Many cancer patients experience lower survival rates if they have less well oxygenated tumors. Lower tumor oxygen levels can lead to a reduced effectiveness of radiation therapy. The ability to overcome this radiotherapy resistance has been severely limited by the lack of a clinically compatible quantitative oxygen sensing technology. We report the design and validation of a silicone-based oxygen sensor measured with MRI for an approved human clinical trial in patients with cervical cancer. The sensor has been validated for compatibility with the clinical workflow and is specifically designed to achieve the endpoints of the trial.