Four-day-old artificial pulmonary micrometastases of two murine fibrosarcomas, designated FSA and NFSA, showed increased sensitivity to ionizing radiation by a factor of 1.13 when animals were exposed to hyperbaric oxygen breathing before and during irradiation, implying the presence of hypoxia in the micrometastases. At the time of irradiation the diameter of FSA and NFSA metastases was smaller than 200 and 100 microns, respectively, which, on the basis of oxygen diffusion, could not be responsible for hypoxia. It is assumed that hypoxia of micrometastases is passive, reflecting the radiobiological hypoxia of lung tissue that could exist under normal breathing conditions.

Milas, Hunter, Ito, Brock, Peters, , , , (). Increase in radiosensitivity of lung micrometastases by hyperbaric oxygen. Clinical & experimental metastasis, ;3(1):21-7. https://www.ncbi.nlm.nih.gov/pubmed/4042454