The radiation responses of EMT6 tumors and tumor cells were examined under a variety of experimental conditions. Survival curves for cultured EMT6 cells irradiated in vitro with 250 kV x-rays under conditions of hypoxia and normal aeration were characterized by D0's of 467 and 155 rad and extrapolation numbers of 3.0 and 2.8 (oxygen enhancement ratio = 3.0). Survival curves of EMT6 cells suspended from solid tumors after irradiation in situ showed the biphasic shape predicted for a mixed population of oxygenated and severely hypoxic cells. Comparison of the survival of cells suspended from tumors irradiated in situ in air-breathing and nitrogen-asphyxiated mice showed the hypoxic fraction of the tumor to be 0.33. The radiation sensitivity of tumors remaining in situ after irradiation, assayed by tumor growth perturbation or by tumor cure, was greater than expected on the basis of the cell survival curves. This was ascribed to tumor-host interactions including immune recognition of the irradiated tumor.

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