Automated system for MCNP input generation and ICRP 110 phantom adaptation in ¹²⁵I prostate brachytherapy with edema

Low dose rate (LDR) prostate brachytherapy with ¹²⁵I seeds requires precise source placement to deliver the prescribed dose. Manual creation of MCNP-compatible input files for such treatments is labor-intensive and prone to errors due to the complex definition of cell and surface cards for each seed. To address this challenge, an automated system was developed to generate MCNP-compatible input files based on clinical planning data. The system automatically adapts the International Commission on Radiological Protection (ICRP) 110 male voxel phantom, utilizing its reference prostate volume of 16.5 cm³, according to user-defined edema percentages (0-50%) and recalculates the spatial distribution of the implanted seeds (73 seeds in the validation case). By producing multiple input files representing different edema conditions, the system enables detailed dosimetric analyses using Monte Carlo simulations without manual recalculation. In this study, the analysis focused specifically on the prostatic edema process and was evaluated using 7 × 10⁸ particle histories to ensure high statistical convergence. Validation was performed using MCNPX 2.5.0, demonstrating that under a 50% edema scenario the mean absorbed dose decreased from the prescribed 144 Gy to 125.28 ± 0.44 Gy, corresponding to approximately 87% of the prescribed dose. The statistical reliability of these results was confirmed by the relative error (R) associated with the MCNPX F8 tally, which remained at 0.28% for the reference case and 0.35% for the 50% edema condition. This tool significantly reduces preparation time, ensures accurate reproduction of clinical plans in the simulation environment, and provides a robust framework for investigating dose variations due to prostate swelling, supporting improved planning strategies for LDR brachytherapy.