Abstract
The biological effects of low-LET ionizing radiation are characterized by dose-rate effects (DREs). It is thought that the frequency of chromosome aberration or mutation induced by low dose-rate decreases rather than high dose-rate. However, recent data show that DREs on chromosome aberration have not been observed in mice receiving the X-rays at dose rate of less than 55 mGy/day. This result indicates that DNA damages induced by very low dose rate may remain unrepaired. The generation and repair of radiation-induced DNA damages in cells have been recently examined by enumeration of phosphorylated histone H2AX (γ-H2AX) foci. Using published data on γ-H2AX foci in this study, we focus here that the number of γ-H2AX foci per cells becomes constant at 24 hours after X-rays irradiation at dose of less than a few ten mGy. We hypothesize that DNA damages induced by X-rays with below a few ten mGy repairs until the same amount of DNA damages at 24 hours after acute irradiation. Consequently this suggests that DRE would not occur among the mice receiving the X-rays irradiation at dose rate of less than a few ten mGy/day. γ-H2AX foci formation may be useful to elucidate the mechanism of DRE.
