Abstract
Intrahepatic cholangiocarcinoma has a poor prognosis. In unresectable cases, the survival period is short despite combination therapy with cytotoxic anticancer agents and immune checkpoint inhibitors. The usefulness of immune checkpoint inhibitors against malignant tumors with microsatellite instability-high (MSI-H) mutations was shown in the KEYNOTE158 study; however, data for intrahepatic cholangiocarcinoma are insufficient. In the present case, a 65-year-old man with intrahepatic cholangiocarcinoma and lymph node metastasis could not be treated with a combination of gemcitabine, CDDP, and S-1. A comprehensive cancer genomic profiling (CGP) test showed MLH1 pathogenic mutation and MSI-H. When pembrolizumab was administered, the tumor shrinkage effect was rapidly observed, which was sustained even after 30 months. No pathogenic mutations were observed in the germline test, and MSI-high was considered to be due to the MLH1 pathogenic mutation occurring sporadically in somatic cells. MSI-H intrahepatic cholangiocarcinoma is extremely rare. However, because pembrolizumab is expected to be effective, CGP testing should be actively performed.