Onychopapilloma (OP) is a tumor of the nail unit that classically presents as longitudinal erythronychia and exhibits histopathologic features of papillomatosis, matrix metaplasia, and subungual hyperkeratosis. Although historically regarded as benign, emerging reports have identified rare malignant variants demonstrating cytologic atypia and altered maturation while retaining the architectural scaffold of an OP. Herein we describe a 52-year-old man with progressive longitudinal erythronychia of the thumbnail with evolving width, darkening, and distal keratotic change that prompted excisional biopsy. Histopathologic evaluation revealed characteristic background features of OP with superimposed full-thickness epithelial atypia exhibiting a Bowenoid pattern, including a disordered loss of keratinocyte polarity, hyperchromatic nuclei, dyskeratosis, and atypical mitotic figures. Immunohistochemistry demonstrated diffuse full-thickness labeling of the nail bed and matrix with Ki-67, and HPV testing was negative. In conjunction with the clinical features, the histopathologic features established a diagnosis of malignant OP. This unique Bowenoid pattern expands the histopathologic spectrum of malignant OP.
Billick et al. (Tue,) studied this question.