The Impact of Bystander Effects and Adaptive Responses in the Health Risks of Low Dose Ionizing-Radiation: the Modulating Effect of Linear Energy Transfer Radiation-induced adaptive and bystander responses

A large volume of laboratory and human epidemiological studies have shown that high doses of ionizing radiation engender significant health risks. In contrast, the health risks of low level radiation remain ambiguous and have been the subject of intense debate. To reduce the uncertainty in evaluating these risks, research advances in cellular and molecular biology are being used to characterize the biological effects of low dose radiation exposures and their underlying mechanisms. Radiation type, dose rate, genetic susceptibility, cellular metabolic state, growth stage, levels of biological organization and environmental parameters are among the factors that modulate interactions among signaling processes that determine the outcome of low dose exposures. Whereas, recommended radiation protection guidelines assume a linear dose-response relationship in estimating radiation cancer risk, investigation of phenomena such as adaptive responses and bystander effects suggest that low dose-induced signaling events act to alter linearity of the dose-response relation as predicted by biophysical argument. Our knowledge of molecular, biochemical and cellular aspects of these phenomena in the context of low dose exposures will be reviewed. The argument that biological responses together with biophysical considerations predict the outcome of cellular exposure to ionizing radiation will be discussed.