Tumor oxygenation during respiratory hyperoxia is dependent on the tumor growth site, on the growth stage, and hence on the vascular pattern. Diffusion of O2 from the surrounding atmosphere contributes considerably to the oxygenation of subcutaneous tumors during normobaric exposure to a pure O2 atmosphere. However, hypoxia is still present in the tumor core under these conditions, and pressurization up to 4 bar is required to completely eradicate these hypoxic areas, thus enhancing the radiosensitivity of the tumors.

Mueller-Klieser, Vaupel, Manz, , , , , , (1984). Effectiveness of respiratory hyperoxia, of normobaric and of hyperbaric oxygen atmospheres in improving tumor oxygenation. Advances in experimental medicine and biology, 1984 ;169():613-9. https://www.ncbi.nlm.nih.gov/pubmed/6731116