Primary adenocarcinoma of the duodenum is a rare neoplasm that is often microsatellite instability-high (MSI-H). Pembrolizumab, a monoclonal antibody, has been recently approved in Japan for treatment of MSI-H solid tumors. Lynch syndrome is a frequent hereditary cancer predisposition syndrome. It is linked to an increased risk of various types of cancer, including colorectal and endometrial cancer, and is closely related to MSI-H. We present the case of a 55-year-old woman who was diagnosed with duodenal cancer. Biopsy findings revealed MSI-H, and pembrolizumab therapy was initiated because the tumor was in contact with the left renal vein and had metastasized to the mesenteric lymph nodes of the small intestine. Subsequently, after completing two courses of pembrolizumab therapy, the patient developed duodenal stenosis and underwent surgery. Pathological analysis of the resected specimen revealed no evidence of malignancy. Given the patient’s previous cancer history and the occurrence of cancer in close relatives, genetic testing of peripheral blood was performed, which revealed the diagnosis of Lynch syndrome. Furthermore, the variant responsible for Lynch syndrome was found to be a mutation of NM_000251.3:c.211 + 1G > C in MSH2.
