A MONTE CARLO BRACHYTHERAPY STUDY FOR DOSE DISTRIBUTION PREDICTION IN AN INHOMOGENEOUS MEDIUM

Abstract The purpose of this study was to present a theoretical analysis of how the presence of bone in interstitial brachytherapy affects dose rate distributions. This study was carried out using a Monte Carlo simulation of the dose distribution in homogeneous medium for 3 commonly used brachytherapy seeds. The 3 seeds investigated in this study are iridium-192 ( 192 Ir) iodine-125 ( 125 I), and palladium-103 ( 103 Pd). The computer code was validated by comparing the specific dose rate (Λ), the radial dose function g(r), and anisotropy function F(r,θ) for all 3 seeds with the AAPM TG-43 dosimetry formalism and current literature. The 192 Ir seed resulted in a dose rate of 1.115 ± 0.001 cGy-hr −1 -U −1 , the 125 I seed resulted in a dose rate of 0.965 ± 0.006 cGy/h −1 /U −1 , and the 103 Pd seed resulted in a dose rate of 0.671 ± 0.002 cGy/h −1 /U −1 . The results for all 3 seeds are in good agreement with the AAPM TG-43 and current literature. The validated computer code was then applied to a simple inhomogeneous model to determine the effect bone has on dose distribution from an interstitial implant. The inhomogeneous model showed a decrease in dose rate of 2% for the 192 Ir, an increase in dose rate of 84% for 125 I, and an increase in dose rate of 83% for the 103 Pd at the surface of the bone nearest to the source.