Repair of radiation-induced DNA double-strand breaks in human fibroblasts is consistent with a continuous spectrum of repair probability

Purpose: To propose a novel interpretation of DNA double-strand break (dsb) repair based on the distribution of energy microdeposition. Materials and methods: Double-strand break repair curves were studied either after irradiation at 4°C or at 37°C (low dose rate). Two human fibroblast cell lines were used: a control line, HF19, and an ataxia telangiectasia repair- deficient line, AT5BI. Irradiations were made with γ-rays or α-particles (²⁴¹Am). Repair data were fitted by the variable repair half-time (VRHT) model. Assuming that each dsb has its own inherent repair half-time (IRHT) and that the VRHT is the average of the IRHT at any time during repair, the distribution of the IRHT was calculated. Results: At the end of the irradiation, the distribution was a continuous asymmetric curve with a maximum of dsb having a short IRHT. After 1h of repair, the curve became bell-shaped. There is a striking similarity between the distribution of dsb repair half-times and that of energy micro-deposition described by Goodhead et al. (1993). Conclusion: This similarity suggests a possible causal relationship between the energy density deposition and the repair rate or the probability of dsb repair.