The radiation inactivation of the proliferative capacity of Chinese hamster fibroblasts is described by the linear-quadratic relationship,$S/S_{0}={\rm exp}(-\alpha D-\beta D^{2})$. Values of α and β have been determined for asynchronous cells irradiated under various conditions of chemical radioprotection and radiosensitization. OH has been found to contribute to cell inactivation by both α (single-events) and β (double-events). Sulfhydryl repair was found to provide radioprotection against single- and double-events while molecular oxygen was found to radiosensitize predominantly by enhancing double-events. Electron-affinic radiosensitizers enhanced the double-event component. These radiation chemical effects on cell inactivation are discussed with reference to potential molecular targets and lesions and the microdosimetry of 250 kV X rays in water.

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