The following CT findings were discussed in favor of focal autoimmune pancreatitis (versus pancreatic carcinoma) with the hepatobiliary surgeons:

• no significant differential enhancement of the head cf. body and tail
• mild CBD dilatation with lack of upstream main pancreatic duct dilatation
• no distal pancreatic atrophy
• no infiltration, or encasement, of peripancreatic fat, vessels, etc.
• no convincing metastases 

Abrupt narrowing of the CBD with wall thickening and enlarged peripancreatic lymph nodes raised suspicion of malignancy, although here, lymph node enhancement was much more intense than that seen in carcinoma.

Ampullary biopsy performed.
Immunohistochemistry highlighted a mixture of CD3- and CD20-positive T and B lymphocytes and numerous CD138- and IgG-positive plasma cells (150/hpf).

IgG4 was positive in scattered plasma cells (8/hpf).

IgG4/IgG ratio was 0.05.

The patient was started on steroids and the bilirubin level returned to normal.

The follow-up CT 6 months later showed near-total resolution of the apparent mass and resolution of the biliary dilatation.