A Case of IgG4-Related Disease That Mimics an Anterior Mediastinal Malignant Tumour

IgG4-related disease (IgG4-RD) is a fibroinflammatory disease characterised by elevated serum IgG4 levels and marked infiltration of IgG4-positive plasma cells. Here, we present the case of a 49-year-old man who presented with dysphagia and hoarseness caused by an anterior mediastinal mass. As the fluorodeoxyglucose positron emission tomography (FDG-PET) SUVmax was 6.68 (early and delayed), malignancy was initially suspected. Serum IgG4 levels were slightly elevated at 148 mg/dL. Surgical biopsy revealed dense lymphoplasmacytic infiltration with storiform fibrosis and numerous IgG4-positive plasma cells, fulfilling the diagnostic criteria for IgG4-RD. Treatment with high-dose prednisolone (60 mg/day) rapidly resolved symptoms and markedly reduced the mass within 6 months. This case highlights the importance of considering IgG4-RD in the differential diagnosis of mediastinal tumours, even when FDG-PET shows abnormal accumulation and serum IgG4 levels are not particularly high.