Tumors with low oxygen levels are associated with poor prognosis and resistance to standard radiotherapy or systemic therapies. The ability to make repeated oxygen measurements in tumors could be used to help select the most effective treatment or the best timing to start therapies. The purpose of this study is to ascertain the safety and feasibility of using an implantable oxygen sensor, known as the OxyChip, to make oxygen measurements in tumors using EPR oximetry, a technique related to magnetic resonance imaging (MRI).