A radiobiologist looks at radiotherapy

The aim of radiotherapy is to destroy all tumor cells without causing lasting damage to the normal tissue. These seemingly contradictory tasks can now be fulfilled on only about 30% of all cases treated. The main problem of tumordestruction are the anoxic tumorcells. The reoxygenation process during treatment and also the efficiency of sensitizers for the anoxic cells therefore determines the best treatment scheme to be used. General guidelines are described. The avoidance of late effects to normal tissue is a complex problem depending largely on the site of the tumor and the sensitivity of the adjacent normal tissue. L. Cohen has shown that we need at least 7 parameters for an analytical investigation. Here only clinical experience can clarify the situation but often the solution is indistinct and the parameters found are ambiguous. For single doses below 600 rad the influence of interphase cell killing (α-effect) is prevailing and treatment with low LET radiation having low α values will therefore give higher tolerance doses than treatment with conventional X-rays and also high LET radiations. This has been proved by extensive clinical trials. To improve radiotherapy close cooperation between clinicans and radiobiologists is needed. Further investigations on cell survivals in vivo are badly needed for tumor cells and also for normal tissue. The prosesses of reoxygenation might be quite different in small animals from man and this might also be so for the cell kinetics of recovery and repopulation after irradiation. (orig./MG)