Longitudinal Melanonychia as a Rare Sign of Invasive Squamous Cell Carcinoma: Case Report and Systematic Review.

A 50-year-old male presented with 4-month history of longitudinal melanonychia on the fourth digit of his right foot. A nail matrix and bed biopsy was performed, which led to the diagnosis of invasive well-differentiated squamous cell carcinoma. Immunohistochemical analysis demonstrated positivity for P16 and P40. Longitudinal melanonychia may be a diagnostic challenge, as it can be associated with both benign and malignant conditions. Given the potential for locally aggressive behaviour, squamous cell carcinoma should always be considered in cases of longitudinal melanonychia. A systematic review was conducted in order to analyse all documented cases of invasive squamous cell carcinomas that presented as longitudinal melanonychia, highlighting any associated clinical, dermoscopic, histological, and immunohistochemical characteristics potentially useful for an early identification of this neoplasm.