[Repopulation in normal tissues].

Apart from dose fractionation, treatment time is a factor that determines the tolerable radiation dose in many normal tissues. Empirical isoeffect formulae have failed to account for this dependency because they are based on oversimplified and non-biological premises. Compensatory repopulation is initiated by tissue damage, presumably cell depletion, and therefore is related to tissue turnover time. Tissues with slow cell renewal thus show little or no benefit from treatment prolongation. In acutely reacting tissues enhanced repopulation is mounted within days or weeks, also depending on the aggressiveness of treatment. Oral mucositis is taken as an example to discuss factors that modify the regenerative response and some aspects how it can be exploited in clinical radiotherapy.