Abnormal gadolinium enhancement has been the standard characteristic for the measurements of brain tumors in clinical trials, but new therapies that target the vascular components of tumors have highlighted the shortcomings of enhancement as a measurement tool. Inhibitors of vascular endothelial growth factor (VEGF), such as bevacizumab, can produce rapid changes in the degree of contrast enhancement within malignant gliomas and in the extent of surrounding hyperintense T2-weighted signal. These effects, which are consistent with decreasing permeability of tumor capillaries, make it exceedingly difficult to employ conventional measures of tumor size in the setting of clinical trials. This chapter will review these issues and describe approaches to tumor measurement in the era of VEGF inhibitors.
