[Cell kinetics of human solid tumors and biological basis for fractionated radiotherapy]

Recent analysis on normal tissue responses to fractionated irradiation indicated that the effect of dose-fractionation is closely related to the cell kinetics of the target cells. Namely, the dose-fractionation spares more late-responding tissues than tissues in early response. The difference between early and late-responding tissues seems to depend not only on intrinsic survival curve of target cells, but also on the capacity of PLD recovery and the redistribution in the cell cycle. Although alpha/beta ratios in LQ model calculated from data of rapidly growing animal tumors have been reported similar to those of early responding tissues, there are few reliable data on slowly growing human tumors. Thus it seems difficult at present to select indicative cases for accelerated or hyperfractionation. Thymidine labeling index of human solid tumors is widely distributed ranged from 0.03% to 57% and the corresponding potentially doubling time can be calculated at p less than 0.001 with an empirical statistical equation. Since the potential doubling time is related to the rate of repopulation of tumor cells during radiotherapy, it may be an useful indicator for selecting individual patient for specific type of dose-fractionation schedule.