Microscopic description and IHC:

The liver biopsy is clearly infiltrated by a tumor composed of mucin secreting glands. The tumor cells are positive for CK20, CDX2; and negative for CK7 and TTF-1. the immunohistochemistry is consistent with previously diagnosed lesion, which was actually cholangiocarcinoma.

Discussion:

CK7/20 are useful in the setting of tumor of unknown primary. CK7-/CK20+ tumors are classically colorectal but can be seen in cholangiocarcinoma, intestinal type. Just like everything else in pathology, exceptions do happen.

Table depicting various stain results
References:

Gyure, Kymberly A., and Alan L. Morrison. "Cytokeratin 7 and 20 expression in choroid plexus tumors: utility in differentiating these neoplasms from metastatic carcinomas." Modern Pathology 13.6 (2000): 638-643.

Rullier, Anne, et al. "Cytokeratin 7 and 20 expression in cholangiocarcinomas varies along the biliary tract but still differs from that in colorectal carcinoma metastasis." The American journal of surgical pathology 24.6 (2000): 870-876.

CASE IMAGES