Oligodendroglioma

Oligodendrogliomas are a type of glioma that are believed to originate from the oligodendrocytes of the brain or from a glial precursor cell. They occur primarily in adults (9.4% of all primary brain and central nervous system tumors) and are only rarely found in children (4% of all primary brain tumors). The median age of diagnosis for oligodendroglioma is 41 years of age.

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Etiology

The aetiology of oligodendrogliomas is unknown. Some studies have linked oligodendroglioma with a viral cause, a single case report has linked oligodendroglioma to irradiation of pituitary adenoma.

Symptoms

In anywhere from fifty to eighty percent of cases, the first symptom of an oligodendroglioma is the onset of seizure activity. They occur mainly in the frontal lobe thus affecting personality. Headaches combined with increased intracranial pressure are also a common symptom of oligodendroglioma. Depending on the location of the tumor, any neurological deficit can be induced, from visual loss, motor weakness and cognitive decline. A Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scan is necessary to characterize the anatomy of this tumor (size, location, heter/homogeneity). However, final diagnosis of this tumor, like most tumors, relies on histopathologic examination (biopsy examination).

Microscopic Appearance

Oligodendrogliomas cannot currently be differentiated from other brain lesions solely by their clinical or radiographic appearance. As such, a brain biopsy is the only method of definitive diagnosis. Oligodendrogliomas recapitulate the appearance of the normal resident oligodendroglia of the brain. (Their name derives from the Greek roots 'oligo' meaning “ few” and 'dendro' meaning “trees”.) They are generally composed of cells with small to slightly enlarged round nuclei with dark, compact nuclei and a small amount of eosinophilic cytoplasm. They are often referred to as "fried egg" cells due to their histologic appearance. They appear as a monotonous population of mildly enlarged round cells infiltrating normal brain parenchyma and producing vague nodules. Although the tumor may appear to be vaguely circumscribed, it is by definition a diffusely infiltrating tumor.

Classically they tend to have a vasculature of finely branching capillaries that may take on a “chicken wire” appearance . When invading grey matter structures such as cortex, the neoplastic oligodendrocytes tend to cluster around neurons exhibiting a phenomenon referred to as “perineuronal satellitosis”. Oligodendrogliomas may invade preferentially around vessels or under the pial surface of the brain.

Oligodendrogliomas must be differentiated from the more common astrocytoma. Non-classical variants and combined tumors of both oligodendroglioma and astrocytoma differentiation are seen, making this distinction controversial between different neuropathology groups. In the US, in general, neuropathologists trained on the West Coast are more liberal in the diagnosis of oligodendroliomas than either East Coast or Midwest trained neuropathologists who render the diagnosis of oligodendroglioma for only classic variants. Molecular diagnostics may make this differentiation obsolete in the future.

Other glial and glioneuronal tumors with which they are often confused due to their monotonous round cell appearance include pilocytic astrocytoma, central neurocytoma, the so-called dysembryoplastic neuroepithelial tumor, or occasionally ependymoma.

Histopathological Grading

The histopathologic grading of oligodendrogliomas is controversial. Currently the most commonly used grading schema is based on year 2007 World Health Organization (WHO) guidelines. Oligodendrogliomas are generally dichotomized into grade II (low grade) and grade III (high grade) tumors. The designation of grade III oligodendroglioma (high grade) generally subsumes the previous diagnoses of anaplastic or malignant oligodendroglioma.

Unfortunately, the WHO guidelines include subjective criteria in differentiating grade II and grade III tumors including the appreciation of “significant” hypercellularity and pleomorphism in the higher grade lesion. In addition, the presence of low mitotic activity, vascular proliferation and necrosis, including pseudopallisading necrosis are insufficient by themselves to elevate the grade of these tumors. This leads to inevitable interobserver variability in diagnosis by pathologists. The ultimate responsibility for making treatment decisions and interpretation of these diagnoses lies with the oncologist in consultation with the patient and their family.

It has been proposed that WHO guidelines should contain a category for grade IV oligodendrogliomas which essentially appear to be glial neoplasms with overwhelming features of glioblastoma multiforme (GBM) arising from known lower grade oligodendrogliomas or GBM with a significant proportion of oligodendroglial differentiation. The diagnostic ultility of this latter category is uncertain as these tumors may behave either like glioblastoma or grade III oligodendrogliomas. As such, this is an exceptionally unusual diagnosis.

The updated WHO guidelines published in 2007 recommends classifying such tumors for the time being as ‘glioblastoma with oligodendroglioma component’. ^[1] It remains to be established whether or not these tumors carry a better prognosis than standard glioblastomas.

Molecular genetics

By far, the most common structural deformity found is co-deletion of chromosomal arms 1p and 19q. The high frequency of co-deletion (60-80%) is a striking feature of this glial tumour, and is considered as a "genetic signature" of oligodendroglioma. 1p/19q deletion has been correlated with both chemosensitivity and improved prognosis in oligodendrogliomas.^[2][3] A t(1;19)(q10;p10) translocation mediates the combined deletions of 1p and 19q. The gene products lost as a consequence of this codeletion may include mediators of resistance to genotoxic therapies. Alternatively, 1p/19q loss might be an early oncogenic lesion promoting the formation of glial neoplasms, which retain high sensitivity to genotoxic stress.

Prognosis & treatment

Oligodendrogliomas are generally felt to be incurable using current treatments. However compared to the more common astrocytomas, they are slowly growing with prolonged survival. In one series, median survival times for oligodendrogliomas were 11.6 years for grade II and 3.5 years for grade III.^[4] Because of the indolent nature of these tumors and the potential morbidity associated with neurosurgery, chemotherapy and radiation therapy, most neurooncologists will initially pursue a course of watchful waiting and treat patients symptomatically. Symptomatic treatment often includes the use of anticonvulsants for seizures and steroids for brain swelling. PCV chemotherapy (Procarbazine, CCNU and Vincristine) has been shown to be effective and is currently the most commonly used chemotherapy regimen used for treating anaplastic oligodendrogliomas.^[5] Temozolomide is a common chemotheraputic drug to which oligodendrogliomas appear to be quite sensitive. It is often used as a first line therapy.

Because of their diffusely infiltrating nature, oligodendrogliomas cannot be completely resected and are not curable by surgical excision. If the tumor mass compresses adjacent brain structures, a neurosurgeon will typically remove as much of the tumor as he or she can without damaging other critical, healthy brain structures. Surgery may be followed up by chemotherapy, radiation, or a mix of both. Oligodendrogliomas, like all other infiltrating gliomas, have a very high (almost uniform) rate of recurrence and gradually increase in grade over time. Recurrent tumors are generally treated with more aggressive chemotherapy and radiation therapy. Recently, stereotactic surgery has proven successful in treating small tumors that have been diagnosed early.

Long-term survival is reported in a minority of patients.^[6] With aggressive treatment and close monitoring, it is possible to outlive the typical life expectancies for both low grade and high grade oligodendrogliomas. In rare cases, patients have survived for up to fifteen years post-diagnosis. Westergaard’s study (1997) showed that patients younger than 20 years had a median survival of 17.5 years.^[7] Another study shows a 34% survival rate after 20 years. ^[8]

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