Case Report: IgG4-Related Pancreatic and Bile Duct Masses Mimicking Malignancy: A Diagnostic Trap Uncovered

Background: IgG4-Related Disease (IgG4-RD) is a systemic fibroinflammatory condition that can involve the pancreas and biliary tree, often mimicking malignancy. Case Presentation: A 78-year-old male with coronary artery disease, type 2 diabetes mellitus, and prior colon polyps presented with jaundice, significant weight loss, and abnormal liver function tests. Imaging revealed a pancreatic tail mass and proximal common bile duct stricture, initially raising concern for malignancy. Endoscopic ultrasound-guided biopsies demonstrated storiform fibrosis, obliterative phlebitis, and increased IgG4-positive plasma cells, confirming IgG4-RD. The patient was treated with corticosteroids with initial improvement, but relapse occurred after stent removal 70 days later. He was subsequently started on rituximab with plans for repeat dosing at six months.