We report on the first-in-human evaluation of a class of silicone oxygen sensors capable of both high sensitivity and repeated and long-term monitoring of tumor oxygen levels. We are evaluating the use of this sensor in patients receiving high dose rate brachytherapy for cervical cancer. This sensor is a direct and quantitative measurement of tumor oxygen. Low oxygen regions of tumors are more resistant to many common forms of treatment. Understanding tumor oxygen levels can enable personalized radiation and chemotherapy treatments to overcome resistance and improve outcomes for patients.