A comparison of the effects of radiation on tumour growth delay and cell survival (The effect of oxygen)

A tumour is described for which it is possible to measure the survival of the tumour cells in vitro following irradiation in vivo. Using this tumour, the relationship between cell survival and alterations in tumour growth after irradiation, and the effect of oxygen on this relationship have been studied. The oxygen enhancement ratio for cell killing was 2·5, and for tumour growth delay was 3·0. This difference in the OER's could not be accounted for in terms of differences in post-irradiation proliferation by cells that were either oxic or hypoxic at the time of irradiation. Evidence is presented which indicates that measurements of cell survival in vitro following irradiations in vivo may lead to inaccurate estimates of the “effective” hypoxic fraction in a tumour. The results suggest that there may be qualitative differences between the types of damage expressed in situ and in vitro, and that techniques which involve the removal of cells from their normal environment may lead to incorrect estimates of cellular radiosensitivity.