Multifractionation of 60Co gamma-rays reduces neoplastic transformation in vitro.

Incidences of neoplastic transformation of 10T 1/2 cells, derived from a C3H mouse embryo, were measured following irradiation of cells with 60Co gamma-rays delivered in single total doses, from 25 to 300 rads, or in different numbers of fractions and with different time intervals between fractions. Throughout the range of total doses that was examined (25-300 rads) exposure of asynchronous, actively growing 10T 1/2 cells to a dose of gamma-rays in five equal daily fractions, rather than in a single exposure, results in a significant reduction in transformation frequency. The initial part of the induction curve fits a straight line quite well. The slope of the line for five daily fractions is approximately 3-fold less steep than that for single exposures. A further increase in the interfraction interval, to 48 h, or further reductions in the dose per fraction, led to additional decreases in the incidence of neoplastic transformation. Our observations are consistent with a net 'error-free' repair of subtransformation damage as a result of protracted irradiation with 60Co gamma-rays.