The Great Masquerader: Autoimmune Pancreatitis Presenting as an Obstructive Pancreatic Head Mass and the Avoidance of a Whipple Procedure

Autoimmune pancreatitis (AIP) is a unique, steroid-responsive focal
form of chronic pancreatitis, particularly in the pancreatic head, often
presents as a mass-forming lesion with obstructive jaundice, perfectly
mimicking pancreatic adenocarcinoma. Distinguishing between these
two entities is paramount, as their management is radically different. A
45-year-old male presented with a three-week history of painless
jaundice, pruritus, and weight loss. Computed Tomography (CT)
revealed a hypodense mass in the pancreatic head measuring 3.5 cm,
with abrupt cutoff of the common bile duct and double-duct sign,
accompanied by proximal biliary and pancreatic duct dilatation. Serum
Carbohydrate Antigen 19-9 (CA 19-9) was elevated at 250 U/mL. The
patient was scheduled for a pancreaticoduodenectomy (Whipple
procedure). Prior to surgery, a review of the CT by a dedicated
pancreatic radiologist noted a subtle “halo sign” around the pancreas.
This prompted further testing, which revealed a serum IgG4 level of 650
mg/dL. A FNA cytology via ultrasonogram showed lymphoplasmacytic
infiltration and >50 IgG4-positive plasma cells per high-power field,
consistent with AIP Type 1. The Whipple procedure was canceled. The
patient was started on oral prednisone (40 mg/day). His jaundice
resolved within two weeks. A repeat CT scan after 4 weeks of therapy
showed a dramatic reduction in the size of the pancreatic head mass
and resolution of the biliary dilatation. CA 19-9 normalized. The steroid
dose was tapered, and the patient was maintained on azathioprine. He
remains asymptomatic at one-year follow-up. This case highlights that
AIP must be a mandatory consideration in every patient with a
suspected pancreatic head malignancy. The presence of a “halo sign”
on imaging and elevated serum IgG4 is crucial diagnostic red flags. A
definitive tissue diagnosis is often necessary to prevent unnecessary and
morbid surgery. A high index of suspicion and a multidisciplinary
approach are essential.