Abstract
Malignant onychopapilloma is a newly described neoplasm of the nail unit. We present the second reported case of malignant onychopapilloma, along with novel correlations related to its clinical, onychoscopic, nail clipping, and soft tissue histopathological features. A 71-year-old man presented with a 2-year history of an asymptomatic lesion affecting his left middle fingernail. Clinical examination revealed 2mm-wide longitudinal xanthonychia extending the length of the nail plate to the proximal nail fold, with distal hyperkeratosis and focal onycholysis. Onychoscopy demonstrated irregular longitudinal nail plate ridging with scattered punctate hemorrhagic foci. Nail clipping histopathology revealed tumoral onychocytes beneath the nail plate intermixed with loculations of parakeratosis and erythrocytes. Histopathologic evaluation of the excisional biopsy specimen redemonstrated these features and further revealed marked cellular atypia and metaplasia of the distal nail matrix. Ki67 staining demonstrated a distinct zone of full-thickness staining with a sharp cutoff to the adjacent unaffected nail unit epithelium. Additional tests, including Fontana Masson, SOX10, and MART-1 staining, as well as fungal stains and HPV testing, were negative. The high Ki67 staining, cellular atypia, and architectural similarities to onychopapilloma led to the diagnosis of malignant onychopapilloma, and the patient was referred for definitive excision. In conclusion, malignant onychopapilloma is a newly described nail unit malignancy, with novel clinical and histopathological features presented here.