MULTIPHASIC SURVIVAL CURVES FOR CELLS OF HUMAN TUMOR-CELL LINES - INDUCED REPAIR OR HYPERSENSITIVE SUBPOPULATION

Survival of the cells of three human tumor cell lines of differing rad iosensitivity was measured after irradiation with single doses of X ra ys (0.05-5 Gy). At doses below 1 Gy, cells were more radiosensitive th an predicted by back-extrapolating the high-dose response. This differ ence was more marked far cells of the radioresistant cell lines than t he radiosensitive cell line so that the ''true'' initial slopes of the survival curves, at very low doses, were similar for the cells of the three cell lines. This phenomenon could reflect an induced radioresis tance so that low doses of X rays are more effective per gray than hig her doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms which can t hen act during the time course for repair of DNA injury.