INTERCOMPARISON OF THE USAGE OF COMPUTATIONAL CODES IN RADIATION DOSIMETRY

Radovan D. Ilić, Milan P. Pešić and Radojko Pavlović The Vinca Institute of Nuclear Sciences

Belgrade, P. O. Box 522, Serbia and Montenegro

Introduction

Our SRNA-2KG [1] software package was modified for this work to include necessary input and output data, and for predicted vozelized geometry as well. SRNA is a Monte Carlo code developed for usege in proton transport, radiotherapy, and dosimetry. Protons within energy range from 100 keV to 250 MeV with pre-defined spectra are transported in a 3D geometry through material zones confined by planes and second order surfaces or in 3D voxelized geometry. The code can treat proton transport in a few hundred different kinds of materials including elements from Z=l to Z=98. Simulation of proton transport is based on the multiple scattering theory of charged particles and on the model for compound nucleus decay. For each part of the range, an average loss of energy is calculated with a fluctuation from Vavilov's distribution. The deflection angle of protons is sampled from Moliere's distribution.

P3. Dose distribution of a proton beam in a water phantom

Task: A parallel beam of protons from a disk source (diameter 15 mm) impinges on a PMMA compensator (cylindrical symmetry) and on a spherical water phantom approximating an eye (Figure 1). All elements are in vacuum. If discrete regions are used for dose calculations (depth-dose and isodose curves), use voxels with dimensions 0.5 x 0.5 x 0.5 mm3. The results should be normalized to one primary proton

Proton beam Compensator Water phantom (eye)

10 mm

3 mm

I'ig.1 : problem scheme (not to scale)

1

Parti'. Consider a 50 MeV monoenergetic beam. Calculate:

1) The depth-dose distribution in the spherical phantom along the diameter D parallel to the proton beam direction;

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SO MeV proton beam in eye water phantom

Beam diameter 1.5 cm

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,„,„•1 l.lwt,-0,00 0,24 0.48 0,72 0,98 120 1,44 1.68 1,92 2,16 2,40

Depth, cm

Figure Pl . l . Depth-dose distribution in spherical phantom

2) Optional - the 90%, 80% and 20% isodose curves on the equatorial plane of the water phantom parallel to the beam direction.

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-1.00 -O.80 •O.fiQ -0.40 -030 0.00 0.30 0^0 0.60 0.80 1,00

Figure PI.2.a. Isodose curves on equatorial eye plane

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Figure P1.2.b. Dose distribution in vozelized eye geometry

Part 2: Simulate the effect of a beam modulator by sampling the source according to the discrete

distribution in energy listed in Tab. 1 (or alternatively calculate the weights for spreading out the Bragg peak referring to energies in the range 40 -50 MeV), calculate:

1) the depth-dose distribution in the spherical phantom on the diameter parallel to the proton beam direction;

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i . i V i 0,00 0.24 0.48 0.72 0.96 1,20 144 1.88 1.92 2.16 2,40

Depth, cm

Figure P2.1. Depth-dose distribution in spherical phantom

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2) optional - the 90%, 80% and 20% isodose curves on the equatorial plane of the water phantom parallel to the beam direction.

-1.00 -0.80 -0.60 -0.4O -0.20 0.00 0.20 0.40 0.60 0.80 IJOO

Figure P2.2.a. Isodose curves on equatorial eye plane

Figure P2.2.b. Dose distribution in vozelized eye geometry

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