ABSTRACT
The response of cells to ionizing radiation is strongly dependent upon oxygen (Gray et al., 1953; Wright and Howard-Flanders, 1957). This is illustrated in Fig. 15.1 for mammalian cells irradiated in culture. Cell surviving fraction is shown as a function of radiation dose administered either under normal aerated conditions or under hypoxia, which can generally be achieved by flowing nitrogen gas over the surface of the cell suspensions for a period of 30 min or more. The enhancement of radiation damage by oxygen is dose-modifying (i.e. the radiation dose that gives a particular level of cell survival is reduced by approximately the same factor at all levels of survival). This allows us to calculate an oxygen enhancement ratio (OER):
for the same level of biological effect. For most cell types, the OER for X-rays is around 3.0. However, some studies suggest that at radiation doses of
Figure 15.1 Survival curves for cultured mammalian cells exposed to X-rays under oxic or hypoxic conditions, illustrating the radiation dose-modifying effect of oxygen. Note that the broken lines extrapolate back to the same point on the survival axis (n 6). OER, oxygen enhancement ratio.
