ABSTRACT
The response of cells to ionizing radiation is strongly dependent upon oxygen. Cell surviving fraction is shown as a function of radiation dose administered either under normal aerated conditions or under hypoxia. The enhancement of radiation damage by oxygen is approximately dose modifying (i.e. the radiation dose that gives a particular level of survival is reduced by 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 cells the maximum OER in culture (comparing doses under 21% air and under anoxia) for X-rays is around 2.5–3. However, some studies suggest that at radiation doses of 3 Gy or less the OER is somewhat reduced. This is an important finding because this is the dose range for clinical fractionation treatments.
