Modification of Lethal Radiation Effects in Rats by Short-Term Protraction of Dosage

Preliminary studies by the present authors indicated that short interval fractionation of lethal doses of radiation (delivered by means of 220-kvp x-rays) increased the survival rate and mean survival time for rats within a thirty-day period of observation. In these studies, the radiation was administered in six equal fractions with intervals between fractions varying from zero to sixty minutes. The mean survival time for a total radiation dose of 786 r (an LD 90 for thirty-day survival as measured in this laboratory) ranged from 6.4 days for zero interval fractionation to 18.3 days for sixty-minute interval fractionation. The thirty-day survival rate for the sixty-minute interval studies was found to be equivalent to an LD 40. In order to determine whether the fractionation of dosage per se or the resulting increased overall period of irradiation was most important in modifying lethal radiation effects, further studies were undertaken. Experiments were set up in which the overall period of irradiation an...