Granulomatosis with Polyangiitis (Wegener Granulomatosis) Presenting as a Pancreatic Mass: A Rare Malignancy Mimic

BACKGROUND Granulomatosis with polyangiitis (GPA), formerly known as Wegener granulomatosis, is a rare autoimmune vasculitis that predominantly affects the respiratory tract and kidneys. Pancreatic involvement is exceedingly uncommon and often misinterpreted as malignancy because of overlapping clinical, radiological, and cytological features. CASE REPORT A 62-year-old man presented with low-grade fever, abdominal bloating, and weight loss for 3 months. Positron emission tomography-computed tomography (PET-CT) revealed a metabolically active mass in the pancreatic body and tail with fluorodeoxyglucose-avid hilar and right cervical lymphadenopathy, raising suspicion of metastatic pancreatic adenocarcinoma. Initial endoscopic ultrasound-guided fine-needle aspiration was interpreted as consistent with well-differentiated adenocarcinoma, representing a false-positive cytologic diagnosis in retrospect, and the patient received 1 cycle of chemotherapy. On referral, systemic evaluation revealed hemoptysis and renal dysfunction, and renal biopsy demonstrated pauci-immune crescentic glomerulonephritis in the setting of high-titer PR3-ANCA positivity, which constituted the decisive diagnostic inflection point overturning the initial malignancy diagnosis. Biopsy from the cervical lump revealed inflamed submandibular gland tissue. A diagnosis of GPA with rare pancreatic involvement was made. The patient responded remarkably to rituximab and corticosteroids, with near-complete disappearance of the pancreatic mass on follow-up imaging. CONCLUSIONS This case exemplifies the diagnostic pitfalls when GPA presents as an isolated pancreatic mass. Fluorodeoxyglucose-PET findings and atypical cytology can mimic adenocarcinoma, leading to inappropriate therapy. Immunologic assays, including C-ANCA, IgG4, and complement levels, alongside histopathological evaluation from multiple sites, are essential for accurate differentiation. Literature review reveals fewer than 15 similar cases, many subjected to unnecessary resections. In patients with systemic features and pancreatic lesions, GPA should be considered a differential diagnosis. Early immunologic testing and multidisciplinary evaluation are key to avoiding misdiagnosis.