Interpretation of the increase in the frequency of neoplastic transformations observed for some ionising radiations at low dose rates.

The anomalous increase of transformation frequency with decreasing dose rate observed by Hill et al. (1982, 1984b) for mouse fibroblast cells irradiated with fission neutrons cannot be satisfactorily explained by current models of radiation action. Recently a new model has been proposed which predicts the enhancement of damage with prolongation of irradiation, for equal doses. This is applied to the transformation studies in an attempt to interpret the enhancement observed for some radiations but not for others. Evidence is presented which suggests that repaired double-strand breaks in the DNA of cells which survive are the precursors of transformation. A critical physical factor is the total irradiation time rather than the dose rate. Approximately 1 per cent of repaired surviving cells go on to transform. From the results an explanation emerges of why transformation enhancement at low dose rates is not observed for natural alpha radiation and for photons or electrons, but is observed for fission neutrons and fast iron ions.