A case of IgG4-related anterior mediastinal sclerosing disease coexisting with autoimmune pancreatitis

Background IgG4-related disease (IgG4-RD) is a systemic fibro-inflammatory condition that predominantly involves exocrine organs. Concerning its thoracic presentation, it often manifests as interstitial lung disease or fibrosing mediastinitis. It is very rare for IgG4-RD to form a well-defined mass in the anterior mediastinum, mimicking an encapsulated thymoma. Case presentation An 82-year-old man with autoimmune pancreatitis under treatment with oral corticosteroids was found to have peripancreatic lymphadenopathy on computed tomography. Subsequent positron emission tomography revealed abnormal uptake (maximal value 3.6) by a thymic mass as well as the peripancreatic lymph nodes. Exacerbation of IgG4-RD was suspected, and we increased the oral steroid dosage. As a result, the peripancreatic lymph nodes, and the anterior mediastinal mass, decreased in size. The thymic mass was suspected to be an encapsulated thymoma because of its lobulated shape, degree of fluorodeoxyglucose accumulation, and response to steroids, and the patient was referred to our department. The serum anti-acetylcholine receptor antibody test was negative. A thoracoscopic tumor resection was performed as diagnostic therapy. Histopathological analysis revealed dense lymphoplasmacytic infiltration with sclerotic stroma within the tumor. Immunohistochemical analysis revealed abundant IgG4-positive plasma cell infiltrates and over 50% IgG4/IgG-positive plasma cells. We did not see either keratin-positive thymocytes or terminal deoxynucleotidyl transferase-positive lymphocytes. Furthermore, deviation in the kappa chain and lambda chain-positive plasma cells was not noted. Accordingly, IgG4-related sclerosing disease was diagnosed. Conclusions IgG4-related sclerosing masses in the anterior mediastinum are very rare, and the effect of tumor resection on prognosis remains unclear. IgG4-RD had potentially been categorized as Castleman's disease.