Dose-fractionation sensitivity of mouse kidney clonogens measured using different interfraction intervals and...

The fractionation sensitivity of kidney clonogenic epithelial cells (X-irradiation in vivo, assay in vitro) was quantified using the linear-quadratic model. The cells were assayed either immediately after the fractionation schedule or after a time delay in order to compare the relative amounts of dose sparing due to fractionation (conventionally sublethal damage repair) and to post-irradiation delay intervals before assay (conventionally potentially-lethal damage repair). As the delay before assay was increased, there was a tendency for both alpha and beta to decrease, and as a consequence the alpha/beta ratio stayed virtually unchanged (3.3-4.4 Gy) regardless of delay time before assay. No change in survival was observed from 12 h to 6 weeks after neutron irradiation (62 MeVp-->Be), suggesting that the change observed after X-rays was due to repair rather than repopulation. As the interfraction intervals in an 8-fraction X-irradiation schedule were increased in steps from 6 h to 5 days, there was improved survival, consistent with the presence of long-term repair. These studies provide further evidence for the potential importance of long-term repair in late reacting tissues not only during but also after multifraction irradiation schedules.