Cytokeratin immunoreactivity in Ewing's sarcoma - Prevalence in 50 cases confirmed by molecular diagnostic studies

Ewing's sarcoma (ES) and primitive neuroectodermal tumor (PNET) are charact erized by the presence of the specific t(11;22)(q24;q12) or variants thereo f, producing diagnostic EWS fusion transcripts. Cytokeratin has been report ed sporadically to be expressed in some cases of ES/PNET. However, its prev alence has not been assessed systematically in a series of cases with confi rmatory molecular or cytogenetic evidence of a diagnostic translocation. We present in detail three index patients in whom strong cytokeratin immunore activity was a confounding factor in the diagnosis. To establish further th e prevalence of cytokeratin immunoreactivity in a series of well-characteri zed ES/PNET, we then performed immunohistochemical studies with antibodies CAM5.2 and AE1/AE3 on 50 cases of ES/PNET diagnosed at Memorial Sloan-Kette ring Cancer Center in which molecular evidence of a specific ES/PNET-associ ated translocation were available. Immunoreactivity to cytokeratin was pres ent in 10 cases (20%), in five diffusely and five focally. There was no sig nificant association between cytokeratin expression and the following param eters: patient age, sex, skeletal and extraskeletal primary site, and the t ype of EWS fusion transcript. Cytokeratin expression, a manifestation of ep ithelial differentiation, is present in as many as 20% of ES/PNET in either a diffuse or focal pattern.