Oligodendroglioma is a rare slow-growing adult-type diffuse glioma that arises from oligodendrocytes typically in the frontal lobes, characterised by 1p/19q codeletion, microscopic ‘fried egg’ cells with ‘chicken-wire’ capillary pattern on microscopy and calcifications on neuroimaging.
Description
Oligodendroglioma is part of the adult-type diffuse glioma tumour group, originating from oligodendrocytes, the cells responsible for producing the myelin sheath in the central nervous system. It is characteristically slow-growing and infiltrative, often found in the cerebral hemispheres, particularly the frontal lobes.
Pathogenesis
The pathogenesis of oligodendrogliomas is not entirely understood. They are believed to result from the transformation of oligodendrocytes or their precursor cells. Genetic alterations, particularly codeletion of chromosome arms 1p and 19q, are common in oligodendrogliomas and are associated with a more favourable prognosis and response to therapy.
Subtypes
The two subtypes of oligodendroglioma are defined based on their level of anaplasia:
- Oligodendroglioma (Classic, WHO grade II): These are slow-growing, less aggressive tumours.
- Anaplastic oligodendroglioma (WHO grade III): These are more aggressive and fast-growing tumours.
Epidemiology, Risk Factors & Associations
- Accounts for 4-15% of all intracranial neoplasms
- Most commonly diagnosed in adults, with a peak incidence in the fifth decade of life
- Slight male predominance
- Associated with losses of chromosome arms 1p and 19q
Clinical Features
- Symptoms depend on the tumour location but may include seizures, headache, and focal neurological deficits such as motor weakness, sensory loss or language difficulties.
Complications
- Risk of malignant transformation to anaplastic oligodendroglioma
- Rarely, they can spread to other parts of the CNS but are unlikely to metastasise outside of it.
Pathological Features
Histopathology
- Macroscopic: Oligodendrogliomas are often soft, grey, and calcified tumours, typically unencapsulated with calcifications and cystic change. Haemorrhage and necrosis are rare.
- Microscopic: Characterised by a chicken-wire capillary pattern and round cells with clear cytoplasm due to high glycogen content with a ‘fried egg’ appearance.
Radiological Features
General Features
- Frequently located in the cerebral white matter, especially the frontal lobes and temporal lobes
- They typically begin in the hemispheric white matter and grow toward the cortex
- Clumped or curvilinear calcification is hallmark, seen in about 90% of cases.
CT
- Non-contrast: Hyperdense compared to the normal brain parenchyma due to frequent calcification.
- C+ Arterial: Variable enhancement
MRI
- T1: Typically hypointense or isointense
- T2/FLAIR: Typically hyperintense with possible areas of cystic degeneration. Vasogenic oedema is classically absent despite large tumour size
- DWI/ADC: No restricted diffusion unless anaplastic transformation has occurred
- GRE/SWI: Internal calcification present
- T1 Gad+: Variable enhancement, more commonly seen in anaplastic oligodendrogliomas. New enhancement suggests transformation to higher grade.
Grading and Staging
Oligodendrogliomas are classified according to the WHO grading system:
- Grade II: Oligodendroglioma
- Grade III: Anaplastic oligodendroglioma
Diagnosis
Diagnosis is confirmed by histopathological examination and molecular analysis, which typically shows combined loss of heterozygosity at chromosome arms 1p and 19q.
Differential Diagnosis
- Astrocytomas: May be indistinguishable. Usually not calcified and involves white matter more so than gray matter. Can be distinguished by the absence of clear cells and the chicken-wire capillary pattern. Also, they lack the characteristic 1p/19q codeletion.
- Ganglioglioma, dysembryoplastic neuroepithelial tumour (DNET), and pleomorphic xanthoastrocytoma (PXA): Typically present at a younger age, often cortical location, usually temporal lobe, less infiltrative, classically mural nodule with a cyst.
- Mixed oligoastrocytoma: Contains elements of both astrocytomas and oligodendrogliomas.
Management
Management typically includes surgical resection to the extent that is safely possible, followed by radiotherapy and chemotherapy, particularly in cases of anaplastic oligodendrogliomas.