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. B oth 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 histioc yte-like and fibroblast-like cells in a capillary-rich stroma. Such cl inical and histologic features fit the original description of this en tity. Immunohistochemically the tumor cells of both cases were immunor eactive for factor XIIIa, Mac 387, and vimentin and gave negative resu lts for factor VIII-related antigen, desmin, actin, and S-100 protein. Ultrastructurally the tumors were composed mainly of siderosome-conta ining histiocyte-like cells, fibroblast-like cells, and intermediate c ells and lacked prominent proliferation of endothelial cells. Thus, th e ultrastructural findings agreed with the immunophenotypes expressed by the tumor cells, supporting the fibrohistiocytic origin of this les ion. In view of the recent rapid growth, the presence of hemorrhagic p seudocysts, the extravasation of erythrocytes, and the high vascularit y of the present tumors, we emphasize the importance of distinguishing this lesion from angiomatoid fibrous histiocytoma and cutaneous malig nancies of mesenchymal origin.