Abstract
The dose-rate effect (DRE) is very important phenomenon to estimate the radiation risk for human health. However the mechanisms of DREs have not been clarified. We thought that dose-rate was fractionated irradiation per unit time, and hypothesized that the DREs might be induced by the accumulation of unrepaired DNA damages. It is necessary to examine the time course of radiation-induced initial DNA damage in detail to prove our hypothesis. First, we investigated the time course analysis of radiation–induced initial DNA damage based on 53BP1 forms discrete foci in primary human fibroblasts (MRC-5). The result showed that the numbers of unrepaired DNA damage were 1.3 per cell at 0 min, 1.33 per cell at 3 min, 1.57 per cell at 5 min, 2.62 per cell at 15 min, 1.2 per cell at 30 min after 20 mGy. Next, we investigated whether unrepaired DNA damages are accumulated in MRC-5 irradiated with 2x20 mGy per 3-10 min. The numbers of unrepaired DNA damage were 2.5-2 per cell after the second irradiation. However, when MRC-5 were irradiated with 2x20 mGy per 15-30 min, the numbers of unrepaired DNA damage were 1.8-1.6 per cell after the second irradiation. This result indicated that unrepaired DNA damage was not accumulated by fractionated irradiation per 15-30 min. Thus, we found that the mechanism of DREs is not able to explain from DNA damage and repair.
