Tissue perfusion and cerebral energy metabolism in brain tumor patients are regionally abnormal, and certain patterns of pathological changes may be demonstrated in and around tumor tissue. Tumor CBF may vary widely between subjects, whereas oxygen extraction is almost always markedly reduced. This is in apparent contradiction with the tumor tissue oxygen tension data (see previous chapter): since these are low, an increase of OER would be expected. Still, as discussed above, oxygen consumption calculated from oxygen tension or PET data yield the same relative decreases (0.3) of tumor oxygen utilization compared to normal brain. This suggests that the characteristic pathological changes of tumor cell energy metabolism have diminished oxygen demand and changed the level of coupling between CBF and OER. Since all published data on OER in tumor patients have been obtained by the steady state method, possibly resulting in an underestimation of OER, it will be mandatory to apply dynamic oxygen uptake measurements to circumvent this methodological problem and e.g. to make use of distribution volume of water to correct for tissue composition as has been reported in one paper studying patients with breast tumors. Because of the conspicuous regional changes in tumor oxygen metabolism it is expected that effectivity of chemotherapeutic measures is directly reflected by characteristical adaptations of metabolism. Systematic studies in this context have not been reported to date. Whether oxygen metabolism changes would be more sensitive than just CBF measurements as indicator of treatment effectivity remains to be seen.