IgG4-related autoimmune pancreatitis mimicking pancreatic carcinoma: a case report

A 59-year-old woman presented with a 5-month history of an incidentally discovered pancreatic mass and episodic epigastric pain. Imaging studies revealed a mass in the pancreatic head/neck and tail with suspected vascular encasement, and the tumor marker CA19-9 was markedly elevated. Due to these findings, pancreatic malignancy was initially suspected. However, serum IgG4 was found to be extremely elevated at 10,022 μg/mL, and an ultrasound-guided pancreatic biopsy showed benign ductal epithelium with low proliferative activity and occasional plasmacytoid cells, with no evidence of adenocarcinoma. A diagnosis of IgG4-related autoimmune pancreatitis was established. The patient was started on oral methylprednisolone (32 mg/day) and pancreatic enzyme supplements. Over 1 year of follow-up, her serum IgG4 level normalized to 489 μg/mL, CA19-9 returned to 16 U/mL, and serial MRI examinations demonstrated marked resolution of the pancreatic head/neck mass and improvement of the biliary stricture. This case underscores the clinical importance of including IgG4-AIP in the differential diagnosis of pancreatic masses, particularly when serologic or imaging features are equivocal, to prevent unnecessary surgical intervention.