HER2 status in extrahepatic cholangiocarcinoma and gallbladder carcinoma: Concordance between immunohistochemistry and chromogenic in situ hybridization in 140 Cases.

HER2 expression was 0 in 72.9% of cases, 1+ in 12.1%, 2+ in 8.6%, and 3+ in 6.4%.

HER2 positivity was observed in 9.2% of eCCAs and 9.4% of GBCs.

ERBB2 amplification was consistently identified in all immunohistochemistry 3+ tumors and in a subset of immunohistochemistry 2+ and, rarely, 1+ cases.

HER2 expression frequently showed intratumoral heterogeneity, often involving less than 50% of tumor cells.