Changes in early and late effects with dose-per-fraction: alpha, beta, redistribution and repair

Both tumor response and the manifestation of "early" and "late" effects in normal tissues as a function of overall treatment time and dose per fraction are thought to reflect an interplay between the inherent sensitivity of the target cells at risk, and their proliferation kinetics. In light of clinical and experimental studies which indicate that differences exist between early and late radiation responses in normal tissues after changes in fractionation pattern, several investigators have postulated that the intrinsic sensitivities of the respective target cells, in particular the alpha/beta ratios, are different. We suggest that the clinical and experimental findings may be explained, at least partly, in terms of cell cycle redistribution, and may also involve changes in the ability of early effects target cells to repair potentially lethal damage.