Oxygenation and Cell Kinetics after Irradiation in a Transplantable Osteosarcoma

In a transplantable osteosarcoma studies were made of the surviving cells after an X-ray dose of 1000 rad. By means of cell labelling in vivo with 125IUDR it was possible to estimate the total cell number per tumour at different intervals after irradiation, and by inoculation of the suspended tumour cells into isogeneic recipients a quantitative assay of cell survival could be performed. Similarly, the oxygenation of the survivors could be assayed by studying the radiosensitivity of the surviving cells in vivo. Before irradiation 14% of the cells of the tumour were found to be anoxic. After irradiation more than 90% of the cells were anoxic for two days, but then the percentage of anoxic cells decreased rapidly to the preirradiation value or below. Over a period of five days after irradiation there was an apparent progressive loss from the tumour of competent surviving cells as estimated by the transplantation assay. Since a similar loss could not be observed after chemotherapy in vivo with cyclophosphamide to a similar cell survival level, it seems extremely unlikely that this cell loss is an artifact induced by trypsinization of damaged cells. It is also not a phenomenon typical of radiation, since it has not been observed in irradiated bone marrow assayed for surviving colony-forming units by the spleen colony technique. To explain the phenomenon a tumour model is proposed in which there is a continuous loss of cells caused by vascular insufficiency. As the tumour cells proliferate in the well-vascularized areas, other cells are pushed to the periphery where they become anoxic and subsequently die. If the cells are brought into suspension immediately after irradiation, most of the survivors are among these anoxic cells. A delay of a few days permits further growth of the tumour; its volume and total cell number increase for 2 to 4 days. This is probably due to the last cell divisions of doomed well-oxygenated cells, and it seems reasonable to assume that this causes the death of the peripheral, previously anoxic cells, since they form the cell compartment which a t the time of irradiation already had a somewhat impaired vascular supply. Simultaneously the few perivascularly located surviving cells proliferate, and thus the well-oxygenated cells increase in number while the initially more numerous anoxic survivors die. The implications of this model for radiotherapy are discussed. (author)