Pancreatic ductal adenocarcinoma with mismatch repair deficiency resected after long-term observation.

Pancreatic ductal adenocarcinoma (PDAC) with mismatch repair (MMR) deficiency is a rare subtype, clinicopathological features of which have not been fully understood. A 70-year-old woman was admitted for the investigation of a 20-mm pancreatic tumor in the pancreatic head, detected during the cause scrutiny of exacerbation of diabetes mellitus and panhypopituitarism. The tumor decreased in size after administration of hydrocortisone for panhypopituitarism. Autoimmune pancreatitis, complicated with hypophysitis, was suspected, and prednisolone treatment was administered. The tumor did not show enlargement for 3 years during which a dose of prednisolone was maintained. However, 1.5 years after the cessation of prednisolone administration, the tumor size increased again. On endoscopic ultrasonography, the tumor was found to be a 25.2-mm mass lesion with almost uniformly low echogenicity and blood flow signal, and anisonucleosis on cytodiagnosis was revealed. Pancreatoduodenectomy was performed, and on histological analysis, moderately differentiated tubular adenocarcinoma with massive lymphocytic infiltration was observed. Immunohistochemistry revealed a concomitant loss of MSH2 and MSH6 in the tumor cells, which implicated mutant MSH2 gene. She has remained well with no recurrence for 2.9 years since her surgery. We herein report a case of PDAC with MMR deficiency, resected after long-term observation.