Answer and Discussion of Quiz Set: N-019

3.    Which of the following combination is most useful in distinguishing diffuse astrocytoma (WHO grade II) from ependymoma?

A.  Epithelial membrane antigen (EMA) and p53

B.  Glial fibrillary acidic protein (GFAP), cytokeratin AE1/AE3

C.  Ki67 (Mib1)

D.  BAF47 (specific for the gene product of hSNF5/INI1 gene

E.  Neurofilament protein cocktail and Neu-N

Answer and Discussion: The answer is (A). EMA is often but not always expressed in ependymomas. In particularly, they are frequently found in ependymal canals and the luminal surface of papillary component of ependymomas. While positive immunoreactivity for p53 is rather uncommon in ependymoma, they can be demonstrated in about 60% of diffuse astrocytomas. Therefore positive immunoreactivity is more in favor of astrocytoma but negative immunoreactivity cannto entirely exclude the possibility of an astrocytoma. Both astrocytoma and ependymoma are strongly positive for GFAP. Ependymoma can be focally positive for cytokeratin. However, immunoreactivity for cytokeratin AE1 and AE3 often produce positive immunoreactivites in astrocytic tumor which render these antibodies useless for this purpose. Ki67 (Mib1) only tells the proliferation rate but not the phenotype of these tumors. BAF47 is negative in atypical teratoid/rhabdoid tumor (AT/RT) but is positive in both astrocytomas and ependymomas. Neurofilament protiens, Neu-N, and other neuronal or neuroendocrine markers are negative in both astrocytomas and ependymomas. The last three choices do not help in differential diagnosis.