RADIATION DOSE DELIVERED BY 125I, 103PD AND 131CS AND DOSE ENHANCEMENT BY GOLD NANOPARTICLE (GNP) SOLUTION IN PROSTATE BRACHYTHERAPY: A COMPARATIVE ANALYSIS BY MONTE CARLO SIMULATION

The energy deposition and radiation dose from commonly used radioisotopes, I-125, Pd-103 and Cs-131, used for brachytherapy of cancers is estimated using Monte Carlo (MC) simulations. To enhance the dose, gold nanoparticle (GNP) solutions are injected into the tumor; this results in more effective and shorter therapy duration. It is thus important to estimate the dose enhancement factor (DEF) achievable by a radioisotope. The research presented in this paper thus focuses on a comparative analysis of radioisotopes. To estimate the radiation dose, the Monte Carlo N-particle code MCNP5 was used for a coupled photon-electron simulation of radiation transport from radiation emanating from seeds of radioisotopes implanted in the prostate at positions prescribed to deliver effective doses to the tumor while protecting neighbouring vital organs such as the rectum and urethra. The quantities tallied were the energy deposition (F6 tally) and the pulse heights (*F8 tally) in specified energy bins. The energy deposited in the tumor was used to estimate the absorbed dose to the prostate incorporating the transformations of the radioisotopes during decay. The absorbed dose was subsequently estimated for a GNP-tissue solution with a concentration of 25 mg Au/g of prostate tissue, modelled as a homogenous mixture. From the simulations, it was found that the lifetime absorbed dose is similar to 96 Gy from 98 seeds, each of 0.31 mCi, of 125I; similar to 102 Gy, from 115 seeds, each of 1.4 mCi, of 103Pd, and similar to 90 Gy from Cs-131 seeds replacing 103Pd seeds of the same initial activity. The main advantage of Cs-131, over I-125 and Pd-103, is observed in the larger dose rate (similar to 26 cGy/hr) delivered initially i.e. in the first few days which is 1.5 and 5.7 times higher than that for (103Pd) and (125I). The absorbed dose for 125I, 103Pd and 131Cs increases to similar to 245, similar to 130, similar to 187 Gy respectively with GNP-tissue solution of 25 mg Au/g tissue. From the analysis, it is found that while the lifetime absorbed dose of all three radioisotopes is of the same order, there are advantages in using Cs-131; these advantages are further quantified.