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

Abstract

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 and the total radiation dosage were kept constant while the number of fractions was varied from 2 to 12. At the same time an additional group of animals was exposed to an equal dose of radiation administered continuously at a low rate over an equal period of time. Separate experiments were performed for overall periods of irradiation of three, six, and twenty-four hours with a total of 647 animals. It was concluded from the data obtained that, within certain limits, fractionation per se is not important, provided the total time of irradiation and the total dose are kept constant and that no individual fraction is so great as to produce significant lethal radiation effects by itself.