Non-IgG4-Related Fibrosing Mediastinitis Diagnosed on Needle Biopsy and Treated with Steroids

Introduction/Objective Fibrosing mediastinitis (FM) is a rare and aggressive fibroinflammatory process of the mediastinum. It is a difficult diagnosis, especially on small biopsies as the presentation and the fibrosing background mimics malignancy. Although the exact etiology is unknown, prior fungal infections, certain medications and autoimmune conditions are documented. Significant histopathologic overlap with IgG4-related disease (IgG4-RD) can complicate the diagnosis. An accurate diagnosis is important for treatment as corticosteroids for IgG4-RD is proven to be effective; however, no effective treatment is described for FM. Additionally, immunosuppression would prove harmful in other conditions with fibrosing morphology. We report a very unique case of non-IgG4-related FM that was diagnosed on a needle biopsy and showed marked improvement with corticosteroids. Methods/Case Report A 65-year-old man with no significant medical history presented with syncope and recent weight loss. Imaging revealed a pericardial effusion and a heterogeneous 8.1 cm mediastinal mass encasing the great vessels and aortic arch. Needle biopsy showed fibrosis with keloid-type collagen, mixed inflammatory cells, and few benign lymphoid follicles. No evidence of hematolymphoid malignancy, carcinoma, or infection was seen. Serum and tissue IgG-4 studies were negative for IgG4-RD. A diagnosis of FM was made, and a trial of corticosteroids was initiated. Interestingly, the patient showed improvement with a 2.3 cm decrease in lesion size and near-complete resolution of pericardial effusion after one month of treatment. Results (if a Case Study enter NA) NA Conclusion FM should be included in the differential diagnosis of infiltrative mediastinal masses which presents a diagnostic challenge on limited needle biopsies. Although effective treatment for non-IgG4 related FM is not clearly established, corticosteroids may prove beneficial, as in this patient. Mortality from FM is directly related to the compressive effect on adjacent structures. Early diagnosis and treatment are therefore crucial. Further studies on the pathogenesis are necessary, as well as better directed therapeutic options.