Histiocytic and dendritic cell neoplasms of the mediastinum.

In this review, we discuss a logical approach to diagnosis of histiocytic and dendritic cell lesions of the mediastinum. We break down the differential diagnosis between true neoplasms of histiocytic and dendritic cells [Rosai-Dorfman disease (RDD), Langerhans cell histiocytosis (LCH), and follicular dendritic cell sarcoma (FDCS)] versus selected neoplasms of other lineages which frequently attract non-neoplastic histiocytes or resemble them morphologically (carcinoma, melanoma, sarcoma, germinoma, mesothelioma, and lymphoma). As neoplasms in the latter category are more common, they should be stringently excluded before diagnosing a lesion in the first group, particularly given enormous differences in clinical management. We also consider histiocytic sarcoma (HS), an extremely rare lesion which, in some cases is likely of intrinsic histiocytic differentiation, whereas in others represents clonal evolution from an underlying non-histiocytic neoplasm.