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Oligodendrogliomas are intracranial tumors that account for 5-25% of all gliomas and 5-10% of all primary intracranial neoplasms.
They are characterized by IDH mutation and 1p19q codeletion and can be WHO CNS grade 2 or 3.
On imaging, oligodendrogliomas commonly present as masses involving the cortex or subcortical white matter, with low attenuation on CT, hypointense compared to grey matter on T1 and hyperintense compared to grey matter on T2-weighted MRI images. GRE/SWI sequence shows blooming due to calcification. The attenuation or signal can be eventually heterogeneous due to calcification, cystic degeneration and hemorrhage.
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Historically, oligodendrogliomas have been defined on purely histological grounds. As of 2016, however, with the update to the WHO classification of CNS tumors the diagnosis of oligodendroglioma is made by identifying a diffuse infiltrating glioma with IDH mutation and 1p19q codeletion 9. This was a very significant change, as the tumors previously diagnosed as oligodendrogliomas on the grounds of histology, and those currently diagnosed on the basis of the molecular markers are not identical, with somewhat different imaging features. As such it is essential when reading about gliomas to ascertain which definition is being used. As a general rule, anything written before 2016 will use histology alone.
The reliance on molecular markers is, however, not foolproof. Sometimes molecular markers are unavailable (in many countries or regional centers) or sometimes equivocal. In such instances, the diagnosis reverts to histological features alone, and are termed not otherwise specified (NOS) 9.
In the 5th edition (2021) of the WHO classification of CNS tumors, the term "anaplastic" has been removed, and tumors are merely graded as 2 or 3 11.
Note, oligoastrocytomas are no longer recognized as a distinct entity. The coexistance of neoplastic astrocytoma and oligodendroglioma (when molecularly defined) in the one tumor is considered extremely rare. More commonly, the term is used to denote tumors that appear histologically to have both astrocytic and oligodendroglial components but that who cannot be fully molecularly characterized (thus oligoastrocytoma NOS) 11.
Oligodendroglioma is considered the third most common glioma accounting for 2%–5% of primary brain tumors and 5%–18% of all glial neoplasms 8.
Oligodendrogliomas are usually tumors of middle-aged adults, occurring most commonly in the 4th and 5th decades of life, somewhat older for grade 3 tumors 10,11. There is a slight male predilection (M:F 1.3:1) 10.
They are rare in children 10,11.
Due to frequent cortical involvement, oligodendrogliomas most commonly (two-thirds of patients) present with seizures 9. Otherwise, the clinical presentation is non-specific with symptoms related to increased intracranial pressure and focal neurological deficits being common.
The vast majority of oligodendrogliomas arise in the cerebral hemispheres, with brainstem, cerebellum and spinal cord being distinctly uncommon 9.
Historically, oligodendrogliomas were considered well-circumscribed, gelatinous, grey masses, and this remains true of histologically defined oligodendrogliomas NOS.
Molecularly defined IDH-mutant and 1p19q co-deleted oligodendrogliomas, however, appear to be a slightly different group of tumors and recent studies suggest that in fact they have poorly defined borders 10.
Oligodendrogliomas are often calcified (70-90% of histological oligodendrogliomas: one of the most frequently calcifying tumors) and also frequently demonstrate focal hemorrhage 9. Cystic change is also quite common. Slow growing cortical tumors can often expand a gyrus and eventually remodel the skull.
Microscopic appearance and classification
Histological features are only one aspect of the diagnosis of oligodendroglioma and to formally make the diagnosis, a tumor must be shown to have both IDH-mutation and 1p19q co-deletion (see Terminology section above).
Neoplastic oligodendrocytes appear as regular cells with spherical nuclei containing finely granular chromatin surrounded by a halo of cytoplasm "fried egg" appearance under the light microscope. Typically there is a delicate network of anastomosing capillaries giving it a so-called "chicken wire" appearance 6. These tumors are slowly growing.
Grade 3 oligodendrogliomas (formerly known as anaplastic oligodendrogliomas) demonstrate increased cellular density, increased mitotic activity, microvascular proliferation and necrosis. Nuclear anaplasia is also common.
Importantly, and unlike astrocytomas, oligodendrogliomas with necrosis and microvascular proliferation are considered only WHO grade 3 and not grade 4 tumors 9.
By definition oligodendrogliomas should demonstrate 11,12:
IDH mutation (either IDH1 or IDH2)
Additionally, a number of other mutations are commonly encountered in oligodendrogliomas 12:
Tumors are of mixed density (hypodense to isodense). High-attenuation areas within the tumor are likely from calcification (70-90% of oligodendrogliomas are calcified) or, less commonly, hemorrhage. Calcification can be located centrally, peripherally or can be ribbon-like 4. The overlying skull may show pressure remodeling.
Approximately 50% of oligodendrogliomas enhance: the degree of enhancement is extremely variable, ranging from no enhancement to strikingly vivid enhancement.
The MRI appearances also vary depending on whether a histological diagnosis or a molecular definition is used.
Oligodendrogliomas NOS, or those tumors that histologically show oligodendroglial features but are 1p/19q intact show more homogeneous signal on T1 and T2 images and have sharper borders than 'true' oligodendroglioma, those with 1p/19q co-deletion 10. In fact, a lesion being well-circumscribed homogeneously T1 hypoattenuating with high T2 signal and T2/FLAIR mismatch without calcification is predictive of not having 1p19q codeletion 10.
Calcification and hemorrhage are difficult to distinguish on MR, appearing as areas of signal loss on T2* sequences, although the phase component of SWI may help. Peritumoral vasogenic edema is minimal in grade 2 tumors.
T1: typically hypointense
T2: typically hyperintense (except calcified areas)
GRE/SWI: calcium can be seen as areas of "blooming"
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
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 lower cellularity and greater hyaluronan proportion 5
MR perfusion (PWI)
increased vascularity "chicken wire" network of vascularity results in elevated relative cerebral blood volume (rCBV)
older literature 1 suggested that this was useful in predicting histological grade of tumor, however, how this relates to modern classification systems based on molecular markers is unclear
11C-Methionine studies can be used to differentiate oligodendrogliomas from anaplastic oligodendrogliomas. FDG uptake of oligodendrogliomas is similar to normal white matter. FDG uptake of anaplastic oligodendrogliomas is similar to normal grey matter.
Treatment and prognosis
As mentioned earlier response to radiochemotherapy and hence prognosis depends significantly on the presence or absence of 1p19q gene deletion 2,7.
Treatment is surgical, with adjuvant radiotherapy and chemotherapy. Although they are macroscopically well delineated, infiltration is present at their margins and local recurrence is common.
Survival statistics are primarily available for histologically defined oligodendrogliomas and tend to demonstrate 10 year-survival of approximately 50% 9.
General imaging differential considerations include:
astrocytoma: difficult to separate from oligodendrogliomas with certainty but some features are helpful
astrocytomas are more common
T2/FLAIR mismatch sign favors astrocytoma
well-defined margins favor astrocytomas
calcification strongly favors oligodendroglioma
CAPNON: entirely calcified
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