Clear cell dermatofibroma. Case report of an unusual fibrohistiocytic lesion.

A case of clear cell dermatofibroma is presented. Clinically, a 41-year-old woman exhibited a hard brown nodule on her instep that was assumed to be a dermatofibroma. Histologically, more than 90% of the lesion was composed of clear cells. Epidermal hyperplasia and a storiform arrangement of spindle cells and sclerotic collagen in some foci at the periphery of the lesion indicated the fibrohistiocytic origin. Moreover, prominent vascularity and some bizarre giant cells in the lower part of the lesion were reminiscent of multinucleate cell angiohistiocytoma. Of a broad panel of antibodies, the lesion was positive only for Factor XIIIa (and vimentin). Ultrastructurally, clear-cell changes corresponded to a mostly translucent cytoplasm, focally with some endoplasmic reticulum and prominent lysosomal structures. A review of 1,496 dermatofibromas seen during the last 15 years at our institute revealed 12 cases (1%) with similar clear-cell changes in a minor part of the infiltrate (less than 10%). The differential diagnosis includes metastases of renal-cell carcinoma, which exhibit more atypia and mitoses and are positive for epithelial cell markers; clear-cell sarcoma, a lesion of tendons or aponeurosis with some moderate cytoplasmic melanin deposition and immunoreactivity with HMB-45; and various non-X histiocytic disorders, such as the predominantly vacuolated type of juvenile (or adult) xanthogranulomas or papular xanthoma, with a mixed infiltrate of various types of mononuclear and multinucleate histiocytes positive with a variety of macrophage markers.