A 75-year-old man presented with upper abdominal pain. CT and MRI showed cystic lesions 27 mm in size with no malignant findings in the head of the pancreas. Under the diagnosis of intraductal papillary mucinous neoplasm (IPMN), the patient was followed-up with imaging studies. Six years after the initial presentation, a contrast CT scan showed an enhanced mural nodule 10 mm in size in the cyst, and the patient was referred to our hospital for further examination. CT and MRI revealed the cysts had shrunk compared to previous imaging findings. Pancreatic juice cytology showed no malignant cells. Based on detailed examination, a diagnosis of IPMN with high risk stigmata was made and laparoscopy-assisted pancreatoduodenectomy was performed. A histopathological examination revealed a cystic lesion consisting of IPMN with low-grade dysplasia, inflammatory infiltrates with lymphoplasmacytes corresponded to nodules in cysts, obliterative phlebitides, and storiform fibrosis. Immunohistochemical staining demonstrated that the infiltrating lymphoplasmacytes were positive for IgG4 and that IgG4+/IgG+ plasm cell ratio was over 40%. Finally, we diagnosed branch duct type IPMN associated with autoimmune pancreatitis type-1, mimicking an enhanced mural nodule. We encountered a rare case of IPMN associated with autoimmune pancreatitis. It can be suggested that serological and histological examinations, including blood IgG4 testing and ultrasound-guided fine-needle aspiration, should be considered in patients with IPMN that show rapid appearance of enhanced nodules and shrinkage of cysts.
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