Radiobiological studies of tumours in situ compared with cell survival.

The dose of radiation needed to eradicate a tumor depends on the radiosensitivity of the individual cells. Direct measurements of tumour cell survival require removal of the cells from their normal environment and may not reflect the course of events in the undisturbed tumour. In situ assays preserve this environment, but only give indirect estimates of tumour cell survival. A major cause of discrepancies between different methods of assaying tumour response is recovery from potentially lethal damage (PLD). For instance, it can affect estimates of the hypoxic fraction and hence of the pattern of reoxygenation. Not all tumours show recovery from PLD. In such tumours, where comparisons have been made between different assays, they have shown differences in the effects of radiation quality and modifying agents and in the capacity for recovery from sublethal damage between tumour cell survival in vivo and in vitro after treatment in vivo. No single assay method can give an adequate description of the response of a tumour to radiation. It is only by comparing different methods of assay that the basic biological properties of tumors that govern their response to therapy will be understood.