Retroperitoneal malignant meningioma. A light microscopic, immunohistochemical, and ultrastructural study.

Malignant meningiomas are uncommon and rarely occur outside the central nervous system. We describe herein a morphologically unusual malignant neoplasm that arose in the retroperitoneum of a 25-year-old woman. The tumor was composed of sheets of epithelioid cells that were frequently arranged in prominent whorls. By electron microscopy, the neoplastic cells had long, tapering cell processes that formed numerous interdigitations; many junctions including desmosomes; and abundant intermediate filaments. Immunohistochemistry showed that the tumor cells expressed vimentin, keratin, and epithelial membrane antigen. Based on these findings, the neoplasm was classified as a malignant meningioma. According to our review of the literature, this is the first reported occurrence of a primary retroperitoneal meningothelial neoplasm and the second reported case of an ectopic meningioma that was malignant.