Oxygen Sensitization of Mammalian Cells under Different Irradiation Conditions

The oxygen dependence of the radiosensitivity of cultured CHO cells was examined in detail with particular attention paid to avoiding possible artifacts due to radiolytic oxygen depletion. Two methods of gas equilibration and irradiation were used. In the first approach, cells were irradiated with 50-kVp X rays in a thin-layer geometry which offered maximum interchange between the cells and the surrounding gas. The second technique employed 280-kVp X irradiation of cells under "full-medium" conditions with mechanical agitation to minimize the effect of radiochemical oxygen consumption by promoting rapid oxygen replenishment. With these techniques oxygen radiosensitization was clearly resolved at an oxygen concentration of 0.03% in the gas phase. The oxygen K curves measured by these two methods were similar in shape over a wide range of oxygen concentration and were in reasonable agreement with calculated values based on the formulas of Alper and Howard-Flanders. No evidence of a plateau in the K curve recently reported for V79-753B cells was observed for the CHO cell line used.