Coexisting Type 1 Autoimmune Pancreatitis and Mixed-type Intraductal Papillary Mucinous Neoplasm.

An 84-year-old man was referred to our hospital for a cystic lesion of the pancreatic head, swelling of the pancreatic tail and hilar biliary stricture, resulting in elevated liver enzyme levels. We suspected branch duct-type intraductal papillary mucinous neoplasm (IPMN) and type I autoimmune pancreatitis (AIP) associated with sclerosing cholangitis because of the high serum IgG4 levels. The main pancreatic duct on the tail side of the AIP lesion was moderately dilated. Although the biliary stricture and pancreatic swelling improved after prednisolone treatment, the pancreatic enzyme levels increased rapidly. The entire main pancreatic duct exhibited remarkable dilatation, which led to the diagnosis of mixed-type IPMN. The clinical characteristics of IPMN in the main pancreatic duct appear to have been initially masked by AIP.