A case of focal autoimmune pancreatitis (AIP) mimicking an intraductal papillary mucinous neoplasm (IPMN).

The present case involved a 76-year-old man with a cystic mass in the head of his pancreas. The cystic lesion, which measured 17.7 × 9.8 mm, was first detected by ultrasonography (US) at the age of 72 years. Follow-up endoscopic ultrasonography (EUS) performed at 4 years after the lesion had first been detected revealed a mural nodule measuring 14.0 × 8.4 mm in the cyst. Endoscopic retrograde pancreatography (ERP) imaging revealed that the main pancreatic duct was in communication with the cyst and that there was no irregular narrowing of the main pancreatic duct. On the basis of these results, the patient was diagnosed with an intraductal papillary mucinous neoplasm (IPMN), and stomach-preserving pancreaticoduodenectomy was performed. A histopathological examination revealed that the interior of the cystic part of the lesion was lined by a pancreatic ductal epithelium. A pathological examination of the nodular lesion detected storiform fibrosis, severe lymphoplasmacytic infiltration, and hyperplasia in the pancreatic duct epithelium together with a small amount of mucus. On immunohistological staining, the infiltrating lymphoplasmacytes were found to be positive for IgG4. Accordingly, the patient was diagnosed with focal autoimmune pancreatitis (AIP). In conclusion, we reported a case of focal AIP mimicking IPMN. This case showed neither enlargement of the pancreas nor irregular narrowing of the main pancreatic duct.