Quantitative comparison between initial DNA-damage RBE of I-125 and Pd-103 brachytherapy sources through a hybrid simulation approach

Abstract

Iodine-125 and palladium-103 are radioactive sources that are frequently used for interstitial brachytherapy purposes. The current study aims to quantitatively compare the initial DNA-damage relative biologic effectiveness (RBE) of these two brachytherapy sources through a hybrid Monte Carlo (MC) simulation approach. Two commercially available brachytherapy sources of SL-125/SH-125 and MED3633 were considered as 125I and 103Pd sources, respectively. At first, geometry of two selected brachytherapy sources was accurately simulated by MCNPX MC code. Then, energy spectrum of released secondary electrons at different distances of 0.5, 1, 3, and 5 cm was scored, and obtained spectra were imported to MCDS MC code. Finally, the RBE values, with respect to Co-60 reference source, were estimated through calculation of induced DNA damages, including single- and double-strand breaks (SSBs and DSBs), by MCDS code for both studied brachytherapy sources at different distances. No significant differences were observed between the calculated RBE values of both considered brachytherapy sources. Based on our findings, the RBE (DSB) for iodine-125 and palladium-103 is, respectively, equal to 1.14 and 1.15, while the RBE (SSB) is the same for both brachytherapy seeds and equal to 0.97.