text: The oxygen effect is quantified by measuring the radiation sensitivity or Oxygen Enhancement Ratio (OER) of a particular biological effect (e.g., cell death or DNA damage), which is the ratio of doses under pure oxygen and anoxic conditions. Consequently, OER varies from unity in anoxia to a maximum value for 100% oxygen of typically up to three for low ionizing-density-radiation (beta-, gamma-, or x-rays), or so-called low linear energy transfer (LET) radiations.
  Radiosensitivity varies most rapidly for oxygen partial pressures below ~1% atmospheric (Fig. 1). Howard-Flanders and Alper (1957) developed a formula for the hyperbolic function of OER and its variation with oxygen concentration, or oxygen pressure in air.
  Radiobiologists identified additional characteristics of the oxygen effect that influence radiotherapy practices. They found that the maximum OER value diminishes as the ionizing-density of the radiation increases (Fig. 2), from low-LET to high-LET radiations. The OER is unity irrespective of the oxygen tension for alpha-particles of high-LET around 200 keV/μm. The OER is reduced for low doses as evaluated for cultured mammalian cells exposed to x-rays under aerobic (21% O2, 159 mmHg) and anoxic (nitrogen) conditions. Typical fractionation treatments are daily 2 Gy exposures, as below this dose the so-called shoulder or repair region of the cell survival curve is encroached upon reducing the OER (Fig. 3).