The neoplastic oligodendrocyte has a small round nucleus and a halo of unstained cytoplasm (“fried egg” appearance). The cell processes are few and stubby, visualized only with silver carbonate stains. Some of the oligodendrocytes have intense immunoreactivity to GFAP, similar to normal myelin-forming oligodendrocytes. Microscopic calcifications are observed frequently, both within the tumor and in immediately adjacent brain tissue.
Genetics A remarkable degree of progress has been made in understanding the genetic aberrations that occur as acquired somatic mutation within these tumors and the relationship of these changes to the prognosis and response to therapy. Specifically, loss of certain alleles on chromosome 1p has been predictive of a high degree of responsiveness to the below-described PCV chemotherapy regimen, and a similar loss on chromosome 19q has been associated with longer survival. The current schemes for classification require that there be a mutation in the IDH gene family (see Yan et al) and a codeletion of 1p and 19q (1p/19q) but some tumors in childhood do not have these changes. What is more important as mentioned, these genetic alterations predict responsiveness to treatment and longer survival (see in the following text).
The most common sites of this tumor are the frontal and temporal lobes (40 to 70 percent), often deep in the white matter, with one or more streaks of calcium but little or no surrounding edema. It is rarely found in other parts of the nervous system. By extending to the pial surface or ependymal wall, the tumor may metastasize distantly in ventricular and subarachnoid spaces, accounting for 11 percent of the series of gliomas with meningeal dissemination reported by Polmeteer and Kernohan (less frequent than medulloblastoma and glioblastoma; see also Yung et al). The tumor does not lend itself easily to any of the grading scales, but a distinction is made between low grade (grade II) and a anaplastic type with degeneration, evidenced by greater cellularity and by numerous and abnormal mitoses (grade III); necrosis may occur in small regions of the tumor in about one-third of cases. In the oligoastrocytomas, either cell type may be anaplastic.
The typical oligodendroglioma grows slowly. As with astrocytomas, the first symptom in more than half of patients is a focal or generalized seizure; seizures often persist for many years before other symptoms develop. Approximately 15 percent of patients have early symptoms and signs of increased intracranial pressure; an even smaller number have focal cerebral signs (hemiparesis). Much less frequent are unilateral extrapyramidal rigidity, cerebellar ataxia, Parinaud syndrome, intratumoral hemorrhage, and meningeal oligodendrogliosis (cranial–spinal nerve palsies, hydrocephalus, lymphocytes, and tumor cells in CSF).
The appearance on imaging studies is variable, but the most typical is a hypodense (on CT) or T2 hyperintense (on MRI) heterogenous mass near the cortical surface with relatively well-defined borders (Fig. 30-6). Intratumoral calcification can be seen in more than half the cases and is a helpful diagnostic sign, but in the context of seizures, this finding also raises the possibility of an arteriovenous malformation or a low-grade astrocytoma. Approximately half of oligodendrogliomas demonstrate some contrast enhancement, and leptomeningeal enhancement adjacent to the tumor can be seen but is rare.