S3382 When IgG4-Related Disease Mimics Metastatic Cancer: A Case of Recurrent Pancreatitis With Mass-Forming Lesions

Introduction: Immunoglobulin G4-related disease (IgG4-RD) is a systemic, immune-mediated fibroinflammatory disorder characterized by tumefactive lesions, storiform fibrosis, and lymphoplasmacytic infiltrates with an abundance of IgG4-positive plasma cells. The wide range of clinical presentations, multi-organ involvement, and tendency to mimic malignancies increase the complexity of diagnosis. Autoimmune pancreatitis (AIP), particularly type 1, is a prototypical manifestation of IgG4-RD, which can present as a pancreatic mass mimicking pancreatic cancer. To date, histopathology remains the only definitive method for differentiating IgG4-RD lesions from malignancy, and the fact that patients can continually develop and present with newly formed masses leads to a vicious cycle of diagnostic dilemmas and unnecessary invasive procedures. Case Description/Methods: A 59-year-old woman presented with abdominal pain. She had a history of multiple emergency department visits for similar episodes, previously diagnosed as idiopathic pancreatitis. Computed tomography imaging revealed multiple ill-defined pancreatic and hepatic lesions, as well as enlarged axillary and paraaortic lymph nodes, raising concern for metastatic disease. Liver and pancreatic biopsies were non-diagnostic but showed atypical lymphocytic infiltrates and fibrosclerotic tissue. Flow cytometry ruled out lymphoproliferative disease. Given the clinical context, she was empirically started on corticosteroids for a presumed diagnosis of IgG4-RD. However, shortly thereafter, new bilateral renal lesions, a pleural nodule, a thyroid nodule, and retroperitoneal stranding were detected, once again raising concern for metastatic disease and prompting renewed diagnostic investigation. A repeat liver biopsy demonstrated a lymphoplasmacytic infiltrate with polytypic plasma cells, consistent with IgG4-RD. Clonality was excluded by a repeat flow cytometry and BIOMED-2 PCR. The patient was discharged on corticosteroids, with plans to start a steroid-sparing immunosuppressant therapy. Discussion: IgG4-RD shares clinical and radiological features with metastatic cancer. Clinicians face significant challenges in weighing the benefits versus the risks of ordering invasive, yet potentially diagnostic, procedures. IgG4-RD responds well to corticosteroid therapy; thus, early recognition and treatment are key to preventing irreversible organ damage and avoiding unwarranted invasive interventions.