Dose-rate effects in mammalian cells. IV. Repairable and nonrepairable damage in noncycling C3H 10T 1/2 cells.

Repairable and nonrepairable components of gamma-ray damage leading to cell reproductive death were determined by measuring the range over which dose rate influenced the response of non-cycling C3H 10T 1/2 mouse cells. Cell proliferation and cell cycle redistribution were eliminated as factors influencing the dose-rate effect in the system by irradiating confluent monolayers of contact inhibited cells. The radiosensitivity of the cells did not change, and no selective loss of damaged cells occurred over the extended treatment times. A pronounced dose-rate effect was observed over the range between 55.6 and 0.29 Gy/hr, but a limit to the repair-dependent dose-rate effect was reached at 0.29 Gy/hr since no further reduction in effect per unit dose was observed when the dose rate was reduced to 0.17 or 0.06 Gy/hr. The survival curves, which were simple exponential functions of dose at dose rates of 0.29 Gy/hr and below, have a common Do of 7.32 Gy and represent an accurate measurement of the nonrepairable component of damage. Log-phase cultures showed remarkably different responses over the range of dose rates, due in large part to cell cycle redistribution and in some cases, cell proliferation during exposures. The results of these studies were compared with time-dose relationships used in clinical brachy-therapy and agree remarkably well with corrections in total dose suggested by R. Paterson [Br. J. Radiol. 25, 505-516 (1952)] and A.E.S. Green [cited in F. Ellis, Curr. Top. Radiat. Res. Q. 4, 357-397 (1968)] when the standard treatment time is changed. Comparison of our data with in vivo isoeffect curves of total dose vs dose per fraction for "early" and "late" tissue responses indicate that cell cycle redistribution should not be ignored as a factor influencing time-dose relationships in radiotherapy.