Which markers comprise the IHC panel for primary cholangiocarcinoma?

• A. CA19-9, CK7, CK20
• B. CA19-9, CK7, CK20, CK5/6
• C. CA19-9, CK7
• D. CK7, CK20, CK5/6

Answer: (A)

Immunohistochemistry for cholangiocarcinoma relies on markers that reflect biliary epithelial origin and help distinguish primary biliary tumors from metastases. CK7 is a key cytokeratin seen in biliary epithelium, so cholangiocarcinomas typically show CK7 positivity. CA19-9 is a tumor-associated antigen that is often elevated in biliary cancers, supporting a biliary origin when seen in the right clinical context. CK20 adds a critical differentiator: colorectal cancers usually express CK20, often with CK7 negativity, so a CK7-positive/CK20 profile rather than CK20-positive alone helps favor a biliary source over colorectal metastasis. Including CK20 in the panel also helps rule out colorectal or other CK20-dominant primaries when CK7 is present. Markers like CK5/6 are not characteristic for cholangiocarcinoma and are less informative in this context. Therefore, CA19-9, CK7, and CK20 form the most informative panel for suggesting primary cholangiocarcinoma and distinguishing it from colorectal metastasis.