An Analysis of the Role of Dose and Dosage Rate in the Early Radiation Mortality of the Chick

Abstract

Many effects of ionizing radiations are known to depend on dosage rate and fractionation (1-11). This dependence is indicative of a partial reversal of radiation effects during, between, and after irradiations which may be expressed not only as recovery of injury sustained but also as nullification of effects leading to injury. Reversal rates are often assumed to be proportional to the amount of injury present, and some of the earlier experimental data were fitted to a theory of exponential reversal (4, 12). Other data do not support such a model (11, 13). In the chicken, time factors have been shown to affect the acute lethal action of X-radiation (8-10). Lethality is based on at least two distinct mechanisms, one of which is responsible for deaths occurring primarily on the first and second day after irradiation, the other expressed through deaths occurring mostly within 3 to 12 days postirradiation. The two modes of death are distinguished not only by the bimodel character of the mortality distribution but also by differences in the physiological failure observed (14-16). We propose to examine the relation between dose, dosage rate, and mortality in young chicks after Co60 -y-irradiation. For the first mode of radiation death, for which the criterion is fatalities occurring within 2 days after irradiation, it will be shown that the assumption of a constant rate of reversal of radiation-induced effects is supported by the data. Variation in dosage rate affects mortality in the second period in a different manner.