Normal and reverse dose-rate effect for the induction of mutants in somatic cells by ionizing radiation

In radiobiology the reduction of the dose-rate in general diminishes the degree of the biological effect per unit dose. This phenomenon is characterized by the dose-rate effectiveness factor (DREF). DREF is the factor by which a risk per unit dose obtained from data at high dose and high dose-rate overestimates the risk at low doses and/or low dose-rates. In general, DREF is in the range of 2 to 10. In the first part of this review, a short survey of the modern microdosimetric approach for a better understanding of radiation load on the cellular level and the significance of dose-rate is given. Experiments on the influence of dose-rate on the mutagenicity of ionizing radiation in cultured cells are reviewed. In contrast to other biological effects, in most experiments the reduction of the dose-rate had no or even a reverse dose-rate effect (DREF < or = 1). In the second part results on the influence of dose-rates on the induction of HGPRT-deficient T-lymphocytes in mice irradiated in vivo are given. Mutagenicity decreases with dose-rate and DREF values between 3-10 were measured. Possible reasons for the discrepancy between in vitro and in vivo experiments are discussed.