Malignant small-cell epithelial tumor of the peritoneum coexpressing mesenchymal-type intermediate filaments.

A 28-year-old man with a large abdominal mass was found to have a tumor involving the peritoneum that had no other apparent primary origin. By light microscopy, the histologic pattern of the tumor was that of a small-cell epithelial neoplasm. Electron-microscopic examination demonstrated desmosomes and paranuclear aggregates of intermediate filaments. Immunohistochemical studies revealed reactivity for keratin, desmin, and vimentin. The globoid staining observed for the last two markers correlated with the paranuclear concentration of intermediate filaments observed by electron microscopy. We believe this case represents an unusual small-cell epithelial tumor, expressing mesenchymal-type intermediate filaments, that should be distinguished from other small-cell neoplasms.