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Clinical trials to ameliorate hypoxia as a strategy to relieve the radiation resistance it causes have prompted a need to assay the precise extent and location of hypoxia in tumors. Electron paramagnetic resonance oxygen imaging (EPR O₂ imaging) provides a noninvasive means to address this need. To obtain a preclinical proof-of-principle that EPR O₂ images could predict radiation control, we treated mouse tumors at or near doses required to achieve 50% control (TCD₅₀). Mice with FSa fibrosarcoma or MCa4 carcinoma were subjected to EPR O₂ imaging and immediately radiated to a TCD₅₀ or TCD₅₀ ± 10 Gy. Statistical analysis was permitted by collection of approximately 1,300 tumor pO₂ image voxels, including the fraction of tumor voxels with pO₂ less than 10 mm Hg (HF10). Tumors were followed for 90 days (FSa) or 120 days (MCa4) to determine local control or failure. HF10 obtained from EPR …