Specific Imaging Findings

Oligodendrogliomas are usually well-demarcated supratentorial lesions that arise from the gray matter, most commonly in the frontal lobes. The typical CT findings include the presence of clumped and nodular calcification within a hypodense to isodense mass involving the cortex. Cystic changes commonly occur, while intratumoral hemorrhages are rare. Oligodendrogliomas are usually T1 hypointense and T2 hyperintense cortical-based lesions with mild mass effect, and areas of signal loss/susceptibility artifact on T2* sequences, caused by calcifications, may be found. Development of new areas of contrast enhancement and edema have been traditionally considered signs of anaplastic transformation in gliomas; however, contrast enhancement has shown very poor accuracy in differentiating anaplastic from lowgrade oligodendroglial tumors. Also, increased relative cerebral blood volume (rCBV) on contrast-enhanced T2* MR perfusion studies, a marker for rapid progression and poor outcome in patients with gliomas, may be found in some low-grade oligodendrogliomas. Increase in tumor size within 6 months appears to be the best prognosticator of rapid tumor progression and poor outcome, better than the high initial tumor volume or high rCBV. Signal heterogeneity, irregular borders, and high rCBV of the tumors are suggestive of the 1p/19q deletion, which is associated with a better treatment response.