INFLUENCE OF RADIATION FRACTIONATION ON SURVIVAL OF MICE AND SPLEEN COLONY-FORMING-UNITS

C57B1/6 mice were given 10 Gy X rays fractionated in several ways. The re was a cyclical pattern of animal survival which was correlated to t he fractionation interval and which indicated a periodicity of 6 h. Te n grays given in a single dose is fatal to 100% of the mice and depres ses the CFU-S to about one per leg with no evidence of proliferation d uring the remaining life. Ten grays given in 2.5-Gy increments at 24-h intervals causes no fatalities and results in a similar CFU-S depress ion but is followed by an exponential increase in CFU-S over the ensui ng 12 days. Although bone marrow from survivors of such treatment was comparable to control marrow in its capacity for short-term rescue, it was clearly inferior in its capacity for long-term rescue. The period icity of 6 h suggests that the cells responsible for survival of the m ice have been synchronized into more or less radiosensitive and radior esistant stages of the cell cycle as a result of the time between the 2.5-Gy increments. Implications for the CFU-S and long-term repopulati ng cells are discussed.