Repair processes and cell survival

A model of cell survival after irradiation is presented which explicitly incorporates a repair process. The model is based on Powers' suggestion of a “pool”, and is developed along the lines initiated by Lajtha and Oliver (1961). The repair process corresponding to the shoulder on the survival curve is distinguished from the process demonstrated by the initial rise observed in the recovery curve. The two processes are separated by the existence of a limited pool of intracellular constituents or states. The time course of the processes of repair or development of damage is expressed in equations which are solved to give the relation between the dose and the final survival observed. The model includes the situation during low dose-rate irradiation when the rate of replenishment of the pool is equal to the rate of depletion due to repair. This is used to analyse experimental results obtained for acute and protracted irradiation. The values of the parameters found can be used in the prediction of the effects of protracted irradiation at other dose-rates. The experimental and conceptual reasons for identifying a repair process other than the well-known Elkind recovery are presented and discussed. The usefulness of the model, incorporating the separation of shoulder and recovery, is demonstrated for the study and prediction of four types of phenomenon: those associated with the size of the pool, the interaction of different agents through the medium of the pool, the rate of action of the pool, and the effects of protracted irradiation.