# Oxygen enhancement ratio

The Oxygen Enhancement Ratio (OER) or oxygen enhancement effect in radiobiology refers to the enhancement of therapeutic or detrimental effect of ionizing radiation due to the presence of oxygen. The effect is most notable in ionizing radiation dose.

The OER is defined as the ratio of radiation doses during lack of oxygen compared to no lack of oxygen for the same biological effect. This may give varying numerical values depending on the chosen biological effect.

$OER = \frac{Radiation\, dose\, in\, hypoxia} {Radiation\, dose\, in\, air}$

Radiation with higher linear energy transfer tends to have a lower OER.

## Uses in medicine

The effect is used in medical physics to increase the effect of radiation therapy in oncology treatments. Additional oxygen abundance creates additional free radicals and increases the damage to the target tissue.

In solid tumors the inner parts become less oxygenated than normal tissue and up to three times higher dose is needed to achieve the same tumor control probability as in tissue with normal oxygenation.