Diffuse thymic fibrosis: histologic pattern of injury or distinct entity?

Thymic fibrosis in the absence of another primary thymic lesion, such as a neoplasm or cyst, is unusual. We identified 6 cases of primary extensive thymic fibrosis that developed in 3 men and 3 women, aged 28 to 60 years, mean 48 years. Patients had limited or no symptoms. Two patients had dyspnea and 1 had cough and hemoptysis. One female patient had myasthenia gravis. The lesions measured 3.5 to 17 cm in greatest dimension, mean 8 cm, and were confined to the anterior mediastinum as determined by imaging, intraoperative notes, and/or gross examination. All cases showed diffuse fibrosis with variable collagen deposition, lymphoplasmacytic infiltrates, and involution/atrophy of thymus. One case had rare IgG4-positive plasma cells and focal obliterative phlebitis. The histology showed overlap with that of IgG4-related sclerosing disease, which to our knowledge has not been documented earlier in this location. Although the etiology of the lesions is undetermined, altered immunity and/or infection may play a role.