COMPARATIVE EFFECTS OF DOSE PROTRACTION BY FRACTIONATION AND BY CONTINUOUS EXPOSURE.

Conclusions from high dose-rate fractionation studies, that mortality response can be effectively estimated solely from the dose and total-time interval of exposure alone, have been broadly interpreted to include a wide dose-rate range. The effects of continuous exposure were compared with those of relatively low dose-rate fractionation, giving equal total doses over the same time span. Fractionated exposure proved to be significantly more biologically damaging than did continuous exposure. Mean after-survival times showed that continuously exposed mice survived approximately 40% longer than those exposed by the fractionation method. It was concluded that extrapolation of dose-rate data on the assumption that fractionation of dose is equivalent to continuous exposure, with time and total dose in each case being equal, is not warranted in all cases.