pcmxc

213

Radiation Protection DosimetryVol. 80, Nos 1–3, pp. 213–219 (1998)Nuclear Technology Publishing

ORGAN DOSE CALCULATION IN MEDICAL X RAYEXAMINATIONS BY THE PROGRAM PCXMCA. Servomaa and M. TapiovaaraSTUK — Radiation and Nuclear Safety AuthorityPO Box 14, 00881 Helsinki, Finland

Abstract — PCXMC is a PC-based Monte Carlo program for calculating patients’ organ doses and the effective dose in medicalX ray examinations. It uses mathematical phantom models, and can be used to compute the doses in 25 organs of patients ofdifferent ages and sizes in freely adjustable X ray projections and other examination conditions of projection radiography andfluoroscopy. The organ doses calculated with PCXMC agree well with the doses calculated by the National Radiological ProtectionBoard (NRPB) for common X ray examinations and with the doses calculated by Sternet al for cineangiographic examinationsof coronary arteries.

INTRODUCTION

X ray diagnostics comprises about 15% of the radi-ation exposure of the Finnish population; annually,about 0.8 X ray examinations per inhabitant are made(1).Average patient doses in different examinations varygreatly. Effective doses range from a small fraction ofa microsievert in peripheral bone radiography to severalhundred millisieverts in complex fluoroscopic examin-ations or interventional procedures(2). The resulting riskvaries also by the age of the patient, and is about threetimes higher per unit effective dose to paediatric thanadult patients. Therefore, specific attention should bepaid to the radiation protection of paediatric patients. Xray examinations should be made using techniques thatkeep the patients’ exposure as low as compatible withthe medical purposes of the examinations(3).

Patient dose is often described by the entrance skindose, primarily because of the simplicity of its measure-ment. In some cases, e.g. quality control measurements,this may be reasonable, but it is not sufficient for com-paring doses of different X ray examinations or differenttechniques of a given examination because entrance skindose is not directly related to the radiation detriment. Incomparisons and in the optimisation of X ray examin-ation techniques, the patient dose should be character-ised by a quantity that better accounts for the radiationrisk. The effective dose has been introduced to expressa radiation detriment-related dose in situations wherethe dose to the body is not uniform, and is defined asa weighted average of the doses to radiosensitiveorgans. In its definition, an equal number of males andfemales and a wide range of ages are assumed in theexposed population. Knowledge of organ doses isnecessary if the risk is to be evaluated with a greaterdetail than allowed by effective dose.

Radiation doses in the various organs or tissues in thebody cannot be measured directly in patients under-going X ray examinations, but they can be calculatedwith a reasonable accuracy if sufficient data on the X

ray examination technique are available. This paperdescribes PCXMC, a PC-based Monte Carlo programthat calculates organ doses in medical X rayexaminations(4).

MATERIAL AND METHOD

Mathematical phantom models

The anatomical data of the phantoms that are usedin PCXMC are from the mathematical hermaphroditephantom models of Cristy(5), and describe patients ofsix different ages: newborn, 1, 5, 10, 15 year old andadult patients. A few changes were made to the Cristyphantoms in order to simulate the irradiation conditionsof X ray examinations better:

(1) the arms of the phantoms can be removed for calcu-lating doses from lateral X ray projections,

(2) the oesophagus has been added to the organs ofinterest(6) to enable the calculation of the effectivedose,

(3) the anterior half of the neck of the phantom hasbeen modified from the Cristy phantom modelswhich have no jaw(7),

(4) the breast material is taken to be a 50:50 mixtureof fat and water, and

(5) PCXMC allows further modification of these phan-toms by letting the user change the weight andheight of the phantoms.

Figure 1 shows (a) the phantom model of an adultpatient of the standard size with the X ray beam andfield size used in cineangiographic examination of thecoronary arteries and (b) the abdomen antero-posterior(AP) projection of a newborn patient.

The bones of the mathematical phantoms aremodelled as a homogeneous mixture of mineral bone,active bone marrow, and other organic constituents ofthe skeleton. The overall composition of the skeleton isapproximated as being constant over all bones in the

A. SERVOMAA and M. TAPIOVAARA

214

body and over all phantoms representing patients ofvarious ages. The amount of active bone marrow isvaried from one part of the skeleton to another, how-ever, and its distribution is different for phantoms ofdifferent ages(5).

Dose calculation

Monte Carlo calculation of X radiation transport isbased on stochastic mathematical simulation of theinteractions between photons and matter, and is themethod of dose calculation in PCXMC. Photons areemitted isotropically from a point source into the solidangle specified by the focal distance and the X ray fielddimensions. The photons are followed while they inter-

Figure 1. (a) Phantom model of a standard sized adult patient with X ray beam and field size corresponding to a projection usedin the cineangiographic examination of the coronary arteries. (b) Phantom model of a standard sized newborn patient with X

ray beam and field size used in the abdomen AP projection.

act with the phantom according to the probability distri-butions of the physical processes that they may undergo:photoelectric absorption, coherent (Rayleigh) scatteringor incoherent (Compton) scattering. A large number ofindividual photon histories is generated and estimates ofthe mean values of energy depositions in the variousorgans of the phantom are used for calculating the dosesin these organs. The cross sections for the photoelectricinteraction, coherent scattering and incoherent scatteringhave been taken from Storm and Israel(8) and the atomicfrom factors and incoherent scattering functions fromHubbell et al(9). PCXMC calculates the organ doses formonochromatic photons of 10, 20% 150 keV energyin ten different batches of each energy value. The finalestimate of the absorption at each energy value is

CALCULATION OF MEDICAL X RAY ORGAN DOSES

215

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216

obtained as the average of these batches, and the statisti-cal error is estimated from their standard deviation. Thesame Monte Carlo data can be used to calculate dosesfor any spectrum of interest. X ray spectra are calculatedaccording to the theory of Birch and Marshall(10) andare specified in terms of the X ray tube voltage (kV),the angle of the tungsten target of the X ray tube, andfiltration. Entrance air kerma (without backscatter) mustbe input by the user, and is specified at the point wherethe central axis of the X ray beam enters the phantom.

PCXMC calculates doses in 25 organs (the activebone marrow, adrenals, brain, breasts, colon (upper andlower large intestine), gall bladder, heart, kidneys, liver,lungs, muscle, oesophagus, ovaries, pancreas, skeleton,skin, small intestine, spleen, stomach, testes, thymus,thyroid, urinary bladder, and uterus). In addition theprogram calculates the effective dose (ICRP 60(11); forlater modification of this quantity see, e.g. ICRP 71(12)),the average whole-body dose, and the fraction of the Xray beam energy that is absorbed in the phantom.PCXMC runs in a PC under Windows 3.1 (and laterversions). The Monte Carlo simulation time depends onthe accuracy required and on the speed of the PC, butranges typically from 1 min to 2 h in a PC with a120 MHz Pentium processor.

900

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700

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(mG

y/G

y)

Figure 2. A comparison of the organ dose conversion factors calculated by PCXMC (white) with the data of Jones and Wall(1985) and Hartet al (1994) (shaded). All doses correspond to an entrance air kerma (free-in-air) of 1 Gy. Adult patient, chest

examination, PA projection, X ray tube voltage 120 kV, filtration 3 mm Al.

RESULTS

The data calculated by PCXMC have been comparedto the organ dose conversion factors for common X rayprojections calculated at NRPB by Jones and Wall(13)

and Hartet al(14,15) and for cineangiographic examin-ations of coronary arteries calculated by Sternet al(16).

Figure 2 compares the organ dose conversion factorscalculated by PCXMC for an adult chest PA examin-ation to those calculated by Jones and Wall(13) and Hartet al(14), and Figure 3 shows a comparison of PCXMCresults with the data of Hartet al(15). The latter compari-son shows data for 37 various X ray spectra and examin-ations: Figure 3(a) conversion factors from entrance airkerma to bone marrow dose and Figure 3(b) conversionfactors from entrance air kerma to effective dose. Theagreement between the NRPB data and the results ofPCXMC is good in all comparison sets consisting of awide range of X ray tube voltages (60 kV% 120 kV),filtration (3 mm Al % 3 mmAl 1 0.2 mmCu), patientage (newborn% adult), and X ray examinations. A fewsmall differences exist between the results, because ofthe small deviations in the phantom models anddefinitions used.

Table 1 shows a comparison of the air kerma-to-organ


217

250

200

150

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(mG

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700

600

500

400

300

200

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Examination condition

(mS

v/G

y)

(b)

Figure 3. A comparison of the conversion factors calculated by PCXMC (white) with the data of Hartet al (1996) (shaded). (a)Conversion coefficient from air kerma to dose in the active bone marrow. All doses correspond to an entrance air kerma (free-in-air) of 1 Gy. (b) Conversion coefficient from air kerma to effective dose. All doses correspond to an entrance air kerma (free-in-air) of 1 Gy. In the examination condition, the first digits show the patient’s age, the letters show the examination (a= abdomen,c = chest, h= head, p= pelvis) and projection (a= AP, p = PA, r = right lateral), and the last digits show the X ray spectrum(60 = 60 kV, total filtration 3 mm Al; 120= 120 kV, total filtration 3 mm Al and 0.2 mm Cu, 17° X ray tube anode angle). The

error bars shown correspond to two standard errors of the data.


218

dose conversion factors calculated by PCXMC and byStern et al(16) for various projections in cineangio-graphic coronary artery examinations. Taking the differ-ences in the phantoms and the different shape of theradiation field (PCXMC: rectangular, Sternet al:circular) into account, the agreement is reasonable inthis comparison.

Figure 4 illustrates the conversion factor fromentrance air kerma to effective dose in the abdomen APprojection of 5 year old patients of variable height andweight. The conversion factors in this figure correspondto a constant X ray field size at the phantom entrance(18.8 cm3 24.6 cm), and it is seen that in this specificcase the conversion factor varies only within615% ofits mean value when the weight of the patient varieswithin 2 standard deviations of its mean value in thepopulation. It is insensitive to the patient’s height whenthe weight is fixed. This result should not be directlyextrapolated to other X ray examinations, however.

14.916.8

19.1

24.821.8

350

340

330

320

310

300

290

280

270

260

250

Phantom weight (kg)

100

109

117

Phan

tom

hei

ght (

cm)

(mS

v/G

y)

Figure 4. The conversion factor from entrance air kerma to effective dose for the abdomen AP projection of 5 year old patientsof variable height and weight. FSD 90 cm, X ray field size at phantom entrance 18.83 24.6 cm2, centre point of the field at thesame anatomical location in all phantoms, X ray tube voltage 70 kV, total filtration 3.5 mm Al, 17° X ray tube anode angle.Basic phantom height 109 cm and weight 19.1 kg. The data in the figure span approximately two times the standard deviation

of patient height and weight.

CONCLUSIONSPCXMC allows the calculation of the doses in 25

organs and the effective dose in paediatric and adultpatients of various age, height and weight in widelyadjustable projections and other conditions of X rayimaging. All exposure parameters, such as the X rayspectrum, focus–skin distance (FSD), field size, beamdirection and location, can be specified. The agreementbetween the results of PCXMC and NRPB is good, andthe differences between them typically fall within thestatistical error that arises from the finite amount of pho-tons simulated. The few exceptions in the otherwiseexcellent agreement can be explained by differences inthe phantom models.

The agreement between the results of PCXMC andSternet al in cineangiographic examinations of coron-ary arteries is reasonable, and the deviations are likelyto result from differences in the phantom models andradiation field shapes in the comparison.


219

REFERENCES

1. Servomaa, A., Heikkila¨, M., Ilus, T. and Parviainen, T.Frequency and Practice of Paediatric X-ray Examinations in Finland1995. Internal report of CEC IV-Framework Research Project (Finnish Centre for Radiation and Nuclear Safety, Helsinki,Finland) (1997).

2. Rannikko, S., Karila, K. T. K. and Toivonen, M.Patient and Population Doses of X-ray Diagnostics in Finland. ReportSTUK-A144 (Radiation and Nuclear Safety Authority (STUK), Helsinki) (1997).

3. International Commission on Radiological Protection.Radiological Protection and Safety in Medicine. Publication 73. Ann.ICRP 26 (Oxford: Elsevier Science) (1996).

4. Tapiovaara, M., Lakkisto, M. and Servomaa, A.PCXMC — A PC-based Monte Carlo Program for Calculating Patient Dosesin Medical X-ray Examinations. Report STUK-A139 (Radiation and Nuclear Safety Authority (STUK), Helsinki) (1997).

5. Cristy, M. Mathematical Phantoms Representing Children of Various Ages for Use in Estimates of Internal Dose.NUREG/CR-1159, ORNL/NUREG/TM-367 (Oak Ridge National Laboratory) (1980).

6. Zankl, M., Petoussi, N. and Drexler G.Effective Dose and Effective Dose Equivalent — the Impact of the New ICRP Definitionfor External Photon Irradiation. Health Phys.62, 395–399 (1992).

7. Kramer, R., Zankl, M., Williams, G. and Drexler G.The Calculation of Dose from External Photon Exposures using ReferenceHuman Phantoms and Monte Carlo Methods, Part I: The Male (ADAM) and Female (EVA) Adult Mathematical Phantoms.GSF-Bericht S-885, reprinted 1986 (Gesellschaft fur Strahlen- und Umweltforschung mbH, Mu¨nchen) (1982).

8. Storm, E. and Israel, H. I.Photon Cross Sections from 1 keV to 100 MeV for Elements Z=1 to Z=100. Nuclear Data Tables,Sect A,7, 565–688 (1970).

9. Hubbel, J. H., Veigele, W. J., Briggs, E. A., Brown, R. T., Cromer, D. T. and Howerton, R. J.Atomic Form Factors,Incoherent Scattering Functions and Photon Scattering Cross Sections. J. Phys. Chem. Ref. Data4, 471–538 (1975).

10. Birch, R. and Marshall, M.Computation of Bremsstrahlung X-ray Spectra and Comparison with Spectra Measured with aGe(Li) Detector. Phys. Med. Biol.24, 505–517 (1979).

11. International Commission on Radiological Protection.1990 Recommendations of the International Commission on Radiologi-cal Protection. Publication 60. Ann. ICRP21 (Oxford: Pergamon) (1990).

12. International Commission on Radiological Protection.Age-dependent Doses to Members of the Public from Intake of Radio-nuclides: Part 4, Inhalation Dose Coefficients. Publication 71. Ann. ICRP25 (Oxford: Pergamon) (1995).

13. Jones, D. G. and Wall, B. F.Organ Doses from Medical X-ray Examinations Calculated using Monte Carlo Techniques.NRPB-R186 (London: HMSO) (1985).

14. Hart, D., Jones, D. G. and Wall, B. F.Normalised Organ Doses for Medical X-ray Examinations Calculated using MonteCarlo Techniques. NRPB-SR262 (NRPB, Chilton) (1994).

15. Hart, D., Jones, D. G. and Wall, B. F.Normalised Organ Doses for Paediatric X-ray Examinations Calculated using MonteCarlo Techniques. NRPB-SR279 (NRPB, Chilton) (1996).

16. Stern, S. H., Rosenstein, M., Renaud, L. and Zankl, M.Handbook of Selected Tissue Doses for Fluoroscopic and Cineangio-graphic Examinations of the Coronary Arteries. HHS Publication FDA 95–8289 (1995).

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