Answer: Subependymoma

Histology: The tumor is highly fibrillary, with areas of clustering of the nuclei. The nuclei show uniform, delicate chromatin and a lack of perinuclear cytoplasm. Calcifications and microcyst formation can be seen.

Discussion: Subependymomas show characteristic clustering of nuclei and highly fibrillated cell processes, along with microcysts, which readily distinguish it from other gliomas. The subependymal giant cell astrocytoma of tubular sclerosis is a cellular spindle and epithelioid lesion with cells showing eosinophilic cytoplasm and vesicular nuclei with prominent nucleoli. All of these features are not seen in subependymomas. Ependymomas may show overlap in architecture with subependymomas. Unlike the ependymoma, subependymoma is less cellular and usually seen in adults.

Subependymomas are slow growing lesions that arise in the wall of the ventricles in the brain, or rarely, in the spinal cord. Usually these lesions are found incidentally, however, they become symptomatic when they occur in the posterior fossa where they can compress brain stem or cranial nerves. The tumor is composed of ependymal and astrocyte-like cells. Subependymomas that occur in the lateral ventricles tend to show prominent micro-cystic changes. Nuclear pleomorphism and occasion mitoses can be seen in these tumors. Larger lesions will often show hemosiderin deposits. Subependymomas of the fourth ventricle usually show less microcystic change, are more fibrillar, and the cells often appear more uniform than those occurring in the lateral ventricles. Subependymomas are immunoreactive for S100 and GFAP. These tumors are one of the few gliomas considered biologically benign.