[Optimal fractionation from the radiobiological viewpoint]

Not only in experiment on animal but also from analysis of clinical data important knowledges could be obtained in the last years, that deal with the course of biological processes in tumor and normal tissue during fractionated irradiation. In centre there are differences in the capacity for recovery from sublethal radiation injury and in repopulation. Chronically reacting normal tissues show a clearly higher repair capacity than tumors, that can be used for hyperfractionation with reduced single doses. With it higher attention must be given to repairing time yet, that the selective benefit is not endangered by incomplete recovery. On the other side clinical analyses have confirmed that the stem cell repopulation--going on in several tumor types, so e.g. in squamous cell carcinomas, during the time of conventional treatment--can make a considerable contribution to radioresistance. The actual level of knowledge justifies further clinical experiments with unconventional fractionation, especially with accelerated hyperfractionation.