Cholangiocarcinoma is a biliary malignant tumor which can arise at any point of biliary tree. Surgical resection is the only curative treatment and chemotherapy is used for unresectable cases, but its prognosis is poor compared with other types of cancer. Recently, pembrolizumab (PEM), an anti-programmed cell death protein 1 (PD-1) antibody, has become available for microsatellite instability (MSI)-high advanced cancers. Here, we report the use of PEM for MSI-high locally advanced cholangiocarcinoma. A 57-year-old man presented to our department with jaundice. Contrast-enhanced computed tomography showed a solitary 28-mm-diameter tumor deep in the anterior segment of the liver. Endoscopic retrograde cholangiopancreatography and intraductal ultrasonography showed narrowing of the common bile duct and absence of contrast in the right hepatic duct, and tumor invaded from hilar region of liver into left hepatic duct. We diagnosed this as double primary cancers, locally advanced intrahepatic and distal cholangiocarcinomas. The patient began gemcitabine in combination with cisplatin therapy as first-line treatment and gemcitabine in combination with S-1 therapy as second-line treatment. However, the tumor gradually grew (maximum 69 mm), intrahepatic metastasis appeared, and tumor marker increased. Because MSI-high was confirmed not only by biopsy specimens but also by liquid biopsy, the patient began PEM (200 mg per every 3 weeks). After 15 cycles of PEM were administered over about 10 months, size of tumor was reduced and tumor marker dramatically decreased. We experienced the rare case which PEM has been successfully used for MSI-high double primary cancers.
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