[Introduction in the problem of the time factor and its therapeutical significance (author's transl)]

Dependencies of biological radiation effects on time distribution of dose may result from a) restitution processes on all levels of effect and b) changes of radiation sensitivity by irradiation itself. In case of fractionated radiation therapy the following mechanisms are important: intercellular recovery, cell renewal, synchronization, reoxygenation and redistribution of cells within the cycle. It has to be proven whether and to what extent the particular processes differ in the radiation reaction of tumours on the side and of normal tissues on the other--so that these differences may be used for the development of optimal fractionation schemes. The Strandqvist concept of dose-time-effect relationship is to be considered as an approximation to complex biological conditions. From recent results of cell kinetic studies and of experimental tumour therapy one may expect that for various tumour types according to their specific growth-characteristics and radiation reactions individual modes of dosage may be developed in future in which the curative dose is lower than expected.