 It has been well established that malignant tumors tend to have low levels of oxygen and that tumors with very low levels of oxygen are more resistant to radiotherapy and other treatments, such as chemotherapy and immunotherapy. Previous attempts to improve response to therapy by increasing the oxygen level of tissues have had disappointing results and collectively have not led to changing clinical practice. Without a method to measure oxygen levels in tumors or the ability to monitor over time whether tumors are responding to methods to increase oxygen during therapy, clinicians reluctance to use oxygen therapy in usual practice is not surprising. 