Rate of Recovery from Radiation Damage and Its Possible Relationship to Life Shortening in Mice

Abstract

The recovery rate of animals exposed to sublethal doses of radiation and subsequently challenged by a second dose of radiation has been studied in detail by a number of authors (1-10). In these studies, which utilize lethality as the biological end point, it has generally been found that in mice and rats recovery can be described as a simple exponential function. When the recovery rate so determined has been utilized to predict median lethal doses for fractionated exposures extending over several days, however, the observed LD50 has generally been lower than that predicted on the assumption of a constant recovery rate. This finding has led to the suggestion that repeated exposures cause a progressive decline in recovery rate (1, 4, 5), but an analysis of the rate of decline has not as yet been performed. Blair (11-13), utilizing the concept of a constant repair rate for reparable radiation damage and the premise that a portion of the damage does not repair, has advanced an interesting hypothesis that attempts to relate the acute LD50 to mean survival time (life shortening) at very long times after radiation exposures. Since there are now a number of published reports on the survival times of mice exposed to a great variety of daily doses of radiation, it was considered worth while to re-examine these data and to calculate mean recovery rate constants for animals exposed to different numbers of daily radiation doses. If a systematic variation in recovery rate with number of fractions could be demonstrated, it was then planned to modify certain aspects of Blair's hypothesis accordingly and to apply the modified hypothesis to certain cases of experimentally produced life shortening from radiation. These calculations and modifications of the Blair hypothesis are the subject of the present report.