1. Diagnostic Challenge Pathology for Neurosurgery & Neurology Residents Department of Pathology University of Oklahoma Health Sciences Center, Oklahoma City, OK, U.S.A. Online Slides Case 2 History: The patient was a 29 year-old woman who presented with prolonged history of headache. CT reviewed an exophytic mass in the 4th ventricle associated with hydrocephalus. As per the radiology report, the mass also extends from the 4th ventricle through the foramen of Luschka. Contributor: Kar-Ming Fung, M.D., Ph.D., karming-fung@ouhsc.edu Last updataded: 4/6/2009karming-fung@ouhsc.edu

2. CT There is a cystic, exophytic mass (arrow) in the 4 th ventricle that is causing hydrocephalus.

3. The nuclei are rather monotonous overall and tends to cluster together. There are some long cytoplasmic processes (arrow) which highly suggest that these cells are glial cells. Cytologic Preparation

4. The low power pattern is that of clusters of nuclei (black arrow) separated by areas with paucinuclear cytoplasmic area (white arrow). In one area there is a epithelioid lining (gray arrow). Frozen Section

5. On higher magnification, there is a layer of epithelioid cells with no basement membrane. This is reminiscent of ependymal cells (gray arrow). The tumor cells also have a perivascular coronary arrangement (black arrows). Frozen Section

6. Permenant Section Many perivascular coronary arrangement of cells with a paucinuclear perivascular mantle are present (pairs of black arrows).

7. There is a lack of mitosis or high grade pleomorphism.

8. Diagnosis: Ependymoma (WHO II). Discussion: The clinical presentation and histologic features are all classic for ependymoma. In general, this is a low grade tumor. There is no high grade pleomorphism, necrosis, or mitoses. The perivascular pseudorosette arrangement of tumor cells as well as the strong immunoreactivity for GFAP are also supportive of the diagnosis of an ependymoma. The epithelioid cells interesting. It may represent neoplastic cells as ependymoma can have extensive papillary structure formation (papillary ependymoma). In this particular case, the papillary structure is not particularly well formed. These epithelial cells may well represent residual ependymal cells of the 4 th ventricle. While many ependymomas are positive for epithelial membrane antigen (EMA), this tumor is negative for EMA. The lack of immunoreactivity, however, does not overturn the diagnosis of ependymoma.