Abstract: To investigate the early radiochemical processes of γ-ray-induced DNA single-strand breaks (SSB), this study employed the Monte Carlo simulation tool TOPAS and its extended module TOPAS-nBio to construct a water sphere-plasmid microscale simulation model. The model was used to simulate the irradiation of supercoiled pUC19 plasmids by 60Coγ-rays in an aqueous environment and the subsequent induction of SSB in plasmid DNA. The study quantitatively compared the SSB yield under systematically varied conditions of radiation dose, DNA concentration, and hydroxyl radical (·OH) scavenger concentration. The simulation results showed that the SSB yield increased with the elevation of radiation dose. A higher DNA concentration in the environment led to an approximately linear increase in the absolute SSB yield, while the damage efficiency per unit mass of DNA decreased with increasing concentration. An increase in the concentration of ·OH scavengers significantly reduced the SSB yield, indicating that the ·OH scavenging capacity exerts a marked inhibitory effect on SSB formation. The findings of this study provide certain theoretical support for the optimization of radiotherapy, the formulation of radiation protection strategies, and the health risk assessment of nuclear environments.
Key words: gamma ray; plasmid DNA; free radical; Monte Carlo simulation; DNA single strand break
（Acta Laser Biology Sinica, 2026, 35(1): 028-034）