CAP16 Abstract Highlights - Extraventricular Neurocytoma: A Diagnostic Pitfall Mimicking Oligodendroglioma

The 2016 annual meeting of the College of American Pathlologists (CAP16) is coming up September 25-28 in Las Vegas. I'll be attending the meeting this year. In this series of posts, I'll be featuring poster abstracts of particular interest to neuropathologists.

Xinhai Zhang and colleagues at Loma Linda University in California discuss in Poster #198 discuss the problem of distinguishing extraventricular neurocytoma from oligodendroglioma:

Context: Extraventricular neurocytoma (EVN) mimics oligodendroglioma in terms of location and morphology. Oligodendrogliomas usually progress, while EVNs, other than atypical EVNs, typically do
not.

Design: We retrospectively analyzed studies encompassing 155 EVN cases published since 2000 for distinguishing features.

Results: Summarized differentiating features include the fact that EVNs have a peak incidence in the third and fourth decade of life, whereas the peak incidence of oligodendrogliomas is from the fourth to sixth decade. EVNs can have small areas of nucleus-free neuropil as well as thickened hyalinized blood vessels, features not seen in oligodendrogliomas. Immunohistochemically, oligodendrocyte lineage transcription factor 2 (OLIG2) is immunopositive in oligodendroglioma but negative in EVNs. In addition, EVNs are consistently positive for synaptophysin but rarely positive for GFAP, whereas oligodendrogliomas are variably positive for synaptophysin and more often positive for GFAP. Many oligodendrogliomas show loss of heterozygosity on
chromosomes 1p and 19q. These changes are not seen in EVNs except for atypical EVN with MIB-1 proliferation indices greater than 3%. IDH1/2 mutations are present in many oligodendrogliomas but not EVNs. Ultrastructurally, EVN tumor cells demonstrate numerous neuritelike cell processes as well as cell bodies with dense-core neurosecretory granules, whereas oligodendroglioma cells show plump and rounded cytoplasm without forming cellular processes, synaptic structures, or dense-core granules. Rare hypothalamic neurocytomas express TTF1 and vasopressin.

Conclusions: While clinicopathologic features are diagnostically helpful, additional molecular analyses are needed to better characterize the spectrum of EVNs and their distinction from oligodendrogliomas.