL

cmRqualityelectronatevaluatedcelreplwater

airL

ecalPPP

PPk

|

| 5.750

•••

••= =

ρ

ρ

--Factor that maps dose at CoFactor that maps dose at Co--60 energy into dose at a reference 60 energy into dose at a reference electron energy (that energy whose depth dose falls to 50% at electron energy (that energy whose depth dose falls to 50% at 7.5 cm depth, ~18 MeV)7.5 cm depth, ~18 MeV)

--Allows a specific electron chamber calibration factor for Allows a specific electron chamber calibration factor for cylindrical chambers (at a later date, when/if available)cylindrical chambers (at a later date, when/if available)

--Allows the calibration of pAllows the calibration of p--p chambers by p chambers by intercomparisonintercomparisonwith a cylindrical chamber in a highwith a cylindrical chamber in a high--energy electron beamenergy electron beam

--kkecalecal is independent of energyis independent of energy



Determination ofDetermination of kkecalecal

�� Cylindrical chambersCylindrical chambers–– look up the value in available tablelook up the value in available table

�� PlanePlane--parallel chambersparallel chambers–– Can use tabled values but Can use tabled values but crosscross--calibrationcalibration

versus a cylindrical chamber is versus a cylindrical chamber is recommendedrecommended

kkecal ecal , Plane, Plane--Parallel Ionization ChambersParallel Ionization Chambers

�� kkecalecal determined from crossdetermined from cross--calibration with calibration with cylindrical chamber at cylindrical chamber at ddrefref in water using highin water using high--energy electrons (recommended):energy electrons (recommended):

( ) ( )( )( )

( )pp

R

cylCowDecalR

Qgr

pp

R

cylw

ppCowDecal

Mk

NkkMP

Mk

DNk

'

,'

',

50

60

50

50

60

=

=

TGTG--51: Dose Equation for Electrons51: Dose Equation for Electrons

[ ]GyNkkMPD CowDecalR

Qgr

Qw

60

50 ,'=

energyat factor n calibratiochamber water todoseabsorbed

energyelectron

referenceat doseintoenergy at dosemapst factor tha

factordependent mber energy/cha

correctiongradient

readingmeter corrected

Qquality,beamat water todose

60,

60

'

60

50

CoN

Cok

k

P

M

D

CowD

ecal

R

Qgr

Qw

=

=

=

=

==

TGTG--51 Calibration51 Calibration

�� So now you have the calibration, or the So now you have the calibration, or the dose rate, at one point, dose rate, at one point, ddrefref

�� However, according to ICRU However, according to ICRU specifications, the prescription dose is to specifications, the prescription dose is to be reported at the be reported at the ddmaxmax pointpoint



Absorbed Dose at Absorbed Dose at ddmax max fromfrom ddrefref

�� Determine the dose at Determine the dose at ddmaxmax from that at from that at ddrefrefusing clinical using clinical %%dddd datadata

�� %%dddd requires stoppingrequires stopping--power ratiospower ratios–– In TGIn TG--51, realistic stopping51, realistic stopping--power ratios (power ratios (SPRsSPRs) have ) have

been used instead of monobeen used instead of mono--energetic energetic SPRsSPRs as in TGas in TG--2525

–– Expression from Burns et al. as a function of Expression from Burns et al. as a function of RR5050

should be usedshould be used–– PPreplrepl (= (= PPflfl X X PPgrgr) is also required (TG21 & TG25)) is also required (TG21 & TG25)

Realistic StoppingRealistic Stopping--Power Ratios for WaterPower Ratios for Water

�� From Burns From Burns et alet al. (in water). (in water)

( ) ( ) ( ) ( )( ) ( ) ( ) ( )

50

50

350

25050

25050

50lnlnln1

lnln,

Rz

Rz

w

air hRgRfRe

dRcRbazR

L

++++

+++=

ρ

Where: Where:

a = 1.0752a = 1.0752 b = b = -- 0.508670.50867 c = 0.088670c = 0.088670 d = d = -- 0.084020.08402e = e = -- 0.428060.42806 f = 0.064627f = 0.064627 g = 0.003085g = 0.003085 h = h = -- 0.124600.12460

These coefficients give an These coefficients give an rmsrms deviation of 0.4% and a max. deviation of 0.4% and a max. deviation of 1.0% for z/Rdeviation of 1.0% for z/R5050 between 0.02 and 1.1. The max. deviationbetween 0.02 and 1.1. The max. deviationincreases to 1.7% if z/Rincreases to 1.7% if z/R5050 values up to 1.2 are considered.values up to 1.2 are considered.

Burns formula for clinical work Burns formula for clinical work

� Accurate enough for clinical work – Exception of magnetically swept beams– Regions close to the surface

Rogers DW, Med. Phys. 2004

Measurement of CA %Measurement of CA %dddd CurveCurve

� Usually done using automated water scanning system– Can use cylindrical or plane-parallel chambers– Each type has its advantages & disadvantages OR

corrections that are neededProblem: if corrections are not applied in the scanner

software, it is difficult to access data and apply corrections



Cylindrical chamber correctionsCylindrical chamber corrections

� Apply gradient correction to raw depth ionization data

� Need to apply Realistic Stopping Power data to depth ionization data

� Need to apply fluence correction as a function of electron energy at depth

PlanePlane--parallel chamber correctionsparallel chamber corrections

� No gradient correction� Need to apply Realistic Stopping Power

data to depth ionization data� No fluence correction except for Markus &

Capintec chambers (TG39)� Pwall small correction (1-2%)� Ppol small effect but needs to be checked

II. B. Cone Factors for Electron BeamsII. B. Cone Factors for Electron Beams

�� Should be determined at Should be determined at ddmaxmax

–– to minimize perturbationsto minimize perturbations–– to minimize positioning uncertaintyto minimize positioning uncertainty

�� Potential problemsPotential problems–– significant contamination by low energy electrons significant contamination by low energy electrons

moving moving ddmaxmax toward surfacetoward surface–– ddmaxmax is broad for high energy electronsis broad for high energy electrons

�� Possible solution (IPEMB, 1996)Possible solution (IPEMB, 1996)–– Use Use ddmaxmax or 0.5or 0.5RR5050 , whichever is greater, whichever is greater

II. C. Measurements in nonII. C. Measurements in non--water phantomswater phantoms

�� WaterWater is recommended for is recommended for absolute calibrationabsolute calibration�� Output checks can be done in plasticsOutput checks can be done in plastics�� Plastic phantoms may be more convenient in Plastic phantoms may be more convenient in

certain situationscertain situations•• low energy electron beamslow energy electron beams•• use of planeuse of plane--parallel chambersparallel chambers

�� There is added complexity to convert dose in There is added complexity to convert dose in plastic to dose in water using ion chambersplastic to dose in water using ion chambers



Use of nonUse of non--water phantomswater phantoms

�� Water substitutes should mimic water across the whole Water substitutes should mimic water across the whole electron energy rangeelectron energy range–– mainly in stopping and scattering powersmainly in stopping and scattering powers

•• thus, both the electron density and the effective atomic thus, both the electron density and the effective atomic number should be matched to waternumber should be matched to water

•• in practical phantoms, this is difficult to achieve (due to the in practical phantoms, this is difficult to achieve (due to the carbon in plastics)carbon in plastics)

–– offoff--thethe--shelf material can have large variations in shelf material can have large variations in density and scattering powerdensity and scattering power

•• Must be careful when using these materialsMust be careful when using these materials•• Check the density of the plastic being used, composition is Check the density of the plastic being used, composition is

more difficult to verifymore difficult to verify

Use of nonUse of non--water Phantom Materialswater Phantom Materials

�� Depths need to be scaledDepths need to be scaled�� Chamber readings need to be multiplied Chamber readings need to be multiplied

by an appropriate fluenceby an appropriate fluence--ratio correctionratio correction�� StoppingStopping--power ratios should be taken at power ratios should be taken at

the scaled depththe scaled depth�� Charge storage effects should be kept in Charge storage effects should be kept in

mind using polystyrene and PMMAmind using polystyrene and PMMA

Measurements of Absorbed Dose in nonMeasurements of Absorbed Dose in non--water Phantomswater Phantoms

�� The SSD and field size are not to be The SSD and field size are not to be scaledscaled

�� Chamber must be positioned with its Chamber must be positioned with its effective point of measurement at the effective point of measurement at the equivalent scaled reference depthequivalent scaled reference depth in the in the plastic phantomplastic phantom

III. D. Dose Determination in Small/Irregular FieldsIII. D. Dose Determination in Small/Irregular Fields

�� Inherent Problems in Dosimetry of Small Inherent Problems in Dosimetry of Small Electron FieldsElectron Fields–– depth of depth of ddmaxmax becomes shallowerbecomes shallower–– the output factor may be significantly the output factor may be significantly

different than the cone factor if the field size different than the cone factor if the field size is smaller than ~ E (MeV) /2.5 cm.is smaller than ~ E (MeV) /2.5 cm.

–– isodose coverage is reduced in all directions isodose coverage is reduced in all directions as the field shrinksas the field shrinks



Change in %DD v. Field SizeChange in %DD v. Field Size

65432100.0

0.2

0.4

0.6

0.8

1.0

10x10 open

3.4cm diam

4 cm diam

9 MeV Fractional Depth Dose vs Field Size

depth (cm)

fdd

1210864200.0

0.2

0.4

0.6

0.8

1.0

1.2

10x10 open

3.4 cm diam

4 cm diam

5 cm diam

16 MeV Fractional Depth Dose vs Field Size

depth (cm)

fdd

When is special dosimetry required?When is special dosimetry required?

� When the minimum field dimension is less than the minimum radius of a circular field that produces lateral scatter equilibrium

20,

0,

0025.098.122.0

88.0

ppp

peq

RRE

where

ER

++=

=

Khan F & Higgins PD, PMB 44 (1999), N77Khan F & Higgins PD, PMB 44 (1999), N77--N80N80

Possible Methods for Small Field DosimetryPossible Methods for Small Field Dosimetry

�� Measurements of output and depth dose Measurements of output and depth dose datadata–– film dosimetryfilm dosimetry–– ionization chamber dosimetryionization chamber dosimetry

�� Model to describe change in output, depth Model to describe change in output, depth of of ddmaxmax, and isodose changes, and isodose changes

Models for Small Field DosimetryModels for Small Field Dosimetry

�� Several publications give Several publications give calculationalcalculationalmethods to approximate outputmethods to approximate output–– Square Root Method (Square Root Method (Phys Med Biol, 1981))–– Khan Model using Lateral Buildup Ratios Khan Model using Lateral Buildup Ratios

((LBRsLBRs) (Phys Med Biol. 1998 43:2741) (Phys Med Biol. 1998 43:2741--54, Phys 54, Phys Med Biol. 44:N77Med Biol. 44:N77--N80, Phys Med Biol. 2001 N80, Phys Med Biol. 2001 46:N946:N9--14)14)

–– Jones approach (Br J Jones approach (Br J RadiolRadiol. 1990 63:59. 1990 63:59--64)64)–– JursinicJursinic approach (Med Phys. 1997 24:1765approach (Med Phys. 1997 24:1765--9)9)



Square Root Method Square Root Method

� Rectangular electron field %dd determination� Take the geometric mean of the percent depth

doses for a square field of length (L) and width (W)

WxWdDLxLdDLxWdD ;(%);(%);(% ⋅=

Hogstrom KR et al., Phys Med Biol, 1981

Khan Model Khan Model -- Lateral Buildup Ratios (Lateral Buildup Ratios (LBRsLBRs))

�� Using this approach and a sector integration Using this approach and a sector integration description for LBR, the depth dose, and dose description for LBR, the depth dose, and dose per monitor unit for irregular shaped electron per monitor unit for irregular shaped electron fields can be determinedfields can be determined

�� Uses Lateral Buildup Ratios (Uses Lateral Buildup Ratios (LBRsLBRs), and ), and �� Pencil Beam Model parametersPencil Beam Model parameters

)()( widthspreadeffectivedr =σ

Basic ProcedureBasic Procedure

�� Separate Cone Factors from inSeparate Cone Factors from in--phantom phantom scatter contributions by measuring scatter contributions by measuring percentage depth doses (percentage depth doses (%%dddd), ), normalized to the surfacenormalized to the surface

�� Measure Measure %%ddsdds as a function of field size, as a function of field size, ranging from very small (e.g. 1 cm radius) ranging from very small (e.g. 1 cm radius) to to ““infinitelyinfinitely”” large fields (e.g. 20x20)large fields (e.g. 20x20)

Basic Procedure Basic Procedure -- LBRLBR

�� For each depth, divide the For each depth, divide the %%dddd of the of the small field by that for the large (reference) small field by that for the large (reference) fieldfield

�� This ratio = Lateral Buildup Ratio (LBR), This ratio = Lateral Buildup Ratio (LBR), or or ( )

( )drdd

drddLBR xx

,%

,%

∞∞

=



LBR v. depthLBR v. depth

1.21.00.80.60.40.20.00.2

0.4

0.6

0.8

1.0

1.2

2 cm

3 cm

4 cm

6 cm

8 cm

LBR vs Depth 20x20 Cone 16 MV

Z/Rp

LB

R

ResultResult

�� Use the values of for 2 cm diameter Use the values of for 2 cm diameter cutout,cutout,

�� Then, for any radius, the value of LBR can Then, for any radius, the value of LBR can be calculated using:be calculated using:

)(dr

σ

−

=),0(1

1ln/)( 22

dLBRrdrσ

Small or Irregular Electron BeamsSmall or Irregular Electron Beams

� New methods of modeling electron depth dose distributions may enable irregular field calculations to become an optionoption, eliminating the need for many clinical measurements.

IV. Electron Beam AlgorithmsIV. Electron Beam Algorithms

�� Short history of electron algorithmsShort history of electron algorithms�� Description of how data should be entered into Description of how data should be entered into

treatment planning computerstreatment planning computers�� What should be done to commission these What should be done to commission these

algorithm (being consistent with TG53)algorithm (being consistent with TG53)–– Describe some of the pitfalls and limitations of Describe some of the pitfalls and limitations of

electron algorithmselectron algorithms–– Discuss normalization of dose distributions for Discuss normalization of dose distributions for

electron algorithmselectron algorithms•• Restricted fieldRestricted field•• Extended treatment distanceExtended treatment distance•• Plans involving inhomogeneitiesPlans involving inhomogeneities



V. ICRU 71 V. ICRU 71 –– Prescribing, Recording, Prescribing, Recording, and Reporting electron beam therapyand Reporting electron beam therapy

� Regular treatments� Intra Operative treatments� Total Skin Electron treatments

VI. Clinically Relevant TopicsVI. Clinically Relevant Topics

�� Inhomogeneities in electron treatmentsInhomogeneities in electron treatments–– The effects of inhomogeneities on dose distributionsThe effects of inhomogeneities on dose distributions–– Computer representation of the effects of dose Computer representation of the effects of dose

inhomogeneitiesinhomogeneities

�� Use of bolusUse of bolus�� Field abutmentField abutment

–– ElectronElectron--electron, same & different energieselectron, same & different energies–– ElectronElectron--photon, standard & extended distancesphoton, standard & extended distances–– Tertiary shielding for field abutmentTertiary shielding for field abutment

VII. Library of Clinical ExamplesVII. Library of Clinical Examples

� Extensive list of clinical applications� Each section to detail the following:

– Introduction and Purpose of the treatment– History and Description– Pre-requisites: special equipment,

measurements– Treatment Planning & Delivery– Quality Assurance

TGTG--70: Conclusion70: Conclusion

�� Task Group report to be finished by end of Task Group report to be finished by end of 20052005

supplement to tg -25bruce j. gerbi, ph.d. tg -51 electron beam calibration 3 tg -51 uses the new...

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