Case Presentation: A 44 year old gentleman with a past medical history significant for Crohn’s disease presented for evaluation of painless jaundice and a pancreatic mass in the setting of significant unintentional weight loss and recent flu-like symptoms including night sweats, low-grade fever, and dizziness.  Prior to admission, the patient underwent computerized tomography of the abdomen indicating the presence of a 4.7x4x2.8cm pancreatic mass. Days later, endoscopic retrograde cholangiopancreatography did not demonstrate any evidence of the previously described mass. The patient was admitted to this institution for a second opinion and was found to have mild jaundice (total bilirubin 3.4 mg/dL, direct bilirubin 2 mg/dL), transaminitis (alanine aminotransferase 752 U/L, aspartate aminotransferase 1142 U/L), and elevated lipase (306 U/L). Endoscopic ultrasound did not detect a mass but did visualize several enlarged lymph nodes. Lymph node biopsy was performed with pathology revealing sheets of non‑reactive lymphocytes concerning for lymphoma.  Serum studies showed elevated total IgG levels with IgG4 levels twice the upper limit of normal. IgG4 staining of the lymph node revealed rare IgG4‑positive cells, and flow cytometry was notable for polyclonal B cells and a slightly increased CD4 to CD8 ratio. Liver biopsy, motivated by the patient’s persistent transaminitis, was consistent with active autoimmune hepatitis and demonstrated increased infiltration of IgG4-positive plasma cells.  Prednisone and azathioprine were initiated. The patient responded well to glucocorticoid therapy.  Post-treatment, liver function labs were normalizing, and constitutional symptoms had resolved.

Discussion: This case exemplifies the value in considering autoimmune pathology in a patient whose presentation initially portended malignancy.  Although IgG4-related disease is relatively uncommon, it has been linked to autoimmune pancreatitis, inflammatory pseudotumor, autoimmune hepatitis, and tubulointerstitial nephritis. Maintaining a high index of suspicion and pursuing evaluation of IgG4-related disease in a symptomatic patient facilitates intervention with glucocorticoid therapy and halts progressive end organ damage.

Conclusions:  Seldom recognized in the general practitioner’s initial assessment, autoimmune pancreatitis, specifically IgG4-related disease, may masquerade as pancreatic adenocarcinoma or lymphoma. With a typically impressive response to glucocorticoid therapy, it is important not to overlook this autoimmune phenomenon in the workup of a pancreatic mass.