The Response of Haemopoietic Colony-forming Units to Single and Split Doses of γ-rays or D-T Neutrons

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Abstract
The effect of single and split doses of 137Cs γ-rays or D-T neutrons on haemopoietic stem cells irradiated in donor mice was investigated, using the spleencolony technique. On the γ-ray survival-curve derived from colony-forming units from the femur, there was a shoulder characterized by an extrapolation number of 3·1 ± 0·7. Split-dose experiments demonstrated a sparing effect of γ-ray dose-fractionation, and survival curves obtained for the survivors at various times after a first dose of 200 rads (γ-rays) showed that a significant shoulder had reappeared within 5 hours of the first dose. An increased radioresistance was indicated, demonstrated by a trend towards higher D0 values for the survivors, when they were irradiated at 5 hours, 8 hours or 20 hours after a 200-rad dose of γ-rays. A lower extrapolation number of 1·7 ± 0·5 was found for D-T neutrons, and split-dose experiments demonstrated that dose-fractionation had no significant sparing effect.