Serum and histological IgG4-negative type 1 autoimmune pancreatitis

A 66-year-old man who was on oral medication for type 2 diabetes experienced a rapid decline in glycemic control (increase in glycosylated hemoglobin level from 7.7 to 10.2% over 3 months). Abdominal ultrasonography revealed a 20-mm hypoechoic mass in the pancreatic tail. Serum tumor marker carbohydrate antigen 19-9 and DUPAN2 levels were within the respective normal ranges; serum IgG4 level was also normal at 21.8 mg/dL. Abdominal contrast computed tomography revealed a 26-mm tumor in the pancreatic tail. Magnetic resonance cholangiopancreatography revealed disruption of the main pancreatic duct and dilation of the caudal pancreatic duct. Endoscopic ultrasonography revealed a near-round-shaped hypoechoic mass with interspersed hyperechoic areas. Endoscopic ultrasonography-guided fine needle aspiration was performed using a 22-G needle, but no malignant findings were observed. There were no signs of sialadenitis, retroperitoneal fibrosis, nephropathy, or other conditions associated with IgG4-related diseases. Distal pancreatectomy was performed; a 23-mm white mass was resected from the pancreatic tail. A histopathological examination showed advanced inflammatory cell infiltration mainly involving lymphocytes/plasma cells along with storiform fibrosis and obliterative phlebitis. No more than five IgG4-positive cells were observed per high-power field. These were level 1 pathological findings, and a definitive diagnosis of type 1 autoimmune pancreatitis (AIP) was made according to the International Consensus Diagnostic Criteria. Type 1 AIP associated with normal serum IgG4 levels and absence of IgG4-positive cells on histological examination is a rare clinical entity, which is very difficult to distinguish from pancreatic cancer. Here we report such a case and present a review of the relevant literature.