Type 1 Autoimmune Pancreatitis with Imaging Appearance Similar to That of Malignant Cystic Tumor

A 79-year-old man was admitted with asymptomatic elevation of liver enzymes and tumor markers. Abdominal contrast-enhanced computed tomography demonstrated swelling of the pancreatic head, and additional blood test showed raised IgG4 levels. Histological examination by endoscopic ultrasonography (EUS)-guided fine needle aspiration for pancreatic head mass revealed storiform fibrosis and IgG4-positive plasma cell infiltration. We diagnosed this case as type 1 autoimmune pancreatitis (AIP). In addition, there was a cystic lesion in the pancreatic body apart from the pancreatic head mass. A mural nodule in the multilocular cyst was detected by EUS, and there was positive uptake of fluorodeoxyglucose in positron emission tomography/magnetic resonance imaging. The preoperative diagnosis of this cystic lesion was intraductal papillary mucinous carcinoma, and distal pancreatomy was performed. Histopathological findings showed various sizes of retention cysts caused by IgG4-positive plasma cell infiltration around the pancreatic branch ducts. The mural nodule was a fibrotic mass with diffuse infiltration of IgG4-positive cells. This cystic lesion mimicking malignant cystic neoplasm occurred in relation to AIP. This case provided important information helping to understand the mechanism of formation of mural nodules in multilocular cysts in patients with type 1 AIP.