Aneurysmal fibrous histiocytoma of the skin. A histological, immunohistochemical, and ultrastructural study.

We present two cases of aneurysmal fibrous histiocytoma of the skin on the lower extremities of a 41-year-old man and a 23-year-old woman. Both the tumors appeared clinically as cutaneous nodules with a recent history of rapid growth and showed microscopically a unique formation of blood-filled tissue spaces and a storiform proliferation of histiocyte-like and fibroblast-like cells in a capillary-rich stroma. Such clinical and histologic features fit the original description of this entity. Immunohistochemically the tumor cells of both cases were immunoreactive for factor XIIIa, Mac 387, and vimentin and gave negative results for factor VIII-related antigen, desmin, actin, and S-100 protein. Ultrastructurally the tumors were composed mainly of siderosome-containing histiocyte-like cells, fibroblast-like cells, and intermediate cells and lacked prominent proliferation of endothelial cells. Thus, the ultrastructural findings agreed with the immunophenotypes expressed by the tumor cells, supporting the fibrohistiocytic origin of this lesion. In view of the recent rapid growth, the presence of hemorrhagic pseudocysts, the extravasation of erythrocytes, and the high vascularity of the present tumors, we emphasize the importance of distinguishing this lesion from angiomatoid fibrous histiocytoma and cutaneous malignancies of mesenchymal origin.