Life-shortening in Female CBA Mice Exposed to Daily Irradiation for Limited Periods of Time

Abstract
Summary Female CBA mice were exposed to daily irradiation by gamma-rays or by fast neutrons at daily doses of from 3–50 rems for periods varying from four weeks to the duration of life. At daily doses between 3 and 30 rems, the mean survival-time reached a near minimum value after an exposure lasting less than half the duration-of-life exposure. Additional exposure had little further effect on survival-time. The shape of the mortality-curve depended systematically on the particular level of daily dose and on the duration of exposure, except possibly at the lowest daily dose. It is concluded that, if experimental support is to be sought for hypotheses relating life-span simply to dose or dose-rate, then experiments must be done with total doses of not more than a few hundred rems or with daily doses smaller than 3 rems. The shape of the curve relating total dose to mean survival-time in duration-of-life exposures can be accounted for on the assumption that all radiation damage is, in a mathematical sense, reparable. There is a fallacy in arguing that the shape of the curve necessitates the assumption that some radiation damage is irreparable. On the other hand, when the biological character of the damage is taken into account in experiments in which a near 100 per cent incident of malignant disease followed a limited period of continued irradiation, it must be considered that almost the whole of delayed radiation damage may be irreparable, even with gamma-rays. The r.b.e. for life-shortening by daily irradiation with fast neutrons was found to be independent of exposure time, but may perhaps vary with dose-rate, possibly because the biological processes mainly responsible for life-shortening differ at different dose-rates.

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