Reduced Oxygen Enhancement Ratio at Low Doses of Ionizing Radiation

Abstract

A decreased oxygen enhancement ratio (OER) at lower radiation doses has been previously reported (B. Palcic. et al. 1984). The question remained whether or not this effect is due to a possible O2 contamination at low doses, which was not the case at high doses. To ensure a sufficient degree of hypoxia prior to the start of irradiation. Chinese hamster cells (CHO) were made hypoxic by gas exchange combined with metabolic consumption of O2 at 37.degree. C. At the same time O2 levels in cell suspension were measured using a Clark electrode. Under experimental conditions used in this laboratory for hypoxic irradiations, the O2 levels before the start of irradiation are always below the levels which could give any significant enhancement to radiation inactivation by X-rays. Full survival curves were determined in the dose range 0-30 Gy [gray] using the conventional survival assay and in the dose range 0-3 Gy using the low dose survival assay. The OER decreases at low doses. Apparently, the dose-dependent OER is a true radiobiological phenomenon and not an artifact of the experiment method used in the low dose survival assay.