The Phenomenon of Tumor Hypoxia Many solid tumors are relatively resistant to treatment with ionizing radiation and certain chemotherapeutic agents such as anthracyclines that are affected adversely by acidic pH. These effects have primarily been attributed to the presence of hypoxic cells within the tumor. The relevance of hypoxia with respect to failure of radiotherapy to cure certain malignancies has had a chequered history. However, in recent years the evidence that hypoxia plays a central role in relative radioresistance has become more compelling.