Biliary cystadenocarcinoma with two types of tumour cells.

We report a rare case of biliary cystadenocarcinoma that occurred in the left hepatic lobe of a 62-year-old man and measured 20 cm in its greatest dimension. The neoplastic epithelium consisted of two types of cells: (1) cells with clear cytoplasm containing abundant mucin, and (2) cells with eosinophilic cytoplasm, which in some areas formed nodules with hepatocytoid features (polygonal cell shape, large nuclei with prominent nucleoli, and pseudoglandular structures). Histochemical stains revealed the presence of cytoplasmic mucin in the hepatocytoid areas, whereas immunohistochemical stains clearly showed a biliary phenotype (diffuse positive staining for "biliary type" cytokeratins, rare foci of positive staining with antibody to human hepatocytes (HEP-PAR1), absence of staining for alpha-fetoprotein, and no evidence of canalicular pattern of staining with polyclonal antibody to carcinoembryonic antigen). These findings indicate that areas reminiscent of hepatocellular carcinoma may occur in biliary cystadenocarcinomas. Histochemical and immunohistochemical stains are useful in reaching a definitive diagnosis in such cases.