Adaptive response to radiation and radiation-induced bystander effect

Contribution of the bystander effect (BE) in the adaptive response (AR) to radiation is reviewed on authors' findings. The biological resistance to radiation is yielded by previous exposure of low dose radiation and this is called as the AR. The BE is the biological effect yielded in cells not exposed but present near the exposure. AR has been considered not inducible by high liner energy transfer (LET) radiation but recent findings suggest that even α-particle can induce. The culture medium of α-ray irradiated cells is shown to induce radio-resistance in bystander cells. There are authors' findings that endogenous NO is a bystander factor induced by high or low LET radiation (carbon beam or X-ray) in human A-172 cells as well as other investigators' similar findings. Despite these, competition of low dose γ-ray in AR and α-ray in BE is also reported in C3H 10T1/2 cells. Utilization of micro-beam radiation in future is expected to resolve the relationship between AR and BE. (N.I.)