Pleomorphic carcinoma of the small bowel. The limitations of immunohistochemical specificity.

We report 3 cases of poorly differentiated tumors of the small bowel with histological, immunohistochemical, and ultrastructural studies. The patients were male, aged 45, 57, and 63. In all 3 cases, histological features of spindle cell, epithelioid cell and giant cell areas favoured a diagnosis of carcinoma, although a malignant stromal tumor could not be firmly excluded. Immunohistochemistry demonstrated in the 3 cases a strong expression of both "epithelial" (cytokeratin) and "stromal" (vimentin) markers; one tumor expressed the epithelial membrane antigen, and another one desmin. Electron microscopy showed no specific features in one case. The case positive for desmin demonstrated intracytoplasmic lumina, allowing the diagnosis of carcinoma. In spite of a non-specific immunohistochemical pattern, we finally considered these 3 tumors as of epithelial origin, corresponding to the rare and recently described pleomorphic carcinoma of the small bowel. This report emphasizes the difficult diagnosis of some poorly differentiated tumors, particularly in the gastro-intestinal tract. Such problems had until recently been resolved by ultrastructural and mostly by immunohistochemical studies. However, an increasing number of reports, together with our 3 cases, show unexpected reactivity of tumors with theoretically specific immunoreactions, such as those directed against intermediate filaments. Coexpression of intermediate filaments could be due to cross reactivity of molecules bearing common epitopes, or to the presence of different filaments in the same cell type; recent immunoblotting studies favour this latter hypothesis.