Monte Carlo quantification of indirect DNA damage from low-energy electrons in 125I prostate brachytherapy

Brachytherapy is a widely adopted technique for treating localised prostate cancer. However, traditional dosimetric models often disregard low-energy electrons emitted by 125 I seeds due to their limited range. This study aims to investigate the biological impact of these electrons, focusing on their potential to indirectly damage DNA via reactive oxygen species (ROS). Monte Carlo simulations using GAMOS/GEANT4 and GEANT4-DNA were performed to quantify the energy spectra and dose contributions from photons and electrons emitted by 125 seeds. Both single-seed and multi-seed (98 seeds) configurations were analysed to evaluate inter-seed attenuation (ISA) and its influence on electron behaviour and dose delivery. The results show a non-negligible presence of ultra-soft X-rays and low-energy electrons outside the seed, capable of initiating water radiolysis and generating OH•radicals. Although their dose contribution is minimal (0.02%), their biological relevance is significant due to ROS-mediated DNA damage. This study presents one of the first quantitative evaluations of indirect DNA damage from low-energy electrons in prostate brachytherapy using Monte Carlo simulations. These findings highlight the need to incorporate biologically weighted effects into treatment planning and support the integration of radiobiological models in future studies.