IgG4-Related Sclerosing Mediastinitis Mimicking Thymoma: A Case Report

IgG4-related disease (IgG4-RD) rarely presents as an anterior mediastinal mass and may radiologically resemble thymic neoplasms. We report a 70-year-old man with an incidentally detected anterior mediastinal tumour on chest CT. The lesion measured 33 mm, was well-circumscribed, homogeneous and partially calcified. Tumour markers and serum IgG4 were within the normal range. The patient underwent thoracoscopic resection under the diagnosis of thymoma. Histopathological examination revealed dense lymphoplasmacytic infiltration and abundant IgG4-positive plasma cells, confirming IgG4-related sclerosing mediastinitis. No corticosteroid therapy was administered, and the patient remained disease-free over 3 years of follow-up. This case highlights the diagnostic challenge of mediastinal IgG4-RD and the importance of considering it in the differential diagnosis of thymic tumours, even when serum IgG4 is normal.