OLIGODENDROGLIOMA:

Oligodendrocytes are specialized neuraglia that form myelin in the CNS, first recognized by Robertson in 1900. In 1926, Bailey and Cushing first described oligodendrogliomas in a histogenic classification of gliomas.

They are a subgroup of gliomas with distinctive morphological characteristics.

At present, oligodendrogliomas are thought to constitute approximately 5% to 10% of all primary tumors of the brain, but by using expanded criteria, they may well represent up to one third. In fact, morphologic criteria are vague and highly subjective and the histologic diagnosis, therefore, remains highly controversial and unsatisfactory. It has been suggested that the traditional 'diffuse fibrillary astrocytoma' are in fact made of isolated neoplastic oligodendrocytes which are entrapped in a fibrillary background composed of axons and fibrillary reactive gliosis.

They are most often diagnosed in patients who are middle aged, though a bimodal distribution has been described with a second, smaller peak among children and adolescents.

It has been demonstrated that oligodendroglial neoplasms usually have loss of 1p and 19q whereas astrocytomas of the progressive type frequently contain mutations of the TP53 gene, and that 9p loss and CDKN2A deletions are associated with progression from well-differentiated to anaplastic oligodendrogliomas.

Oligodendrogliomas most commonly involve the cerebral hemisphere. Involvement of the frontal and temporal lobes is particularly common. Rare cerebellar oligodendroliomas have been described.   Patients usually present with a history of seizures.   CT scanning, typically, reveals a supratentorial lesion with nodular calcifications and surrounding edema. Contrast enhancement is seen in 60% of cases. On MRI, they are hypo to isodense on T1 and hyperdense on T2 images with heterogenous enhancement.   Oligodendrogliomas generally expand the cortex diffusely giving it a hypertrophic appearance, while the underlying white matter may show cysts of varying sizes. While they appear grey and friable, calcified granules can often be felt when cut with the knife.  However, in the oligodendrogliomas seen in India, the calcification has been found to be minimal in contrast to that seen in similar tumors in the west.

Oligodendroglioma with calcification-MRI T2

Grading of these tumors is difficult as most of the oligodendrogliomas show monomorphism as regards the oligodendroglial tumor proper, but frequently they are associated with an astrocytoma grade II.

Anaplastic oligodendrogliomas (AO) are uncommon. They have the features of high grade neoplasm e.g. increased mitotic activity, necrosis, vascular endothelial proliferation and hypercellularity. Their natural history, prognosis, and optimal management are not yet fully understood. These tumors still maintain the rounded shapes of their nuclei and the arciform vasculature. These tumors can progress sometimes to Glioblastoma multiforme and, in the process, incorporate many astrocytes. However, they are associated with a better prognosis and response to multimodality therapy based on specific molecular changes.

Management include, aggressive excision  and post operative radiation.

Chemotherapy with PCV (procarbazine, CCNU, and vincristine) has also been advocated, particularly in mixed and anaplastic variants. Oligodendrogliomas have been the focus of considerable interest over the past decade, ever since they were recognized as chemosensitive tumors. A recent report suggests that alterations of chromosome arms 1p and 19q are associated with chemotherapeutic response and overall survival in anaplastic oligodendroglioma patients treated with procarbazine, lomustine, and vincristine chemotherapy.

While postoperative survival is generally longer with oligodendrolioma, its prognosis is ‘notoriously uncertain’. The morbidity and mortality profile for oligodendrogliomas is much better than for astrocytic tumors. However, it also depends on tumor location and pressure effects, as with any other intracranial lesion.

5 year survival rates for patients with oligodendrogliomas are commonly in the range of 50% to 65%. Early diagnosis, complete excision, and low grade histology predict a better prognosis.

 Glial tumors of unknown origin:

They include Gliomatosis Cerebri, Astroblastomas, and Choroid gliomas.

1) Gliomatosis Cerebri (diffuse cerebral gliomatosis):

The term "gliomatosis cerebri" is used to describe a diffusely proliferative glial neoplasm involving large regions of the brain, characterized by an infiltrative nature rather than by the formation of a distinct tumor mass.

They are rare. Although found anywhere throughout, the CNS, multilobar hemispheric involvement is most common.

Usually, the involvement is diffuse.

Symptoms at presentation are frequently those of seizure or headache. Most patients present between third and fourth decades.

The duration of symptoms may be several months to many years.

CT may reveal subtle hypo to isodense changes with diffuse mass effects. MRI may be more illustrative.

Occasionally, there may be minimal contrast hetereogenous enhancement.

Multiple sclerosis, encephalitis, and leukodystrophies must be ruled out. CSF cytology is unremarkable.

Positron emission tomography (PET) scans using ¹¹C-L- methionine show isotope accumulation in the diffusely infiltrative tumorous area with greater accuracy than CT or MRI.

Many are diagnosed at autopsy.

Grossly, there is diffuse enlargement. The gyri may be slightly enlarged with normal architecture.

Biopsy is required for diagnosis.

Histologically, a diffuse infiltration of astrocytic cells bearing a spindle like pattern  is seen;

the infiltration occurs along the white matter tracts.

Radiation is often of limited benefit. Steroids help in palliation. Surgery, obviously, is not possible, except for hydrocephalus,if any. 

The prognosis is dismal.

2) Astroblastomas:

They are rare tumors believed to arise from astroblasts, the precursors of adult astroglia.

They are not well encapsulted, soft to firm in consistency.

Histologically, there are prominent elongated neoplastic cells with footplates forming pseudorosettes around blood vessels. Limited mitosis may be present. Necrosis is seen in up to 70% of the tumors; but does not appear to have prognostic significance.

GFAP positivity may be present occasionally.

Surgical resection is the primary treatment.  Role of radiotherapy and chemotherapy is not clear.

The prognosis is hard to predict; it appears to be intermediate between that of astrocytoma and glioblastoma.

3) Choroid gliomas:

They are rare, have been reported in anterior third ventricle only.

Radiologically, they mimic  meningioma, and pit .adenomas.

They are slow growing and present with hydrocephalus.

Histologically they have glial features with ependymal differentiation, and there is GPAF positivity. The cell of origin is not known.

The treatment is surgery. The role of radiotherapy is not clear. The outcome is generally favorable.