Pathologic features of ependymoma: Histologic patterns and a review of the literature

The pathologic features of ependymoma, which are necessary for making a histological diagnosis, are reviewed on the basis of a series of the authors' personal cases and each histological pattern is discussed with reference to descriptions found in the literature. Diagnostically, the most useful features of ependymoma are, in order of importance, perivascular pseudorosette, epithelial structure (including ependymal lining formation) and ependymal rosette in addition to the well‐demarcated margin of the tumor. Blepharoplast is not an essential feature. Clear cell pattern, trabecular pattern, Homer Wright rosette, papillary structure and piloid tumor cells are occasionally seen in limited areas of ependymoma. Piloid cells within ependymoma (connoting so‐called tanycytes) are discussed. In some cases, tumor cells contain eosinophilic round inclusion bodies that are occasionally accompanied by a few radial lines. These are consistent with intracytoplasmic lumina with microvilli containing granular and filamentous materials observed in electron microscopy. The inclusion bodies, which have not been well recognized in the past, often show additive histologic features indicating the epithelial nature of the tumor cells. Immunohistochemically, glial fibrillary acidic protein and vimentin are generally positive and epithelial membrane antigen is positive only in the epithelial structures. Comments are made on the World Health Organization classification of ependymal tumors in contrast with previous classification systems and on the features suggesting the tumor's malignant biological behavior.