CLEAR-CELL EPENDYMOMA - A MIMIC OF OLIGODENDROGLIOMA - CLINICOPATHOLOGICAL AND ULTRASTRUCTURAL CONSIDERATIONS

Although clear cells resembling oligodendrocytes are known to occur in ependymomas, tumors composed primarily of such cells, i.e., clear cel l ependymoma (CCE), are rare. Herein we characterize the clinicopathol ogic features of eight tramples of CCE encountered at Mayo Clinic from 1983 to 1996. The tumors occurred in patients 3-31 years of age and p resented as well-demarcated, deeply situated, contrast-enhancing masse s, all of which were supratentorial. All but one case lacked classic l ight microscopic features of ependymoma, Although nearly all were immu noreactive for glial fibrillary acidic protein, it was electron micros copy that showed the diagnostic hallmarks of ependymoma, including com plex intercellular junctions, surface microvilli and cilia, and micror osette formation, thus underscoring the importance of electron microsc opy in the diagnosis of clear cell ependymomas. The differential diagn osis of CCE includes not only oligodendroglioma, but central neurocyto ma and glioneurocytoma. Unlike oligodendrogliomas, CCEs are characteri zed by their sharp circumscription, hyper vascularity as reflected in contrast enhancement on computed tomography and magnetic resonance ima ging, their noninfiltrative pattern of growth that displaces parenchym a, and the occasional formation of vague perivascular pseudorosettes. Unlike central neurocytomas and glioneurocytomas, CCE lack secretory g ranules, vesicles, and synapses by electron microscopy and neuroendocr ine markers by immunocytochemistry. In summary, the diagnosis of CCE r equires neuroimaging, histologic, and ultrastructural correlation. The latter is essential in a limited biopsy. Ultrastructural studies also play a role in identifying glioneurocytomas. CCEs behave like more or dinary ependymomas. The importance of their recognition is the avoidan ce of alternative diagnoses and inappropriate therapies.