Rare coincidence of intraductal papillary mucinous neoplasm and type 1 autoimmune pancreatitis.

The present case involved a 60-year-old man with autoimmune pancreatitis (AIP). While his AIP was in remission for 6 years, a follow-up CE-CT revealed a dilated main pancreatic duct (MPD) and an enhanced mural nodule. Fluorodeoxyglucose uptake was positive on positron emission tomography. Endoscopic retrograde pancreatography showed a filling defect of the MPD, and pancreatoscopy revealed a nodule partially covered with papillary lesions. Although a repeat biopsy revealed no evidence of malignancy, we speculated that there was a high likelihood of main-duct-type intraductal papillary mucinous neoplasm (IPMN)-derived carcinoma concomitant with AIP. Subsequently, the patient underwent subtotal stomach-preserving pancreaticoduodenectomy. A surgical specimen showed a 35 mm protuberant papillary lesion with abundant stroma, located in the main duct of the pancreas. Further histological evaluation revealed that the nodule was predominantly composed of IPMN with low-grade dysplasia, which was accompanied by abundant IgG4-positive lymphoplasmacytic infiltration, and fibrosis existed predominantly around the IPMN. The epithelium of the cyst showed mucinous hyperplasia with focal papillary structures of gastric phenotype (MUC5A+, MUC6+, MUC1-, MUC2-, CDX-). After surgical resection, we did not find any imaging evidence suggesting a recurrent tumor and AIP relapse in the remnant pancreas. In conclusion, we report a case of IPMN coincidentally found in a patient with type 1 AIP. Active AIP may exaggerate the morphology of IPMN, and careful evaluation should be performed to select appropriate management.