Kolasa, K.

Abstract Fibrosing mediastinitis is an infiltrative disease which may develop secondary to a variety of causes, including autoimmune, infectious, and malignant processes. Here, we present a case of fibrosing mediastinitis, with initial concerns for IgG4-related disease. A 36-year-old man presented to his primary provider with three months of intermittent dyspnea. CT chest was significant for a large, infiltrative mediastinal and left hilar mass with calcifications and involvement of the left upper and lower lobe bronchi, lingular bronchus, and left main pulmonary artery. Subsequent autoimmune and fungal studies were negative. The patient underwent bronchoscopy, with biopsy of a lymph node at station 11L showing increased IgG4 on immunostain (30 cells/hpf), though IgG immunostaining was not completed for comparison. Despite initial concerns for fibrosing mediastinitis, the patient was diagnosed with IgG4-related disease and initiated on rituximab and mycophenolate. Notably, the patient's serum IgG4 concentrations were consistently within normal limits before and after initiation of rituximab. The patient was referred to our clinic after lack of response to therapy on imaging. Laboratory evaluation redemonstrated a normal serum IgG4 concentration and fungal studies were consistent with previous histoplasma infection, suggesting fibrosing mediastinitis secondary to histoplasmosis. IgG4-related disease was determined to be unlikely considering repeatedly normal IgG4 levels and weakly positive IgG4 staining. Repeat CT chest demonstrated fibrosis of the left upper lobe; compression of the left atrial appendage; and occlusion and severe obstruction of the left superior pulmonary vein and left lower lobe pulmonary artery, respectively. VQ scan demonstrated significantly reduced perfusion of the left lung and the patient was referred for stenting of the affected vasculature. PET-CT revealed moderate FDG avidity of the left hilar nodes, concerning for ongoing active fibrosing mediastinitis, for which he was continued on rituximab. The indolent nature of fibrosing mediastinitis often leads to advanced disease at the time of presentation. Hilar structures are commonly involved, as in the case of this patient with a severe perfusion defect secondary to obstruction of the pulmonary vessels. A diagnosis of fibrosing mediastinitis merits careful investigation of the underlying cause. Fungal infections, specifically histoplasmosis in North America, are a leading cause of this condition. IgG4-related disease is also associated with fibrosing mediastinitis, and both conditions are driven by fibroinflammatory processes. However, it is important to note that IgG4 immunostaining is non-specific, and diagnosis of IgG4-related disease requires the meeting of rigorous criteria. Differentiation between these diagnoses is important given potential differences in management.