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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.

Transformation frequencies induced in cell systems by low- and high-LET radiations are compared and analysed as a function of LET and dose protraction. At acute dose rates, the dose-response curve with low-LET radiations shows a complex shape: the transformation frequency increases with the dose by a power of less than 1 in the interval (0.25÷1.5) Gy; a power of about 2 from 1.5 Gy to 4.5 Gy, where it reaches its maximum value and then remains constant. High-LET radiation transformation frequencies increase with dose by a power of 2 from 0.1 to 2 Gy after which they remain constant. RBE values increase with LET up to 140 ke V/μm and then decrease. The transformation frequency is reduced following dose fractionation or dose rate reduction with low-LET radiation, whereas it is enhanced with high-LET radiation, such as 0.85 MeV neutrons. Values of RBE for transformation up to 50 can be evaluated for neutrons. Various models that have been proposed to explain these results are analysed and compared.