Serum IgG4-Negative Focal Autoimmune Pancreatitis Type 1 That Was Difficult to Diagnose Preoperatively Even with Frequent Endoscopic Ultrasound-Guided Fine-Needle Aspiration and Fine-Needle Biopsy: A Surgical Case Report

Introduction: Focal autoimmune pancreatitis (AIP) without elevated serum IgG4 levels presents a diagnostic challenge compared to pancreatic tumors, often leading to surgical intervention. Case Presentation: We report a case of serum IgG4-negative focal AIP type 1 in a 52-year-old male. Despite repeated endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) and fine-needle biopsy (FNB), preoperative diagnosis was elusive. Initially, a 30-mm hypoechoic mass in the pancreatic head was detected by ultrasonography, with dynamic computed tomography revealing well-defined borders and homogeneous delayed enhancement. Serum IgG4 levels were within the normal range (115 mg/dL). Two EUS-FNAs and one EUS-FNB failed to provide a definitive diagnosis, leading to suspicion of a solid pseudopapillary neoplasm and subsequent pancreaticoduodenectomy. Postoperative histopathology confirmed focal AIP type 1 with IgG4-positive plasma cells. After 38 months, there is no recurrence, and serum IgG4 levels remain normal. Conclusion: Diagnosis of focal AIP, particularly when serum IgG4 is negative, warrants consideration despite its difficulty. Imaging findings, such as a well-defined mass with homogeneous delayed enhancement, should prompt evaluation for characteristic features like capsule-like rim, pancreatic duct penetration, and biliary tract wall thickening.