Oligodendrogliomas are a type of gliomas accounting for 5-10% of all primary intracranial malignancies. They commonly present as space occupying lesions involving the cortex or subcortical white matter.  Radiologic attenuation can be heterogeneous due to calcifications, cystic degeneration and haemorrhage. Oligodendrogliomas usually occur in the cerebral hemispheres. Infratentorial locations in brainstem and spinal cord are very uncommon.

Historically, oligodendrogliomas have been defined on histological grounds. As of 2016, however, with the update to the WHO classification of CNS tumours the diagnosis of oligodendroglioma is made by identifying a diffuse infiltrating glioma with IDH mutation and 1p19q codeletion. 

Epidemiology

Oligodendroglioma is the third most common glioma accounting for 2%–5% of all primary brain tumors. It presents in middle-aged adults with a peak incidence in the 4th and 5th decades with a slight male preponderance (1,3 : 1). Oligodendrogliomas are rare in children. 

MRI features

The appearance of oligodendrogliomas on MRI vary depending on whether a histological diagnosis or a molecular definition is used.

Tumors that histologically show oligodendroglial features but are 1p/19q intact show a more homogeneous signal on T1 and T2 images and have sharper borders than 'true' oligodendrogliomas, those with 1p/19q co-deletion. A well-circumscribed homogeneously hypo-attenuating non-calcified cortical tumor may be predictive of not having a 1p19q codeletion. Up to 90% of oligodendrogliomas show calcifications, best seen on CT imaging. 

T1: T1 imaging usually shows a hypointense signal

T2: Aside from calcified areas, T2 signal is typically hyperintense

T1 C+ (Gd): contrast enhancement is common but it is not a reliable indicator of tumor grade, with only 50% of oligodendrogliomas enhancing to a variable degree, and usually heterogeneously. 

DWI: These tumors usually do not show diffusion restriction. It may be used, however, to distinguish oligodendrogliomas from astrocytomas (that in general have a higher WHO grade). 

MR perfusion: Perfusion imaging shows an increased vascularity "chicken wire" network that results in elevated relative cerebral blood volume (rCBV) of grade II versus grade III tumors.