Combined tanycytic ependymoma and subependymoma, (WHO grade II).
Tanycytic ependymoma (WHO grade II) is a rare subtype of ependymoma arising from tanycytes. This tumor is composed of clusters of elongated cells forming nuclear dense zones and streaming cell processes forming fibrillary zones. Typical ependymal rosettes are absent, and perivascular rosettes are vague (10).
Subependymomas (WHO grade I) are slow growing, benign neoplasms typically attached to a ventricular wall composed of clusters of glial tumor cells embedded in an abundant fibrillary matrix with frequent microcystic change (5).
A tumor showing features of both tanycytic ependymoma and subependymoma raises an interesting question regarding its histogenesis. The following hypotheses have been proposed: (A) these two components can reflect a "collision" phenomenon between two separate neoplastic clones, (B) both elements can be derived from a single totipotent stem cell of a common progenitor "combination phenomenon", (C) or one component can reflect metaplasia or dedifferentiation from another "conversion phenomenon".
The origin of ependymoma is a matter of debate. There seems to be an agreement that the ontogeny of ependymoma is radial glia, which is the stem cell of tanycyte and glia. Tanycytes are interspersed among the columnar ependymal cells. Studies suggested that tumors with histomorphologic and ultrastructural characteristics similar to tanycyte also include pilocytic astrocytoma, myxopapillary ependymoma, astroblastoma, and subependymoma (10). Interestingly the precursor cells of subependymomas have not been conclusively identified although some candidates have been proposed: subependymal glia, astrocytes of the subependymal plate, ependymal cells and a mixture of astrocytes and ependymal cells (6) .
Subependymomas represent only 0.2 to 0.7% of intracranial tumors and are often clinically silent (7) . In 1978, Scheithauer reviewed a total of 95 cases from the literature, the largest study to date, including 21 symptomatic cases from his personal files (9). In a clinicopathological evaluation of 83 cases (8), 43 tumors arose in the posterior fossa, 37 in lateral ventricles, 2 in spinal cord and one in the temporal horn. Eighteen-percent (15/83) of subependymomas exhibited a mixed histollogic pattern: that is, subependymoma together with another glial tumor. The most common mixture (13/83) was subependymoma and ependymoma and the remaining two cases were subependymoma mixed with astrocytoma.
Tanycytic ependymomas generally do not show typical ependymal rosettes and these have been defined as "pure type" (3) . Another type was defined as "mixed type" and this contained both true rosettes and perivascular pseudorosettes. In Friede and Pollak's initial report of 11 tumors 6 of them were likely mixed type with the remaining 5 being pure type (1).
The occurrence of subependymal giant cell astrocytoma (SEGA)-like pleomorphic cells was an additional unusual feature. In a previous report (2) a case of a right frontal ganglioglioma with a tanycytic ependymoma as the glial component has been described in a 51 year old man. In our case the large pleomorphic cells with a ganglionic appearance were focally positive for GFAP and negative for synaptophysin.
Subependymomas and tanycytic ependymomas are strongly reactive for GFAP as in our case. In some reported cases, tanycytic ependymomas were positive for EMA and in other cases as in our case were negative for EMA (4).
Tanycytic ependymomas must be differentiated from other tumors which are composed of elongated piloid cells mainly pilocytic astrocytomas, diffuse astrocytomas and schwannomas. Electron microscopy helps in this differential diagnosis with neoplastic tanycytes showing microvilli, cilia, cell junctions, intracytoplasmic filaments and absence of basal lamina. Pilocytic astrocytomas, diffuse astrocytomas and schwannomas are devoid of microvilli and cilia, whereas in schwannomas tumor cells are surrounded by basal lamina.
Thus, the present case is an unusual example of tanycytic ependymoma; which to the best of our knowledge has not been associated with subependymoma. The occurrence of SEGA-like pleomorphic cells was an additional unusual feature.
Contributed by Leonidas D. Arvanitis, MD, Paolo Gattuso, MD and Sukriti Nag, MD