Mucin-positive epithelial mesothelioma of the peritoneum: Small bowel involvement

Mucin‐positive epithelial mesothelioma has been reported in the peritoneum only once, and that mainly involved the stomach wall. We report the second peritoneal case and this is the first case mainly involving the small bowel wall. A 65‐year‐old man showed diffuse involvement from the duodenum to the ileum and metastatic masses in the left adrenal gland. Segmental resection of the small bowel was performed; 2 months later the patient died. Light microscopy showed diffusely anaplastic epithelioid cell proliferation and foci of glandular formation with granular mucinous materials in the cytoplasmic vacuoles or within glandular lumina. Histochemically, these mucin materials were PAS‐positive and diastase‐resistant. Immunohistochemically, the various mesothelial markers were positive, and a few adenocarcinoma markers were focally positive. Ultrastructurally, the tumor cells showed long slender microvilli on the apical surface, consistent with mesothelioma. Electron microscopy can play a decisive role in the case of ambiguous histochemical and immunohistochemical results.