Abstract

A 37-year-old female presented with onychodystrophy with mild hyperkeratosis and onychoschizia of the left 4th toenail. Histopathology showed an exophytic dermal tumor with verrucous epidermal hyperplasia and hyperkeratosis without epidermal atypia. The dermal tumor was characterized by a densely cellular proliferation of spindled cells with some areas of fascicular growth and many multinucleated cells with a floret appearance. In some lesional areas, the spindled cells were densely crowded and atypical mitoses were noted. The lesional cells were CD34 positive and Factor XIIIa, S100, p63, caldesmon, ERG, and EMA negative. PHH3 highlighted a few scattered mitotic figures and Ki67 showed an overall relatively low proliferation rate. FISH DNA break apart signal analysis for detection of the translocation associated with PDGFB was negative. These histopathologic features were diagnostic of an atypical unguioblastic fibroma. Complete excision was recommended. Onychomatricoma is a benign biphasic fibroepithelial nail unit tumor. Three subtypes have been described in the literature including unguioblastoma, with a predominant epithelial component; unguioblastic fibroma, with a predominant cellular stroma; and atypical unguioblastic fibroma, with a cellular lesional stroma showing nuclear pleomorphism, nuclear atypia, and increased mitoses. Atypical unguioblastic fibromas are rare, with only two cases reported in the literature. One case presented as a subungual tumor and one case presented as a nail lesion. Dermatologists and dermatopathologists need to be aware of this tumor residing in the nail organ to avoid misdiagnosis of other cellular acral tumors.

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