Oligodendrocytes are the second most common glial cell, and function as the myelinating cells of the central nervous system. Oligodendrogliomas have features similar to these cells and are thought to arise from them.
A related entity is the oligoastrocytoma, which has appearances and prognosis between that of astrocytoma and oligodendroglioma.
Oligodendroglioma occurs at a younger age than the diffuse astrocytomas, usually afflicting young to middle aged adults. They account for about 5% of gliomas (compared with glioblastoma, accounting for 50%).
Aetiology and Pathogenesis
There are no specific causes of these tumours.
These tumours almost always arise in the cerebral hemispheres. Progression to higher grade disease occurs but is typically slower than astrocytoma; average survival after surgical resection is usually over 10 years. Oligoastrocytoma is usually more aggressive with poorer results.
Symptoms and Signs
Oligodendrogliomas commonly present with seizure (about 80% of cases); other symptoms such as focal neurological symptoms are not uncommon.
Calcium deposits may occasionally be visible on CT scanning before contrast is given.
Tumour/Normal Tissue Features
The tumour is usually grey to pink, with a gelatinous consistency. Foci of calcium may be deposited, giving a gritty texture.
The classic feature of oligodendroglioma is of uniform nucei with perinuclear halo formation (artefactual). This appearance as known as "fried egg"; it is not seen in frozen sections. Anaplasia (nuclear pleomorphism, hyperchromatism etc) suggests a higher grade disease, which is uncommon. The extensive capillary network gives a "chicken wire" appearance.
Higher grade oligodendrogliomas may show areas of necrosis and increased microvascular proliferation, usually associated with high grade glioblastoma.
There are no specific stains for oligodendroglioma; they usually stain positively for S-100 but this is not specific.
About half to two thirds of oligodendrogliomas will show a characteristic loss of chromosomal arms 1p and 19q. This is an important prognostic marker, and tumours that have finding on FISH or SISH will typically respond more favourably to chemotherapy (eg. temozolomide, PCV combination)
Oligoastrocytoma is a diagnosis given to gliomas that show features intermediate between oligodendroglioma and the more common astrocytomas. On microscopic examination, the oligodendroglioma component and astrocytic component may be biphasic/compact or intermingled/diffuse. Diagnosis is often difficult. Necrosis or microvascular proliferation may be present, indicating a higher grade of disease. Genetic analysis is thought to be relatively important; tumours that possess a genotype similar to oligodendroglioma (1p, 19q loss) will tend to behave less aggressively.
Staging / Classification
These tumours are usually Grade II, with an average survival of over 10 years (for pure oligodendrogliomas). Anaplastic features or astrocyte phenotype confer a worse prognosis. There is debate regarding grading of oligodendroglioma/oligoastrocytoma with necrosis; there appears to be no difference in survival between these patients and those with other anaplastic features.