Answer: Central neurocytoma

Histology: The lesion is composed of uniform, round cells. The architecture is predominantly solid, but also contains nucleus-free areas of fibrillary neuropil as well as oligodendroglioma-like regions with a honeycomb appearance. Nuclei are round-to-oval with finely speckled chromatin and a “neurocytic” appearance. Microcalcifications are scattered throughout. The MIB-1 index is low (1-3%) and mitoses are not easily found. Immunostains for synaptophysin are strongly positive, consistent with neuronal differentiation.

Discussion: Central neurocytomas are WHO grade II/IV neoplasms occurring predominantly in the supratentorial ventricles of young adults. Immunohistochemical and ultrastructural studies have confirmed the neuronal nature of these tumors. The differential diagnosis includes other tumors composed of cells with small, round nuclei, e.g. oligodendrogliomas, ependymomas and dysembryoplastic neuroepithelial tumors (DNT). The perivascular orientation of tumor cells seen in ependymomas is lacking in this case. The specific glioneuronal elements seen in DNT’s, composed of oligodendroglia-like cells and “floating” neurons, are also absent. Staining for neuronal markers such as synaptophysin and NeuN is characteristic in central neurocytomas, and helps differentiate them from the glial tumors mentioned above. Calcifications are seen in approximately 50% of central neurocytomas. The clinical course of central neurocytomas is usually benign, however more aggressive histological features such as microvascular proliferation and increased mitotic activity are occasionally seen, and are associated with worse clinical outcomes. Tumors with similar histology occasionally arise outside the ventricles and are known as extraventricular neurocytic tumors.