Response of mouse jejunum to multifraction radiation

The dose survival curve for clonogenic jejunal crypt cells appears to bend continuously until doses of at least 1100 rad. This suggests that a proportion of cells has a large capacity for repair of sublethal injury and that the survival curve is better described by a quadratic than a multitarget function. Repair of sublethal injury is dose-dependent. Quantitative estimates of repair per fractionation interval, expressed as (D/sub N/-D1)/(N-1), when related to size of the dose/fraction, suggest that the repair capacity of most jejunal crypt cells may not be appreciably different from that of the cells of many other tissues. After low doses (less than 300 rad) surviving crypt cells, through repair of sublethal injury, shed damage equivalent to about 60 to 65 percent of the preceding dose. There appears to be some extra sparing of the crypt cells due to repair of sublethal damage even when doses/fraction of 200 rad or less are further fractionated. Determining accurately the dose below which repair is maximal, and all cell killing results from ''single-event'' nonrepairable injury, is difficult using present assays. Regeneration of surviving jejunal crypt cells is rapid after radiation injury. Its onset and rate may depend upon degree of injury (size of dose), and is probably facilitated by lengthening the fractionation interval, even if the overall treatment time is unchanged. No general formula can be applied to relate number of dose fractions and treatment duration to the total dose needed for an iso-effect in jejunal mucosa. Thus, the NSD formula, for example, should not be considered relevant to the response of intestinal mucosa to multifraction radiation