Monte Carlo in assessment of low dose rate prostate brachytherapy dose distributions

Abstract

The purpose of this study was to investigate differences in dose distributions when Monte Carlo models included interseed attenuation of I125 seeds and tissue heterogeneity effects and when these interactions are ignored. The Monte Carlo calculations were performed using the PENELOPE/penEasy system code. The code is implemented in FORTRAN 77 and its structure is based on a set of subroutines that are invoked from a main program written by the user. The code simulates incoherent scattering, coherent scattering and Bremsstrahlung X-ray production at energies ranging from 1 keV to 1 GeV for photons and from 0.1 keV to 1 GeV for electrons. For simulations, a simplified array of 3 × 3 × 3 seeds was placed in a sphere of water (radius of 5 cm). The geometry and energy spectrum of individual seeds was based on the manufacturer’s description. The seeds were positioned in order to mimic a simplified clinical arrangement for various separations and tissue heterogeneities. All simulations were conducted using an off the shelf laptop computer of 1.8 GHz AMD A6 processor with a RAM memory of 6 GB. From the results observed it is clear that there are differences in dose distributions between simulations that account for interseed attenuation and tissue heterogeneity and those that do not. Future work would be to develop more accurate voxel based anatomical phantom models for simulations, with more particle histories and the inclusion of other Monte Carlo tallies would allow for a more robust quantitative analysis of the results and a thorough assessment the differences observed.