Keratin in epithelioid sarcoma. An immunohistochemical study.

Intermediate-sized filaments have been noted in epithelioid sarcoma by previous investigators, two of whom have reported that the filaments represent vimentin. We utilized polyclonal antibodies directed against keratin and immunoperoxidase techniques (PAP) to stain 32 of the more than 300 cases accumulated at the AFIP . All of our material was formalin-fixed, paraffin-embedded. Seventy-five percent of our cases (24/32) showed positive immunoreactivity, a feature that may be of diagnostic help in distinguishing epithelioid sarcoma from modular fasciitis, benign and malignant fibrous histiocytoma, malignant melanoma, and necrotizing granuloma. In these cases, the reaction was enhanced using predigestion with trypsin. The immunoreactivity varied from tumor to tumor, perhaps due to formalin fixation. Since synovial sarcoma and mesothelioma may also be cytokeratin-positive, our findings indicate that keratin immunoreactivity is not confined to epithelial tumors and may also occur in neoplasms traditionally regarded as mesenchymal.