Calculation of Absorbed Dose of <sup>103</sup>Pd and <sup>131</sup>Cs Sources in Prostate Cancer Brachytherapy Simulation Using Monte Carlo N-Particle 6 (MCNP-6) Software

Implant brachytherapy is one of the therapies for Low Dose Rate (LDR) in prostate cancer. This type of therapy uses a source with low energy and penetration rate so that the organs around the prostate do not receive much of the absorbed dose. This study uses a calculation method Monte Carlo N-Particle 6 (MCNP-6) to calculate the interaction of photons with organ materials. The main objective of this research is to determine the number of seeds and the ideal source activity to achieve the optimal absorbed dose in the prostate and provide the minimum absorbed dose in the surrounding organs. Organs around the prostate include the testes, small intestine, descending colon wall, descending colon, sigmoid colon wall, bladder wall, and bladder. This study uses two types of radioisotope sources namely 103Pd and 131Cs which each has a photon energy of 21 keV and 30 keV. Variations made is the addition of the number of seeds from 60 to 100 at intervals of 8 seeds symmetrically away from the center of the prostate and variation of source activity from 0.1 mCi to 0.6 mCi at intervals of 0.1 mCi for each type of source. Results of this study obtained the relationship that the more the number of seeds the greater the dose received by the prostate and surrounding organs, as well as the addition of source activity. The effect of increasing the number of seeds can increase the absorbed dose more significantly than the effect of adding activity to the organs around the prostate. Optimal absorbed dose for 103Pd is 125 Gy and 115 Gy for 131Cs. Based on the simulation results with MCNP-6, it is obtained that the ideal combination for the optimal absorbed dose is obtained from the source 103Pd is the number of seed 60 with 0.3 mCi activity, and source 131Cs is the number of seed 76 with an activity of 0.5 mCi. Source 103Pd provides a lower absorbed dose to the organs around the prostate compared to the source 131Cs.