For some radiation types, there is a great difference in the biological effect seen in oxic versus anoxia conditions. This is known as the oxygen effect. It is particularly important for radiation types that cause damage through indirect action (eg. photons, electrons). This is because oxygen is required to 'fix', or make permanent, the damage caused by free radicals. Without oxygen, the DNA radical that is created after reacting with the hydroxyl radical is simply repaired.
The Oxygen Enhancement Ratio (OER)
The OER is the ratio of effect caused by oxic versus anoxic conditions, given by the formula:(1)
For photons and electrons, the OER approaches 3 for most cell types.
Cell killing is three times more effective in oxic conditions versus anoxic conditions for indirectly acting radiation
Radiation types that rely more on direct action (eg. protons, alpha particles, heavy ions) show minimal changes in the presence or abscence of oxygen. This is because oxygen is not required to 'fix' the damage caused by these radiations, as they interact directly with the DNA molecule to cause damage.
Importance of the Oxygen Effect
Many tumours are hypoxic due to:
- Chronic hypoxia from poor vascular supply
- Acute hypoxia due to temporary occlusion of tumour vessels
Hypoxic tumour cells are significantly more resistant to the effects of radiation. There are numerous ways to overcome hypoxia, of which fractionation is the most commonly used in the modern setting. Fractionation of dose kills oxygenated cells first, allowing the hypoxic population to migrate closer to the vascular network and become oxygenated. The remaining fractionated dose then kills these previously hypoxic cells.