Repair and proliferation: major determinants of the multifraction radiation response.

An analysis of multifraction radiation survival data for V79 monolayers and spheroids, in which measured responses to each treatment were intercompared with the alpha/beta ratio derived using the common "reciprocal-dose" methodology at selected survival levels, resulted in quantitatively and qualitatively different conclusions. These discrepancies were the result of non-constant toxicity of each radiation dose in the multifraction scheme, which was obvious in the multifraction dose response curves, but not in the reciprocal dose plots. These data appear to indicate incomplete recovery between radiation exposures. However, under some circumstances, lack of repair was offset by cell proliferation, thus emphasizing the roles of repair, repopulation, and redistribution in the multifraction radiation response.