Fontana–Masson Staining in Aneurysmal and Hemosiderotic Dermatofibromas: A Study of 79 Cases

BACKGROUND: A dermatofibroma is a common benign cutaneous neoplasm composed of variable combinations of fibroblasts, histiocytes, and coarse collagen fibers. The aneurysmal/hemosiderotic (HDF/ADF) subtypes have characteristic pseudo-vascular spaces with hemosiderin-laden histiocytes and reactive spindled and epithelioid cells. These subtypes histologically can resemble melanocytic and vascular tumors, sometimes necessitating the use of immunohistochemical and histochemical stains to appropriately differentiate these entities. To our knowledge, Fontana-Masson staining has not previously been investigated in HDF/ADF. METHODS: A search of the SLUCare Dermatopathology Laboratory Information System was performed to identify cases signed out as aneurysmal and/or hemosiderotic dermatofibroma. These were then stained with Fontana-Masson stain and SOX-10 or S100 protein and evaluated by two board-certified dermatopathologists. RESULTS: A total of 79 cases were included, 48 females and 31 males aged between 12 and 84 years. Ninety-five percent of ADF and 98% of HDF cases were positive for Fontana-Masson stain. SOX-10 or S100 protein stain was negative in all cases. CONCLUSIONS: This study demonstrates that HDF/ADF stain with Fontana-Masson suggests the presence of melanin in these dermatofibroma subtypes, although Fontana-Masson is not entirely specific for melanin. These findings indicate that ADF/HDF are often highlighted by Fontana-Masson, suggesting that Fontana-Masson may not be a reliable tool for distinguishing HDF/ADF from melanocytic lesions. SOX-10/S100 may be useful in differentiating HDF/ADF from melanocytic lesions.