Influence of tumour size on radiosensitization by misonidazole.

The administration of misonidazole shortly before irradiation increased the radiation response of the Lewis lung tumour growing either as approximately 500mm3 subcutaneous implants or as 15mm3 lung colonies. In the subcutaneous tumours, the dose enhancement ratio was approximately half the oxygen enhancement ratio. In the lung colonies the sensitization of hypoxic cells appeared to be complete. Similar studies on subcutaneous B16 melanoma showed that the drug increased radiosensitivity to a level comparable with that of cells in small pulmonary nodules, whose hypoxic fraction appears to be below 10(-3). The results indicate complete dose modification of hypoxic cells where their natural proportion is small with incomplete enhancement of radiosensitivity where the natural hypoxic fraction is larger.