Life Shortening in Mice Exposed to Fission Neutrons and γ Rays: I. Single and Short-Term Fractionated Exposures

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Abstract
Quantitative dose-response relationships are presented for life shortening in mice from all causes of death after irradiation with 0.85 MeV fission neutrons or 60Co .gamma.-rays, delivered either in a single exposure (neutrons, 20-240 rad; .gamma.-rays, 90-788 rad) or in 24 weekly fractions (total doses: neutrons, 20-320 rad; .gamma.-rays, 206-5110 rad). The effectiveness of a given dose of .gamma.-rays was decreased when the dose was fractionated; the effectiveness of a given dose of neutrons was significantly increased. For both single and fractionated exposures, the relative biological effectiveness (RBE) of neutrons (calculated on the basis of either radiation-specific excess mortality rate or days of life lost per rad) varied inversely with the square root of the neutron dose over the range of doses studied. Depending on the method of calculation, the coefficient of the RBE-dose relationship (the RBE at 1 neutron rad) for single exposures was predicted to be 28 or 33 for males and 40 or 44 for females. For fractionated exposures, the extrapolated values were 62 or 91 for males and 41 or 79 for females. Experiments are in progress that will test the validity of these predictions.