The importance of microdosimetry for radiation biology and radiation protection

In this contribution several features will be discussed of relations between biological effects produced by ionizing radiations and the spatial distributions of energy deposition of these radiations. Effects produced by high-LET radiations are generally found to be less dependent on dose-rate, dose-fractionation and on the influence of cellular conditions and of various compounds in the cell environment, than effects of low-LET radiations. Relations between the relative biological effectiveness of ionizing radiations and the distributions of local energy densities will further be shown to provide information which is required to test various hypotheses about radiobiological mechanisms. This is especially of importance for the extrapolation of experimental results obtained at high doses and dose-rate to the low-dose rates and low doses of interest in radiation protection. It is finally pointed out that measurements of energy deposition patterns for different radiations may provide the basis for selecting the types of radiation which should be compared in order to obtain information about differences in dose-effect relations for those biological endpoints, e.g. tumour induction and genetic effects, that can only be investigated in experiments with large numbers of animals and long observation periods