Primary unifocal thymic Rosai-Dorfman disease: an extremely rare challenge in diagnostic practice.

Rosai-Dorfman disease (RDD) is currently considered a group of neoplastic diseases of unknown etiology, with monoclonal proliferation of histiocytes, showing unique histopathologic features and varying clinical presentation. Primary thymic RDD is an extremely rare extranodal form of this disorder. In this study, we describe the case of an otherwise healthy 64-year-old Chinese man who presented with an isolated, asymptomatic soft tissue density lesion in the anterior mediastinum detected by computed tomography. Histology of the surgical specimen revealed infiltration of thymic tissue by sheets of large histiocytes with mixed lymphocytes and plasma cells, and background fibrosis. Immunohistochemical staining of the histiocytes was positive for S100, CD68, CD163, OCT2 and cyclin D1, but negative for CD1a and BrafV600E expression, thus supporting a diagnosis of RDD. Primary thymic RDD is extremely rare and may be a diagnostic challenge when presenting as mediastinal lesion.