Pleomorphic Onychomatricoma Mimicking a Malignancy

Summary: Pleomorphic onychomatricoma, a rare variant of onychomatricoma, is characterized by cellular stroma with nuclear atypia, pleomorphism, and mitoses. A 71-year-old male had an increasingly painful mass on the 5th toe of the nail bed for one year. Onychodystrophy with thickened, yellow over-curved nail plate and nail plate perforations were seen. Histopathology showed an exophytic biphasic fibroepithelial neoplasm originating from the nail matrix. The tumor showed finger-like projections lined by papillomatous matrix epithelium. The fibrous stroma was composed of spindle cells, some of which showed moderate to severe cytologic atypia, and pleomorphism, and multinucleated cells arranged in a floret-like pattern. Brisk mitotic activity was seen. The lesional cells were positive for CD68 and Factor 13a. The following stains were negative: CD34, CD10, S100, SOX10, desmin, MelanA, HMB45, p63, ERG, and SMA. Sarcoma panel molecular testing detected no fusions. A diagnosis of a Pleomorphic onychomatricoma was rendered. This neoplasm is part of the spectrum of onychomatricoma, distinguished by its pronounced atypical histological features. It is important to recognize this tumor due to its potential to be mistaken for malignant neoplasms, although it remains a benign entity. Proper identification and differentiation from other nail matrix tumors are crucial to avoid unnecessary aggressive treatments.