PROMINENT MYOFIBROBLASTIC DIFFERENTIATION - A PITFALL IN THE DIAGNOSIS OF DERMATOFIBROMA

Myofibroblastic differentiation occurs in 10-20% of all dermatofibroma s, affecting <25% of cells. We report on a series of 36 dermatofibroma s collected from >1,500 fibrohistiocytic lesions (2%), with mon promin ent (>25%) myofibroblastic differentiation characterized by markedly s lender and elongated spindle cells positive for smooth muscle markers. While most of the lesions did not otherwise differ from ordinary derm atofibromas, three cases (0.2%) from the neck-shoulder region of male adults showed extensive myofibroblastic features (>90%). Clinically, t hese three lesions measured approximately 1 cm and had a firm consiste ncy, with the differential diagnosis of some fibrohistiocytic tissue r esponse. Histologically, densely packed cells and prominent, partially nodular, stromal sclerosis with focal palisading of nuclei indicate s ome overlap with other rare variants of fibrohistiocytic tissue respon se, such as cellular benign and palisading cutaneous fibrous histiocyt oma. Yet, these features together with focal whorled nesting of more e pithelioid cells (one case) also caused considerable diagnostic proble ms to exclude other myofibroblastic as well as (malignant) spindle cel l lesions such as (palisaded) myofibroblastoma, dermatofibrosarcoma pr otuberans, and neurothekeoma. Immunohistochemically, all lesions were markedly (>90%) labeled for smooth muscle markers (HHF35, anti-SMA) an d with NK1C3 (CD57), while a broad panel for other spindle cell tumors , such as pan-keratin, S100 protein, EMA, desmin, CD34, CD31, and KiM1 p, were negative. Electron microscopy of two cases revealed prominent endoplasmic reticulum and Golgi complex, numerous intermediate filamen ts, attachment plaques, and focal basal lamina formation. No recurrenc e was seen during a follow-up of 9 (two cases) and two years, respecti vely.