Repair of cell killing and neoplastic transformation at reduced dose rates of 60Co gamma-rays.

Using the C3H/10T1/2 mouse embryo-derived cell line, we measured the survival and incidence of neoplastic transformation following exposures of the cells to 60Co gamma-rays at an acute (100 rads/min) and at reduced (2.5, 0.5, and 0.1 rad/min) dose rates. Typical of what is generally observed with mammalian cells, at reduced dose rates, the survival of 10T1/2 cells increases as the dose rate is decreased due to the repair of sublethal damage during irradiation. The induction curve following acute exposures of 60Co gamma-rays is similar to that previously reported for 50-kV X-rays; the frequency initially rises rapidly, reaching a plateau of about 3 X 10(-3) transformants/survivor. The transformation induction curves for the reduced dose rates, while similar in shape to that for the high dose rate, exhibit a strong dependence on dose rate. All of the induction frequencies for reduced dose rates lie below those for 100 rads/min. In the low-dose region (i.e., 0 to 150 rads), our results are most extensive at 100 and at 0.1 rad/min. While each set of data in the low-dose region is fitted quite well by a linear dose dependence, the slope of the regression line at 0.1 rad/min is about one-half of that at 100 rads/min. We conclude that subeffective transformation damage, as well as sublethal damage, is repaired during low-dose-rate irradiation.