L.H. Gray Medal lecture: cell kinetics and radiation oncology.

Development of the study of human tumor cell kinetics during the past decades has deepened our understanding of the natural history of human cancers. Mathematical models based on the data which have been accumulated may help to evaluate, for the various types of human tumors, the time during their growth that the dissemination process occurred and to calculate the size distribution of the subclinical metastases at the time of treatment of the primary tumor. The perturbations caused by radiotherapy and chemotherapy are complex and include reassortment of surviving cells and repopulation. Currently it appears difficult in clinical practice to take advantage of cell reassortment while the differences in the rate and in the duration of repopulation between normal and neoplastic tissues are exploited in most therapeutic regimens. A better knowledge of cell and tissue kinetics following treatment may help to optimize the treatment scheduling in particular during combined administration of radiotherapy and chemotherapy. The study of the kinetics of proliferation in normal tissue has shown the existence of several types of inhibitory and stimulatory humoral factors. These, when purified, can be used to manipulate the proliferation of critical normal tissues in order to protect them during the administration of cell cycle specific drug or to accelerate their regeneration after treatment.