Oligodendrogliomas are cerebral tumors that are differentiated from other gliomas on the basis of their unique genetic characteristics and better response to chemotherapy. These tumors are classified according to their grade (low grade oligodendrogliomas: grade II of the WHO classification and anaplastic oligodendrogliomas: grade III of the WHO classification) and according to their pure or mixed histology (oligoastrocytomas).
Until now, the incidence of these tumors has been largely underestimated. Oligodendrogliomas may represent up to 30% of all adult gliomas. Annual incidence can therefore be estimated at around one new case per 100,000 individuals per year. Prevalence is estimated at 1/300,000.
Low grade tumors are usually diagnosed after a prolonged history of seizures and headaches. Clinical presentation of malignant forms is usually more acute with rapidly progressive neurological deficits and signs of intracranial hypertension (headaches and vomiting). Malignant forms can arise from transformation of a benign tumor several years after its diagnosis. These tumors may develop anywhere in the central nervous system but arise most frequently in the cerebral hemispheres.
The etiology is not known but no hereditary forms of oligodendroglioma have been reported. However, specific genetic characteristics, such as loss of chromosomes 1p and 19q, are observed.
MRI may reveal infiltration of the brain parenchyma. Enhancement after contrast injection is indicative of an aggressive and malignant oligodendroglioma.
The differential diagnosis should include low grade astrocytomas (see this term).
Treatment starts with surgery. Radiotherapy is mandatory for malignant tumors, regardless of the extent of the surgical resection. Chemotherapy is beneficial and is often used during treatment of the initial tumor and for treatment of a recurring malignant oligodendroglioma. In the case of low grade partially resected tumors, chemotherapy now tends to replace radiotherapy as thefirst-line treatment. Management requires a multidisciplinary team with experience in neuro-oncology. The treatment protocol should follow the recommendations resulting from prospective studies.
The prognosis is usually better than that for other gliomas, such as astrocytomas. As a result of combined treatment with surgery, chemotherapy and radiotherapy, survival rates in patients without recurrence are also good, even for patients with malignant (anaplastic) tumors.
- Dr Jacques GRILL