Background and purpose: Tumor oxygenation is recognized as an important determinant of the outcome of radiotherapy and possibly also of other treatment modalities in a number of tumor types and in particular in squamous cell carcinomas. The hypoxic status of various solid tumors has been related to a poor prognosis due to tumor progression towards a more malignant phenotype, with increased metastatic potential, and an increased resistance to treatment. It has been demonstrated in head and neck cancer that hypoxic radioresistance can be successfully counteracted by hypoxia modifying approaches. The microregional distribution and the level of tumor hypoxia depend on oxygen consumption and temporal and spatial variations in blood supply. It is unclear if severely hypoxic cells can resume clonogenicity when O(2) and nutrients become available again as a result of (treatment related) changes in the tumor microenvironment. Non-terminally differentiated hypoxic cells that are capable of proliferation are important for outcome because of their resistance to radiotherapy and possibly other cytotoxic treatments. Various exogenous and endogenous markers for hypoxia are currently available and can be studied in relation to each other, the tumor architecture and the tumor microenvironment. Use of nitroimidazole markers with immunohistochemical detection allows studying tumor cell hypoxia at the microscopic level. Co-registration with other microenvironmental parameters, such as vascular architecture (vascular density), blood perfusion, tumor cell proliferation and apoptosis, offers the possibility to obtain a comprehensive functional image of tumor patho-physiology and to study the effects of different modalities of cancer treatment.
Conclusion: A number of functional microregional parameters have emerged that are good candidates for future use as indicators of tumor aggressiveness and treatment response. The key question is whether these parameters can be used as tools for selection of treatment strategies for individual patients. This requires testing of these markers in prospective randomized clinical trials comparing standard treatment against experimental treatments targeting the relevant microregional constituent.