Tumor hypoxia originates from the inability of neovasculature to provide an adequate blood supply to accommodate the metabolic demands of the tissue. Hypoxia is operationally defined as a spectrum of reduced levels of oxygen that result in the impairment of tissue function. The upper end of this reduced oxygen spectrum plays important physiologic functions in maintaining blood pressure and stimulating erythropoiesis. The lower end of this spectrum approaches a complete lack of oxygen (defined as anoxia) and is uniquely found in solid tumors. In between the two ends of the spectrum in solid tumors lie a large number of cells at intermediate oxygen tension that have been hypothesized to present the greatest therapeutic problem for radiotherapy and chemotherapy (1 ).
