Dose optimization in 125I permanent prostate seed implants using the Monte Carlo method

The aim of this work consisted in using the Monte Carlo code MCNP and computational phantoms to assess the absorbed dose distributions in the prostate, due to a radiotherapy treatment using 125I radioactive seeds. The intention was to develop a tool that can serve as a complement of the treatment planning system of radiotherapy procedures, reproducing accurately the exact geometry of the sources and the composition of the media where the seeds are inserted. The radiation activities of the simulated seeds varied from 0.27 mCi to 0.38 mCi, for hypothetical treatments employing 80, 88 or 100 125I sources, typical parameters for this technique. The prostate volumes where the seeds were virtually inserted were simulated with spherical or voxel computational phantoms. The configuration containing 88 seeds with initial radiation activity of 0.27 mCi resulted in a final absorbed dose near 144 Gy, in accordance with the recommendations of the American Association of Physicists in Medicine (AAPM). Based on this configuration, it was possible to obtain the radiation absorbed dose distributions for the voxel phantom, which allowed the determination of treatment quality indicators. The obtained results are in good agreement with experimental data presented by other authors.