A model of cell survival after irradiation is proposed which differs from previous models in that it includes the possibility of sublethal damage becoming fixed and available for interaction with the next dose increment even after a prolonged recovery interval. The model represents a modification of that proposed by Lajtha and Oliver for dose-rate effects and cell survival after continuous irradiation. It was applied to the analysis of in vitro experiments with Chinese hamster cells incubated at temperatures low enough to prevent cell division. The most direct evidence of incomplete recovery due to fixation of sublethal damage was obtained in split-dose experiments and the hypothesis was consistent with the results of experiments with continuous and fractionated irradiation performed while cells were incubated at temperatures between 11 and 24°C. The survival curves for cells which were given a conditioning dose followed after a time interval by graded acute doses were more adequately described by a model with linear and square dose terms in the exponent rather than a two-component model including single- and multitarget terms.

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