A case of autoimmune pancreatitis showing narrowing of the main pancreatic duct after cessation of steroid therapy in the clinical course.

Autoimmune pancreatitis (AIP) is characterized by irregular narrowing of the main pancreatic duct (MPD), and narrowing of the MPD is an essential criterion for the diagnosis of focal/segmental AIP according to the Japanese diagnostic criteria 2011. We report a 55-year-old man in whom magnetic resonance imaging showed two masses in the pancreatic head and tail and whose pancreatogram was normal at first. However, the two masses changed into diffuse swelling of the pancreas, and diffuse narrowing of the MPD appeared after the cessation of steroid therapy. Finally, we could diagnose this patient as having AIP according to the Japanese diagnostic criteria.