Polypoid and Cystic Presentation of Type 1 Autoimmune Pancreatitis at the Duodenal Papilla: A Diagnostic Pitfall Mimicking Intraductal Papillary Mucinous Neoplasm.

We report a rare case of type 1 autoimmune pancreatitis (AIP) forming a polypoid mass at the major duodenal papilla in continuity with a cystic lesion of the pancreatic head, initially misdiagnosed as an intraductal papillary mucinous neoplasm (IPMN). A man in his 70s underwent pancreaticoduodenectomy after imaging revealed a mural nodule within a cystic lesion. Gross examination revealed a papillary polypoid lesion with continuity to a firm nodule in the pancreatic head. Histologically, the lesion exhibited glandular proliferation with pyloric and Brunner's gland-like differentiation in the fibrotic stroma, with dense IgG4-positive plasma cell infiltration and obliterative phlebitis. An incidental 6-mm high-grade pancreatic intraepithelial neoplasia was also identified adjacent to the IgG4-rich fibroinflammatory lesion. The surrounding pancreas showed acinar atrophy, vanishing duct change, and features consistent with resolving inflammation. The postoperative serum IgG4 levels remained elevated. Awareness of this unusual papillary, lumen-extending morphology may prompt consideration of type 1 AIP in the differential diagnosis and guide preoperative evaluation, including serum IgG4 testing and tissue sampling when clinically feasible.