Dose-fraction size and the extent of cellular recovery from sublethal radiation injury

The clonogenic survival or Chinese hamster cell cultures was studied after exposures to two unequal fractions of X-ray doses in a direct or reverse sequence. When, under hypoxic conditions, exposure to the larger fraction was made first, the survival was consistently decreased in comparison to the survival when the smaller fraction was given initially. The difference was interpreted to indicate a gradual inhibition of the recovery capacity of cells with increasing first-exposure doses. The observations are compatible with the concept of a cellular pool of chemical compounds which are capable of repairing sublethal radiation damage. It is suggested that large doses of irradiation may damage the mechanism which refills the pool. (author)