An inadequate supply of oxygen, hypoxia, is an important factor contributing to resistance to treatment in a number of tumor types, including head and neck cancer. Novel imaging methods have been applied to studies of this important prognostic factor. Mammalian cells need oxygen to live but O2 also participates in the cytotoxic effects of ionizing radiation. Hypoxia is often the result of abnormal blood vessels supplying the tumor, increased diffusion distances to tumor cells, and reduced O2 transport capacity of the blood. Its consequences are mediated by a series of hypoxia-initiated genomic changes activating angiogenesis, glycolysis, and other processes that enable tumor cells to survive or escape the O2-deficient environment. Hypoxia has been shown to be important in overall diminished therapeutic response, malignant progression, increased probability of recurrence, locoregional spread, and distant metastases. Strategies are being developed to surmount the cure-limiting consequences of hypoxia, but methods are needed to select patients most likely to benefit from these new treatments. Even though hypoxia is a common tumor phenotype, it is by no means universal and is often heterogeneous within an individual patient. This review considers the biology of hypoxia, its consequences with respect to treatment, methods for measuring oxygenation in tissues, modern techniques for imaging of regional hypoxia, and how information about the oxygenation status of tumors might impact treatment.
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