Intrahepatic clear cell cholangiocarcinoma: immunohistochemical aspects in a very rare type of cholangiocarcinoma.

Clear cell cholangiocarcinoma is a very unusual variant of peripheral bile duct carcinoma. We present 3 cases on which we performed a broad spectrum of immunohistochemical analysis. The tumors showed a glandular and trabecular growth pattern with abundant desmoplastic stroma and clear cell change of about 80% of the tumor cells. Positive expression of CK7 indicated a cholangiocellular origin. A primary hepatocellular carcinoma and metastatic clear cell tumors of the kidney, gastrointestinal tract, and the thyroid gland were excluded by absence of CK20, CD10, HepPar1, and TTF1. No mucin could be detected within the cytoplasm of the clear cells. Electronmicroscopy revealed only a few glycogen granula, but numerous cytoplasmic lipoid vacuoles as a possible explanation for the clear cell phenotype. All 3 tumors exhibited positive expression of CD56 (NCAM) in a significant amount of the clear cells. Beside the clear cell component, one tumor also showed an adenocarcinomalike and a well-differentiated tubular component. CD56 expression was detected in all 3 tumor areas. This report of 3 cases demonstrates that clear cell cholangiocarcinomas are not only of unusual histomorphology. They also show CD56 expression which is a very uncommon finding for intrahepatic cholangiocarcinomas. As CD56 expression is also found in reactive bile ducts and bile duct adenomas, one may speculate that these rare neoplasms may originate from reactive bile ducts or cholangiomatous lesions.