Autoimmune pancreatitis limited to the main pancreatic duct without mass formation.

We present a rare case of autoimmune pancreatitis (AIP) confined exclusively to the subepithelial layer in the main pancreatic duct (MPD) without associated pancreatic enlargement or mass formation. A 73-year-old man presented with elevated serum pancreatic enzymes and IgG4 levels. Imaging revealed a short-segment MPD stenosis in the body of the pancreas without pancreatic swelling. Endoscopic ultrasonography showed localized periductal thickening of the MPD, raising suspicion of pancreatic carcinoma in situ (CIS). Cytological analysis of pancreatic juice identified atypical cells, prompting surgical resection. Histopathological examination of the resected specimen revealed characteristic features of type 1 AIP, including dense lymphoplasmacytic infiltration, storiform fibrosis, and obliterative phlebitis, which were strictly confined to the subepithelial layer in the MPD. No malignant cells were found, and the surrounding pancreatic parenchyma showed only minimal fibrotic changes. This case may broaden the recognized clinical spectrum of AIP by demonstrating a rare presentation without mass formation or pancreatic swelling, which closely mimicked pancreatic CIS. Further accumulation of cases may enhance our comprehensive understanding of this disease entity.