Radiocarcinogenesis in vitro: «Inverse dose-rate effect»

Summary

There is substantial experimental evidence that protracted exposure to high-LET radiation can have a greated effect than single exposure in inducing cell transformation, the so-called «inverse dose-rate effect». The magnitude of this enhancement is due to the complex interplay between dose, dose rate and radiation quality. We have developed a model that explains the complex trend of the experimental results. This model is based on the assumption that there is a brief period of high sensitivity to transformation in the cell cycle as proposed in the literature and takes into account the saturation observed at high doses in the dose-effect curves. Specific equations are reported for acute, protracted and fractionated irradiation. Findings with C3H10T1/2 cells were analised in the light of this model. Assuming best fitted parameters of the model obtained from acute-irradiation data, tranformation frequencies due to protracted or fractionated exposure were predicted and compared with experimental findings on fission and monoenergetic neutrons and on charged particles of LET between 20 and 150 keV/μm. The model’s predictions were found to be closely consistent with the available experimental data.