Cholangiocarcinoma - Morphology, Immunohistochemistry, and Genetics

Cholangiocellular carcinoma (cholangiocarcinoma, CCA) is a heterogeneous group of malignant epithelial tumors of the bile ducts, with varying classifications and molecular characteristics. CCA can arise in intrahepatic, perihilar, or extrahepatic bile ducts, with its incidence rising globally, particularly in Southeast Asia due to liver fluke infections. Other risk factors include primary sclerosing cholangitis, viral hepatitis, gallstones, congenital bile duct malformations, and cirrhosis. CCA is often diagnosed at advanced stages and has a poor prognosis. This article summarizes the complex classification systems of CCA and biliary precancerous lesions (biliary intraepithelial neoplasia, BilIN), focusing on morphology, molecular profiles, and clinical implications. It briefly addresses the differential diagnosis between CCA and BilIN, distinguishing intrahepatic from extrahepatic CCA, as well as differentiating CCA from hepatocellular carcinoma (HCC) and metastatic adenocarcinomas. Alongside selected literature, experiences from our institution using various immunohistochemical methods (CRP, S100P, IMP3) are presented, highlighting their relevance in routine practice. The majority of the article focuses on the molecular pathology of CCA, where mutation profiles differ according to anatomical subtypes. Intrahepatic CCA more frequently harbors mutations in IDH1/2, FGFR, and BAP1, while perihilar and extrahepatic CCAs are more likely to exhibit mutations in TP53, KRAS, and ERBB2. Alterations in FGFR2 and IDH1 are associated with better prognosis, while TP53 and KRAS mutations indicate worse outcomes. The article provides an overview of genetic alterations that are targetable with current oncological therapies, including FDA-approved inhibitors for FGFR2 (pemigatinib, futibatinib) and IDH1 (ivosidenib), along with inhibitors targeting BRAF, HER2, NTRK, and immunotherapies for MSI-high and TMB-high tumors. Intrahepatic CCA presents a broader spectrum of therapeutic targets, including rare fusions (ALK, RET), compared to perihilar and extrahepatic CCA, which share a poor prognosis and limited therapeutic options with pancreatic cancer. In this regard, intrahepatic CCA may become the "non-small cell lung cancer of gastrointestinal oncology."