Composite small cell and mucinous carcinoma originating from the intrahepatic bile duct: report of a case.

The biliary tract is a very rare site for the occurrence of extrapulmonary small cell carcinoma. A 68-year-old Japanese female was being followed up for autoimmune hepatitis, and was referred to our hospital because segmental intrahepatic bile duct dilation was found on routine imaging studies, suggesting intrahepatic cholangiocarcinoma. She underwent left lobectomy of the liver and concomitant resection of the caudate lobe. Microscopic examination of the explanted liver showed a primary composite tumor comprising small cell and mucinous carcinomas that originated in the intrahepatic bile duct. Further immunohistochemical studies, including cytokeratin-19 and chromogranin-A staining, showed the two cellular components of the tumor to have similar characteristics. The amphicrine properties indicated that the tumor had a monoclonal origin but with biphenotypic differentiation, which was responsible for the histogenesis of this tumor.