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Hypoxia, a partial pressure of oxygen (pO2) below physiological needs, is a limiting factor affecting the efficiency of radiotherapy. Indeed, the reaction of reactive oxygen species ~ ~(ROS, produced by water radiolysis) with DNA is readily reversible unless oxygen stabilizes ~ ~the DNA lesion. While normal tissue oxygenation is around 40 mm Hg, both rodent and ~ ~human tumors possess regions of tissue oxygenation below 10 mm Hg, at which tumor cells ~ ~become increasingly resistant to radiation damage (radiobiological hypoxia) (Gray, 1953). ~ ~Because of this so-called “oxygen enhancement effect”, the radiation dose required to ~ ~achieve the same biologic effect is about three times higher in the absence of oxygen than in ~ ~the presence of normal levels of oxygen (Gray et al., 1953; Horsman & van der Kogel, 2009). ~ ~Hypoxic tumor cells, which are therefore more resistant to radiotherapy than well ~ ~oxygenated ones, remain clonogenic and contribute to the therapeutic outcome of ~ ~fractionated radiotherapy (Rojas et al., 1992).