However, two years later, Petry (1923)[5] first attributed oxygen tension as affecting ionizing radiation effects on vegetable seeds.
[12] Another hypothesis posits that ionizing radiation provokes mitochondria to produce reactive oxygen (and nitrogen species), which are leakage during oxidative phosphorylation that varies with a hyperbolic saturation relationship observed with both the oxygen and nitric oxide effects.
Radiobiologists identified additional characteristics of the oxygen effect that influence radiotherapy practices.
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.
[17] 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.
Relative sensitivity. This figure illustrates the typical change in the relative radiosensitivity for a biological effect such as cell death when exposed to radiations of low ionizing density (e.g. x-rays). The hyperbolic relationship shown has a maximum OER of 2.70 for 100% oxygen (at 760 mmHg), with a half-range OER value at 4.2 mmHg or 0.55% of oxygen.
Change with ionizing density. This figure illustrates the trend in the relative radiosensitivity or OER with oxygen tension for radiations of different ionizing density or linear energy transfer (LET, keV/μm). The inhibition of clone-formation by cultured human cells was measured after exposure to alpha-particles, deuterons and 250 kVp x-rays by Barendsen et al. (1966). The range of the maximum OER for 100% oxygen (at 760 mmHg) was 2.7 for 250 kVp x-rays dropping to 1.0 for 2.5 MeV alpha-particles. In each case the OER curves shown assume a half-range OER value of 4.2 mmHg or 0.55% oxygen.
Cell survival. This figure is illustrative of the reduction in the OER from aerobic to anoxic conditions for lower compared to higher doses, which has a bearing on the choice of dose fractionation exposures for radiotherapy of tumours.
