Voxel phantoms and Monte Carlo methods applied to internal and external dose calculations.

Transcription

1 Voxel phantoms and Monte Carlo methods applied to internal and external dose calculations. J.G.Hunt, E.G. Cavalcanti, D.S. dos Santos e A.M.G. Azeredo. Instituto de Radioproteção e Dosimetria IRD/CNEN Av. Salvador Allende, s/n - Via 9 - Jacarepaguá CEP Rio de Janeiro RJ, Brazil.. john@ird.gov.br Abstract. Voxel phantoms and the Monte Carlo technique are applied to dose calculations due to external and internal sources of radiation. Each voxel is a cube of side of 0.36 cm. The phantom is presented in a format of 488 "slices" each of 192 x 96 picture elements. An adult man standing up of height 1.76 m is represented. The organs and tissues in the Yale phantom of interest to the calculation of the effective dose as defined in the ICRP 60 were maintained. The adipose tissue was also maintained. The other organs and tissues were grouped together into the rest. The Monte Carlo program used was Visual Monte Carlo (VMC), developed at the IRD. The program was written with the specific aim of transporting photons and electrons through voxel geometries. Photon energies between 15 kev to 2 MeV are considered. VMC was developed to calculate external doses due to environmental, occupational or accidental exposures. The program calculates tissue and effective dose for the following geometries: cloud and water immersion, ground contamination and a point source irradiation. Multienergy gamma emission is simulated. VMC results for dose calculations involving semi-infinite cloud immersion and ground irradiation geometries are presented and compared with previously obtained dose factors. Immersion in water is also included. A close agreement between the dose factors was found between the results obtained in this work and the previously obtained results. Dose calculations for an internally deposited point source are also presented. 1. Introduction The Monte Carlo technique and voxel phantoms have been applied previously to the problem of dose calculations [1,2]. The Monte Carlo code Visual Monte Carlo dose calculation (), which uses voxel phantoms based on the Yale whole body voxel phantom, was designed and written to permit the easy establishment of the irradiation geometry and to report the absorbed dose to each organ and tissue relevant to the calculation of the effective dose as defined in ICRP Publication 60 [3]. has been extensively validated against physical phantoms and other Monte Carlo programs [4]. In this paper, dose calculations for semi-infinite cloud immersion and ground irradiation geometries and water immersion geometries are presented and compared with previously obtained dose factors. The program was then applied to the calculation of doses due to an internally deposited point source of depleted uranium. 2. Materials and methods 2.1. was written with the specific aim of transporting photons and electrons through voxel geometries. Photon and electron energies from 15 kev to 2 MeV are considered. The elemental compositions of the organs and tissues were taken from the tissue compositions given in the ICRU report number 44 [5]. The program XGEN 3.0 [6] was used to obtain the mass absorption coefficients. The first application of VMC, VMC-in vivo, simulates mathematically Whole Body Counter systems [7,8]. VMC-in vivo was then modified to simulate a point, plane, air cloud or water immersion source of photons and to transport the photons and electrons through an anthropomorphic voxel phantom. The modified program is called Visual Monte Carlo dose calculation, or. Multi-energy gamma emission is simulated. The point source can be placed anywhere inside or outside the voxel phantom. For the simulation of the immersion in water containing gamma emitters, the minimum distance between the outside of the voxel matrix and the skin of the voxel body was increased to 40 cm, which is 6.6 mean-free-paths in water for photons of 0.1 MeV and 8.3 mean-freepaths in water for 0.05 MeV. This coverage of water is sufficient to simulate the body immersion in an infinite pool source. The calculations were made using 5 x 10 6 photon histories. 1

2 2.2. The voxel phantoms Two similar voxel phantoms were used for this work. The voxel phantoms were derived from a whole body magnetic resonance image (MRI) scan obtained from the Yale University voxel phantom library that is maintained by I. G. Zubal [9,10]. For the two voxel phantoms, each voxel is a cube with 3.6 mm side. For the dose calculations due to immersion in air, ground contamination and the point source of depleted uranium, the complete Yale phantom was used to represent a man of height 1.76 m standing up, with 488 "slices" each of 192 x 96 picture elements. The organs and tissues in the Yale phantom of interest to the calculation of the effective dose as defined in the ICRP Publication 60 were retained. The adipose tissue was also retained. The other organs and tissues were grouped together into the rest, see Figure 1. Two cubes representing the left eye lens and H p (10) were also included. In this work, this phantom is called the Yale-ICRP 60 phantom. FIG. 1. Front view of the Yale-ICRP 60 voxel phantom. For the dose calculations for water immersion, the voxel matrix of the Yale-ICRP 60 phantom was increased to 708 slices of 392 x 296 picture elements and the voxels outside the body were converted from air to water. This simulates the immersion of the Yale-ICRP 60 phantom in water Definition of the air and ground sources The main problem with simulating the irradiation from a semi-infinite cloud source or from an infinite ground source is to determine the extent of the source that has to be simulated. For the air source, if the air attenuation is not considered, the dose received by the simulator is approximately proportional to r, where r is the radius of the hemisphere of the air source centred on the phantom. For the ground source, not considering the air attenuation, the dose received by the phantom is approximately proportional to log e (r), where r is the radius of the circle centred on the phantom. 2

3 When the air attenuation is taken into consideration, there will be a value of r, (r max ) such that the dose received will be constant for all values of r larger than r max. As the mean free path of a photon of energy 0.1 MeV in air is around 50 meters, and for 2 MeV the mean free path is around 200 meters in air, r max is rather large. As r max is large, the volume of air or area of ground that has to be simulated as the source is large also. This means that the probability that a primary or scattered photon will hit the voxel phantom is small. Therefore, the number of photon histories that has to be run is large, above It was found that, for photon energies below 2 MeV, it is necessary to simulate a 200 m radius air or ground source. VMCdc calculates the equivalent dose to the two largest tissues or organs, the muscle and skin, and then takes the average of the two doses. The result is called here the average dose. As the number of photon interactions in the muscle and the skin is much higher than that in the other organs and tissues, the average dose gives an approximation to the effective dose much more quickly, after 10 6 histories, or 10 minutes running time on a 1 GHz clock micro-computer Definition of the depleted uranium source For the calculation of the dose from a fragment of depleted uranium (DU), the fragment was considered to be a point source. Estimates for the mass of DU fragments encountered in wounds are in the region between 0.1 g 100 g [11]. For this simulation, a fragment of DU of mass 10 g was placed in the muscle in the middle of the right thigh, 3.5 cm under the skin surface. The specific activity of DU is taken to be 13 kbq g -1. It was assumed that all the daughters the U 238 decay chain were in secular equilibrium, and all the photon energies from the U 238 decay chain with an intensity above 1% were taken into consideration. The calculation was made for 10 7 histories, each history representing one Bq of U 238 and daughters. 3. Results 3.1. Semi-infinite cloud source and plane source at the air-ground interface was run for a mono-energetic photon sources and the results were compared with the values given in the Federal Guidance Report No. 12, External Exposure to Radionuclides in Air, Water, and Soil. [12]. Comparisons of the results are shown in Tables I and II. It is pointed out that the two phantoms used of the adult body FGR: geometric, VMC: voxel are quite different. Table I. Comparison of FGR 12 and results for mono-energetic semi-infinite cloud source. Photon energy Dose (Sv/Bq s m ) (MeV) FGR 12 (Effective dose) (Effective dose) (Average dose) Table II. Comparison of FGR 12 and results for mono-energetic plane source at the airground interface. Photon energy Dose (Sv/Bq s m ) (MeV) FGR 12 (Effective dose) (Effective dose) (Average dose)

4 Immersion in water containing gamma emitters The results obtained using VMC dc for immersion in water containing gamma emitters are given in Table III where they are compared with their equivalent values in the Federal Guide Report N o 12. It can been seen that there is a good agreement between the two sets of dose coefficients for 0.05 MeV and for 0.1 MeV for the external organs such as skin, muscle, testes and brain, but for the more internal organs such as bone surface, marrow and lungs, the dose coefficients for the two energies are about a half the FGR 12 report s values. The main sources of divergence between the two results are the difference in the geometries used in the two systems: a geometrical phantom for the FGR 12 report, and the voxel phantom for this work, the fact that the dose to the breast was not considered in the compilation of the effective dose for this work, and the different methods of calculating the dose to the bone surface. calculates the dose to the bone surface as the average of the dose to the hard bone and the dose to the bone marrow. In the FGR 12 report, the dose to the bone marrow and bone surface was calculated using a collision-density fluence estimator for the skeleton combined with fluence-to-dose conversion factors for active marrow and bone surface. Table III. Comparison of FGR 12 and results. Organ dose rates and effective dose rates due to immersion in an infinite pool of water containing a gamma emitter of mono-energetic photons. Organ dose rates and effective dose rates for immersion in water containing gamma emitters (Gy/Bq s m or Sv/Bq s m ) Organ 0.05 MeV photons 0.1 MeV photons FGR 12 FGR 12 Brain Bone Surface Lungs Muscle Red Marrow Skin Testes Effective dose Dose due to an internally deposited fragment of depleted uranium The geometry of the irradiation is shown in Figure 2. Organ doses and the effective dose due to the presence of 10 g of depleted uranium in this geometry are given in Table IV. The dose rates are very low due to the low specific activity of DU. The local dose to the muscle due to alpha end beta particle emission was not taken into account in this calculation. The effective dose received per year for such a fragment is approximately 0.2 msv. FIG. 2. Transverse slice through the thighs, showing the location of the DU source. The circles 4

5 indicate the location of Compton or photoelectric interactions in the leg. Table IV. Organ dose rates and effective dose rates due to a point source of 10 g of depleted uranium in the right thigh. 4. Conclusions and future work Organ Dose rate for a 10 g fragment of DU in thigh, (µgy s or µsv s ) Lungs 0.11 Muscle 17 Red Marrow 5.4 Skin 6.2 Testes 22 Effective dose 5.6 For the source geometries considered, the doses calculated using and the doses reported in the Federal Guide Report N o 12 are in good agreement, considering the different phantoms and calculation method that were used. With the advent of higher resolution voxel phantoms, and quicker personal computers, the calculation of the dose factors may be further refined. Future work in includes the transport of protons and alpha particles through nonhomogeneous materials, and the application of the program to areas of nuclear medicine. 5. References 1. Veit, R., Zankl, M., Petoussi, N., Drexler, G., Dose equivalents in anthropomorphic phantoms and their relation to the ambient dose equivalent H*(10) for external exposure. Radiat. Prot. Dosim. 28(1-2): (1989). 2. Zankl, M., Panzer, W., Herrmann, C., Calculation of Patient Doses Using a Human Voxel Phantom of Variable Diameter. Radiat. Prot. Dosim. 90(1-2): , (2000). 3. International Commission on Radiological Protection Recommendations of the International Commission of Radiation Protection. ICRP publication 60. Annals of the ICRP, No. 21(1-3), Pergamon Press, Oxford and New York (1990). 4. Hunt, J.G., da Silva F.C.A., Mauricio C.L.P., dos Santos, D.S.. The validation of organ dose calculations using voxel phantoms and Monte Carlo methods applied to point and water immersion sources. Radiat. Prot. Dosim.108:85-89, (2004). 5. International Commission on Radiation Units and Measurements. Tissue substitutes in Radiation Dosimetry and Measurements. ICRU publication 44. Pergamon Press, Oxford (1989). 6. Halbleib, J.A., Kensek, R.P., Mehlhorn, T.A., Valdez, S.M., ITS version 3.0: The integrated TIGER series of coupled electron/photon Monte Carlo transport codes. Sandia National Laboratories, SAND Albuquerque (1992). 7. Hunt, J.G., Malátová, I., Foltánová, S. Calculation and Measurement of Calibration Factors for Bone-Surface Seeking Low Energy Gamma Emmiters and Determination of 241 Am Activity in a real case of Internal Contamination. Radiat. Prot. Dosim. 82(3): , (1999). 8. Hunt, J.G., Bertelli, L., Dantas, B.M., Lucena, E. Calibration of in vivo measurement systems and evaluation of lung measurement uncertainties using a voxel phantom. Radiat. Prot. Dosim. 76(3): , (1998). 5

6 9. Zubal I.G. The Zubal phantom data, voxel-based anthropomorphic phantoms. Webpage: (2001). 10. Zubal I.G., Harrell C.R., Smith E.O., A computerized three-dimensional segmented human anatomy Med. Phys. 21: , (1994). 11. Eckerman K.F., Ryman, J.C. External exposure to radionuclides in air, water and soil. Federal Guidance Report N 0 12, EPA-402-R , USEPA, (1993). 12. Guilmette, R.A., Los Alamos National Laboratory, private communication. (2003). 6