Contributors: Diva R. Salomão and R. Jean Campbell
A fibrous histiocytoma (fibroxanthoma, dermatofibroma) is a benign mesenchymal tumor. It usually presents in adult life, but have been observed in infancy. There is no gender predilection. Fibrous histiocytoma has an apparent predilection for the orbit and is the most common mesenchymal tumor in this site in adults. Orbital tumors involve especially the upper nasal quadrant. Proptosis and diplopia are caused by an infiltration of the extraocular muscles.The tumor is believed to arise from facultative phagocytes rather than primary histiocytes. A fibrous histocytoma lacks a true capsule and is composed mainly of spindle-shaped cells that form a characteristic storiform or cartwheel pattern. Extracellular connective tissue is variable but rarely conspicuous. Cells with a histiocytic morphology complemented by appropriate enzyme properties are an inconstant feature. Scattered multinucleated giant cells are sometimes present. Mitotic activity is minimal. The tumor cells have interdigitating processes and form rudimentary desmosomal junctions. This morphology is typical of fibroblasts rather than of macrophages. Most fibrous histiocytomas are benign, but some are locally-aggressive or malignant. A benign and locally aggressive fibrous histiocytoma may recur following incomplete excision. This occurs in ~30% of cases. Rarely a fibrous histocytoma undergoes malignant transformation into a malignant fibrous histiocytoma, but even then metastases seldom occur. The epithelium over a dermatofibroma may undergo basal cell hyperplasia and need tobe differentiated from a basal cell carcinoma.