Oligodendroglioma

Oligodendrogliomas account for 5-25% of all gliomas and 5-10% of all primary intracranial neoplasms. These tumours are typically located in supratentorially (85%), involving the white matter and overlying cortex.  They are most commonly found in the frontal lobes.

Demographics

These are usually tumours of middle-aged adults, occuring most commonly in the fourth and fifth decades of life. The average age of presentation is 35.

Aetiology

Unknown.

Gross Appearance

Generally, oligodendrogliomas are well-circumscribed, gelatinous, gray masses.  They are usually calcified (70 - 90%), often with focal hemorrhage.  20% of these tumours are cystic, and they can often expand a gyrus and remodel the skull. They can occur essentially anywhere, including intraventricularly where they are difficult to distinguish from central neurocytomas on light microscopy.

Microscopic Appearance and Classification

Oligodendroglioma (WHO grade II/IV)

Sheets of regular cells with spherical nuclei containing finely granular chromatin surrounded by a halo of cytoplasm.  Typically contains a delicate network of anastomosing capillaries. These tumours are slowly growing.

RADIOGRAPHIC IMPLICATIONS: dense, finely branching capillary network give “chicken-wire appearance on contrast-enhanced MRI and MRP (more prominent in anaplastic).

Anaplastic oligodendroglioma (WHO grade III/IV)

Increased cellular density, mitotic activity, microvascular proliferation and necrosis.  Nuclear anaplasia is also common.

RADIOGRAPHIC IMPLICATIONS: increased vascularity results in elevated relative cerebral blood volume (rCBV) of grade II vs grade III on PWI.  PWI has a sensitivity of 95% and PPV or 87% for distinguishing grade II from grade III Oligodendrogliomas [1].  Contrast enhancement is heterogenous and variable for both grades.

Anaplastic oligodendrogliomas make up 20 - 50% of all oligos. Oligoastrocytomas, a tumour of mixed oligodendroglioma and astrocyoma cell populations are common, and carry a prognosis equivalent to their astrocytoma component. (worse grade for grade).When necrosis is present in an anaplastic oligo then they are considered WHO Grade IV lesions and referred to a GBM with oligodendroglioma component.

Molecular genetics and subclassification

1)1p/19q chromosome loss: excellent response to radiation and chemotherapy.

2)1p/19q chromosome intact: refractory to radiation and chemotherapy [2].

RADIOGRAPHIC IMPLICATIONS: 1p/19q loss has significantly higher rCBV than 1p/19q intact [29]. Intact 1p/19q tumours show more homogenous signal and have sharper borders on T1W and T2W images than those with chromosome deletions [3].

Radiographic findings

CT

NECT: mixed density mass (hypodense to isodense).  High-attenuation areas within the tumour are likely from calcification or, less commonly, from hemorrhage.  70 to 90% of oligodendrogliomas are calcified.  Calcifications can be located centrally, peripherally or they can be ribbon-like [4].

CECT: 50% of oligodendrogliomas enhance.  Degree of enhancement extremely variable (no enhancement to striking)

MRI

hypointense on T1W and hyperintense on T2W and FLAIR.  Intact 1p/19q tumours show more homogenous signal on T1W and T2W images and have sharper borders than tumours with 1p/19q deletions.  Calcifications and hemorrhage are difficult to distinguish on MR.  Contrast enhancement is common but it is not a reliable indicator of tumour grade.

T1 C+: 50% of oligodendrogliomas enhance.  The enhancement is typically heterogenous.

T2*GRE: Calcium seen as areas of "blooming."

MR Diffusion (DWI): typically no diffusion restriction.  DWI can be used to help differentiate oligodendrogliomas (generally lower grade) from astrocytomas (generally higher grade).  Astrocytomas have higher ADC values probably because of their higher cellularity [5].

DSC-MR (PWI): can be used to help differentiate low-grade oligodendrogliomas from higher grade oligodendrogliomas or from higher grade astrocytomas.  Caution: foci of elevated rCBV can mimic a high grade tumour.  A threshold of 1.75, rCBV above this threshold demonstrate more rapid tumour progression [1]. 

Nuclear Medicine

PET: 11C-Methionine studies can be used to differentiate oligdendrogliomas from anaplastic oligodendrogliomas.  FDG uptake of oligodendrogliomas is similar to normal white matter.  FDG uptake of anaplastic oligodendrogliomas is similar to normal gray matter.

Clinical Presentation and Prognosis

Due to their usual cortical involvement, presentation is most frequently as a result of seizures. Although they are macroscopically well delineated, infiltration is present at their margins and local recurrence is common. 5 year survival rate is 50-75%.

Treatment

Treatment is surgical, with adjuvant radiotherapy and chemotherapy.

DDx

1. Astrocytoma - centered on white matter. Not as frequently calcified. May have identicle appearances.
2. Ganglioglioma - sharply demarcated temporal lobe cortical lesion.
3. Dysembryoplastic neuroepithelial tumour (DNET) - sharply demarcated "bubbly" cortical neoplasm
4. Pleomorphic xanthoastrocytoma (PXA) - enhancing mural nodule, usually cystic, potentially with a dural tail.
5. Cerebritis / Herpes encephalitis
6. Ischemia / infarction - confined to vascular territorries
7. Arteriovenous malformation (AVM) - if thrombosed, may be indistinguishable