Dosimetric impact of prostate edema in 125I LDR brachytherapy using MCNP simulations and an adapted ICRP 110 voxel phantom.

Low Dose Rate (LDR) prostate brachytherapy with permanent 125I seed implants is frequently associated with prostatic edema, an inflammatory response that increases the gland's volume and alters source geometry. This study quantifies the impact of post-implant edema on dose distribution using the MCNP Monte Carlo code integrated with the ICRP 110 reference adult male voxel phantom. The ICRP 110 voxel phantom was globally rescaled by modifying the voxel grid resolution to emulate prostate volume expansion scenarios ranging from 10% to 50%. Although this transformation affects the entire voxelized geometry, dosimetric evaluation was restricted to the prostate region containing the implanted 125I seeds. A dedicated computational system was employed to automatically generate MCNP input files, recalculating the 3D positioning and inter-seed distances for 73 125I seeds (model 6711) in each scenario. The simulations revealed a progressive reduction in dose coverage governed by the Inverse Square Law, as the volumetric expansion increases the distance between sources. While the non-edematous reference case was normalized to the prescribed dose of 144 Gy, the relative absorbed dose decreased to 99%, 97%, 94%, 91%, and 87% for edema levels of 10%, 20%, 30%, 40%, and 50%, respectively. These results indicate that edema exceeding 20% leads to progressive reductions in absorbed dose and increasingly unfavorable DVH characteristics. Considering the 59.4-day half-life of 125I, this dosimetric deficit is particularly critical during the first few weeks post-implant, when source activity is at its peak. Based on the observed data, there is a clear necessity to incorporate edema-aware strategies into brachytherapy planning and post-implant evaluation to ensure therapeutic efficacy and target coverage.