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Repair of neoplastic transformation damage following protracted exposures to 60Co gamma-rays.

The incidences of neoplastic transformation induced by 60Co gamma-rays in exponentially growing mouse embryo 10T1/2 cells were measured following acute and protracted exposures. Delivery of 60Co gamma-rays at a low dose rate (0.1, 0.5, 2.5 cGy min-1) compared with a high dose rate (100 cGy min-1) results in appreciable, dose rate dependent reductions in cell killing and, independent of the effect on cell survival, reduces significantly the incidence of neoplastic transformation. Exposure of exponentially growing 10T 1/2 cells to a dose of gamma-rays in 5 equal daily fractions also significantly reduces transformation frequency, compared with delivery in a single dose, throughout the dose range examined (25-300 cGy). The initial parts of the induction curves are fitted quite well by a linear dose dependence. The slopes of the regression lines for multifractionation delivery or irradiation at 0.1 cGy min-1, are one-third and one-half, respectively, of those for single exposures at a high dose rate. Increasing the inter-fraction interval up to 48 h, or reduction of the dose per fraction further reduce incidence of neoplastic transformation. We conclude that protracted exposures of low LET radiation result in a net "error-free" repair of subtransformation damage.