[Rational dose fractionation schedule 1. Each fractional dose].

The dose per fraction which will produce the maximum therapeutic ratio in clinical radiotherapy is discussed, using the data on the variation of the oxygen enhancement ratio with dose size and the difference in the dose response curves between tumor cell lines and late effects in normal tissues. Isoeffect doses for late injury increased more than that for tumor response with decreasing dose per fraction. When late effects in normal tissues are dose-limiting for radiotherapy, the highest therapeutic ratio (normal tissue tolerance dose/dose for a given probability of local tumor control) would be obtained when the dose/fraction is equal to the flexure dose. Lengthening the interfraction interval to allow complete repair of sublethal injury in the normal tissues responsible for late effects may allow some repopulation of tumor cells. In this case it may be necessary to increase the dose/fraction slightly above the flexure dose.