Autoimmune Pancreatitis Mimicking Pancreatic Cancer: It Can Happen

Autoimmune pancreatitis (AIP) is a steroid-responsive chronic fibro-inflammatory disease of the pancreas. AIP was initially described by Sarles et al. as “primary inflammatory sclerosis of the pancreas”. We present a unique case of AIP mimicking a pancreatic malignancy. A 70-year-old female presented to the emergency department with abdominal pain. Physical exam was unremarkable except for generalized abdominal tenderness. The murphy's sign was negative. Laboratory workup, including serum lipase, liver function tests, serum IgG-4 levels, and serum bilirubin were within normal limits. A Computed tomography (CT) and magnetic resonance imaging (MRI) of abdomen revealed a 2.2 cm mass in the pancreatic tail along with occlusion of splenic vein at the same level (figure 1). Gastroenterology was consulted and an endoscopic ultrasound (EUS) was performed which showed a mass in pancreatic tail and possible splenic involvement. EUS guided fine needle aspiration (FNA) showed normal pancreatic tissue. However, due to persistent concerns for malignancy and after discussion with the patient, decision was made to pursue surgery. A laparoscopic surgery was performed which was later converted to an open distal pancreatectomy and splenectomy due to intra-operative splenic laceration. Pathology of pancreatic tissue revealed sclerosing fibrotic tissue (figure 2), lymphoplasmocytic infiltrates and > 50 cells/HPF of IgG4 positive cells (figure 3), consistent with a diagnosis of IgG-4 mediated AIP. The patient did well post-operatively and was discharged with outpatient follow up. Pharmacotherapy was not initiated since no objective follow-up data was available to assess treatment response. Follow-up MRI of abdomen three months after surgery showed no evidence of disease recurrence. Although the most common presentation of AIP is obstructive jaundice, our patient presented with an incidental finding of pancreatic tail mass. The diagnosis of this radiographic entity remains a diagnostic dilemma as it can mimic pancreatic cancer. Almost one third of patients with focal AIP underwent pancreatic resection due to presumed diagnosis of pancreatic cancer on imaging. Type 1 AIP can present with normal serum IgG4 levels, as was the case in our patient. Steroids are the corner stone in treatment, however, initiation of therapy is not suggested if no objective data for follow-up is available.Figure: A 2.2 cm mass in the pancreatic tail is visualized under abdomen MRI.Figure: Inflammatory infiltrates (red cursor) and fibrotic tissue (blue cursor) under H&E stain (40x magnification).Figure: Plasma cells stained positive under IgG4 Immunohistochemical stain (100x magnification).