Limiting Values for the RBE of Fission Neutrons at Low Doses for Life Shortening in Mice

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Abstract
Recently published data on the effects of low doses of fission neutrons on the mean survival times of mice was analyzed. The analysis for single-dose exposures was confined to doses of 20 rad or less, while for fractionated exposures only total doses of .ltoreq. 80 rad were considered. The data were fitted to the frequently used power function model: life shortening = .beta.D.lambda., where D is the radiation dose. Apparently, at low doses per fraction, either the effects are not additive or the dose-effect curve for single exposures cannot show a greater negative curvature than about the 0.9 power of dose. Analysis of the data for .gamma. rays showed that an exponent of 1.0 gave an acceptable fit. The RBE [relative biological effectiveness] for neutrons evidently cannot change more rapidly with neutron dose than about RBEN .simeq. .**GRAPHIC**. This conflicts with the more widely accepted relationship, RBEN .simeq. .**GRAPHIC**. Because of the inherent implausibility of exponents < 1.0 for the neutron dose-effect curves at low doses it was concluded that at neutron doses of .ltoreq. 20 rad the RBE for the life shortening is constant and ranges from 13 to 22 depending on mouse strain and sex.

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