Background: We experienced a rare case of intraductal papillary mucinous neoplasm arising from Santorini's duct (SD) forming a tumor protruding into the duodenum. Case presentation: A 71-year-old woman was incidentally diagnosed with a 3 cm type Isp polypoid tumor in the second portion of the duodenum at another hospital. Enhanced CT and endoscopic ultrasound revealed that the origin of this protruding tumor was arising from SD and that the tumor mimicked a pedunculated duodenal tumor. Our preoperative diagnosis was a malignant pancreatic tumor arising from SD with invasion into the duodenum. She underwent a subtotal stomach-preserving pancreaticoduodenectomy, and the resected specimen showed a 25 mm tumor protruding into the duodenum with a villous surface. The pathological findings revealed that the tumor was intraductal papillary mucinous adenoma (IPMA) arising from SD. Conclusions: To the best of our knowledge, this is the first case of IPMA protruding into the duodenal lumen from SD, although most of the tumors arising from SD have been reported to be malignant.
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