Oxygen Sensitization of CHO Cells at Ultrahigh Dose Rates: Prelude to Oxygen Diffusion Studies

Abstract

The response of cultured [Chinese hamster Ovary] CHO cells to ultrahigh dose rate (.apprx. 1011 rad/s) radiation, as measured by colony-forming ability, has been studied under various O2 concentrations with single electron pulses of 3 ns duration. A thin-layer technique was designed for these experiments to permit rapid equilibration of the cells with the ambient O2 containing gas and to ensure that no segments of the cell population were shielded from the relatively low energy electrons (average energy, .apprx. 450 keV), thus eliminating the possibility of artifacts influencing surviving fraction. Dosimetry was performed using a combination of calorimetric, charge-measuring and thermoluminescent techniques. At a concentration of 0.44% O2 breaking behavior of the survival curve was observed in accord with the radiolytic depletion of intracellular O2 by the radiation pulse. The sensitivity for doses greater than the breakpoint dose followed an anoxic response, clearly different from that observed when the cells were irradiated at conventional dose rates in the same gas under the identical geometry. The breakpoint dose was approximately linear over the O2 concentration range 0-0.7%. These results are consistent with recent observations of mammalian cell response under high-intensity irradiation conditions and provide a basis for the design of a double-pulse experiment.