IMMUNOHISTOCHEMISTRY OF DERMATOFIBROMAS AND BENIGN FIBROUS HISTIOCYTOMAS

Dermatofibromas (DF) are common, benign skin tumors composed predomina ntly of cells having elongated nuclei and very scant cytoplasm (i.e., fibroblasts) and capillaries in a collagenous stroma. Some authors dis tinguish DF from benign fibrous histiocytomas (BFH), which are compose d of cells with round to oval nuclei and abundant cytoplasm (i.e., his tiocytes). In general, this group of tumors expresses factor XIIIa but not the antigen recognized by MAC 387. However, immunohistochemical d ifferences specifically between DF and BFH have not been reported. We have studied the immunophenotype of 23 lesions having morphologic feat ures predominantly either of DF (17 cases) or BFH (6 cases) using anti bodies against desmin (muscle marker), alpha-smooth-muscle actin (musc le and myofibroblast marker), CD68 and HAM56 antigen (markers commonly expressed by macrophages, so called ''histiocytic'' markers), CD34 (a marker present in hematopoietic, vascular, and occasional dermal dend ritic cells), and factor XIIIa (a transglutaminase present in many cel ls including dermal dendrocytes). Many spindle-shaped cells expressed alpha-smooth-muscle actin while many large, round cells expressed the histiocytic markers. However, most lesions expressed at least focally both alpha-smooth-muscle actin and ''histiocytic'' markers. Thus a cle ar-cut distinction between DF and BFH could not be made based on immun ophenotype alone. Additionally, the prominent alpha-smooth-muscle acti n immunoreactivity and desmin non-reactivity suggests myofibroblastic differentiation in the spindle-cell regions of these tumors, and indic ates that expression of alpha-smooth-muscle actin cannot be used as de finitive proof of muscle differentiation in spindle-cell tumors. We co nclude that DF and BFH are not discrete entities, but represent polar expressions of one nosologic entity exhibiting both myofibroblastic an d ''histiocytic'' differentiation.