Normal tissue radioresistance in clinical radiotherapy

It is more logical to view the late structural and functional effects of radiation as the result of direct parenchymal depletion than as changes secondary to vascular damage. The dose-survival characteristics of the ''target'' cells in late-effects tissues are different from those for the ''target'' cells in acutely responding tissues. (The accumulation of sublethal injury with increasing dose assumes more importance in the killing of the target cells for late effects than it does in the stem cells of acutely responding tissues.) Thus, the ''sparing'' effect of dose fractionation is greater for late than for acute effects, suggesting that hyperfractionation may increase the therapeutic ratio by sparing late-effects tissues relative to acutely responding tumors