The Effect of Dose-rate and Fractionation on Acute Mortality in X-irradiated Mice. Part II

Summary

1. Changing the exposure time from 15 min to less than 1 min and the dose-rate from 54 to 1100 rads/min did not reduce the LD_50/30 by more than a few per cent. Changes in the uniformity of tissue dose, as by changing the direction of the x-ray beam, were quantitatively much more important.

2. The effect of increasing exposure time from 6 to 36 hours with continuous irradiation was compared with other exposures spread over similar periods of time but divided into 2, 4 or 18 equal fractions, each at a relatively high dose-rate. Neither with continuous nor with fractionated exposures did the LD_50/30 increase indefinitely as overall exposure time was prolonged. The rapid phase of decay of radiation damage appeared to stop early, at 6 hours, after a single dose but not until after 18 hours of prolonged exposure.