Nuclear imaging with hypoxiaspecific tracers should be an important tool for selecting patients who might benefit from this treatment. Stugeron Contrary to expectations, there is now abundant evidence that tumor hypoxia does not correlate with tumor size, grade, and extent of necrosis or blood hemoglobin status. Moreover, most of the commonly used clinicopathologic parameters for evaluation of tumor hypoxia are not strong indicators of prognosis.Most of these studies, however, have shown widespread heterogeneity in tumor hypoxia within a tumor, between tumors, and between patients with the same tumor type. Although hypoxia generally resolves when a tumor shrinks after treatment with either radiotherapy or chemotherapy, it may show paradoxical results in some tumors, perhaps because of hypoxic cell sparing by the treatment. Currently available assays for tumor hypoxia can be categorized as in vivo biopsy. Experience has shown that regional levels of hypoxia should be measured for individual patients and tumor sites.To be maximally successful, hypoxiadirected imaging and treatment should target both chronic hypoxia and hypoxia resulting from transient interruption of blood flow. Other desirable characteristics for an ideal clinical hypoxia assay include rapid and easy to perform with consistency between laboratories, and the ability to quantify without the need for substantial calibration of the detection instrumentation.Location of the tumor in a patient should not be a limiting factor for the assay.Lastly, spatial heterogeneity in the distribution of hypoxia dictates that the ideal assay must provide a complete locoregional evaluation of the tumor.All of these requirements suggest an important role for imaging in evaluating hypoxia.This assay can be calibrated in units of millimeters of Piperonyl butoxide mercury and has been referred to as a gold standard.The electrodes do not provide full maps of a tumor area; they only provide a histogram of the distribution of cell regions as a function of the electrodes reading.There may also be interlaboratory variations in calibration of the electrodes. Polarographic electrodes measure oxygen tension in a group of cells and the readings can be influenced by the presence of blood. Although imageguided sampling can be used to select the path and depth of electrode deployment to avoid blood vessels, anatomic imaging methods are notoriously limited in identifying areas of viable tissue within a tumor.Hypodense areas visualized within a tumor on CT can indeed have measurable levels of oxygen, but it also might compromise patient compliance.In addition, electrode measurements are limited by the need for accessible tumor location and difficulties with serial measurements.These limitations prompted the search for a noninvasive method that could be done serially to characterize and quantify hypoxia in cancer patients.Imaging methods for hypoxia provide a complete anatomic map of relative oxygenation level in tumor regions with good spatial resolution and in an environment that tends to be highly heterogeneous.Direct methods were started with largely fluorescent techniques, such as intravital fluorescent video microscopy, fluorophore coupling of fibronectin, quenched nearinfrared fluorochromes to matrix metalloproteinase substrates, MR imaging. The simplicity of dynamic contrastenhanced MR imaging has led to fairly widespread use of this technique. It provides a signal that effectively integrates vascular blood flow, blood volume, and vascular permeability.