Figure 1.

Example of oligodendroglioma, not elsewhere classified (NEC). In the hematoxylin-eosin–stained section, the histology of this tumor in the temporal lobe of a 40-year-old woman is fully compatible with a diffuse glioma (arrows indicate preexistent neurons that are overrun by diffusely infiltrating tumor cells). The tumor cells have a prototype oligodendroglial phenotype: “fried-egg” appearance with round nuclei and a clear halo. However, unlike “canonical” oligodendrogliomas as defined in the World Health Organization 2016 classification, this tumor was isocitrate dehydrogenase (IDH) wild-type and did not show codeletion of chromosome arms 1p and 19q. After ruling out other tumors that may have an oligodendroglial phenotype (especially dysembryoplastic neuroepithelial tumor, [extraventricular] neurocytoma, clear cell ependymoma, and pilocytic astrocytoma) and demonstrating that this tumor did not have molecular features of IDH–wild-type glioblastoma (see Consortium to Inform Molecular and Practical Approaches to CNS Tumor Taxonomy—Not Official WHO update 3), the tumor was signed out as “oligodendroglioma, NEC.”