IgG4‐associated fibrosing mediastinitis requiring differentiation from posterior mediastinal tumour: A case report

A 71‐year‐old man, who was found to have a posterior mediastinal tumour, was referred to our hospital. Contrast‐enhanced computed tomography (CT) showed a 15‐cm soft tissue shadow in the posterior mediastinum, with many affected areas and a gradually increasing pattern. We also detected oligemic areas with poor contrast‐filling. There was no invasion into the adjacent vertebral body and the blood vessels penetrating the interior were intact. Positron emission tomography–CT revealed a high maximum standardized uptake level of 4.53 in the mediastinal masses. We performed thoracoscopic surgery for the biopsy. Histological findings showed lymphoplasmacytic infiltration in the fibrous stroma as well as storiform fibrosis. Immunohistochemical examination revealed abundant infiltration of immunoglobulin G4 (IgG4)‐positive plasma cells and 40% IgG4/IgG‐positive plasma cells. Postoperative serum examinations showed a high serum IgG4 level (570 mg/dl). Accordingly, we diagnosed the patient with IgG4‐related fibrosing mediastinitis, a rare manifestation of IgG4‐related disease.