Atypical extraventricular neurocytoma.

Extraventricular neurocytoma (EVN) is a rare brain tumor that poses diagnostic difficulty. Described herein is a case of atypical EVN arising in a 54-year-old woman. A well-circumscribed lesion (3.0 x 3.0 x 3.0 cm) in the right parietal lobe showed diffuse proliferation of monotonous tumor cells with perinuclear clearing within a delicate fibrillary matrix similar to neuropil. Tumor also showed vascular proliferation and high mitotic activity. Immunohistochemically, these tumor cells were strongly positive for synaptophysin both in the neuropil and in the perinuclear cytoplasm, and were negative for glial fibrillary acidic protein and Olig2. Ki-67 labeling index was 13.0% in the most stained areas, but accumulation of p53 was not observed. These findings were compatible with those of EVN with histological atypia. EVN should be considered as a candidate in the differential diagnosis of parenchymal brain tumor, especially oligodendroglioma. The important features are the delicate fibrillary matrix similar to neuropil, diffuse and strong immunoreactivity for synaptophysin, and negative immunoreactivity for Olig2. High proliferative activity without accumulation of p53 suggests that other factors are involved in oncogenesis of atypical EVN.