Squamous cell carcinoma (SCC) is the most common malignant tumor of the nail apparatus. It is a low-grade malignancy that progresses slowly. SCC in situ (Bowen disease) is confined to the epidermis and does not invade the basal membrane. SCC of the nail unit is more commonly observed in males aged 50-70 years. Fingernails are more commonly affected than toenails.

Predisposing factors include trauma, immune suppression, human papillomavirus (type 16), smoking, chronic bacterial or viral infection, tar, arsenic, chronic radiation exposure, radiation therapy, or inherited dermatologic disorders such as ectodermal dysplasia, dyskeratosis congenita, or dystrophic epidermolysis bullosa. TP53 mutations have been found in SCC of the nail bed. 

SCC of the distal digit typically begins in the lateral nail fold or distal groove and invades the nail unit by extension; nevertheless, SCC may occasionally arise in the nail bed. The early appearance may mimic that of more benign conditions such as verruca, chronic paronychia, or infection.

A diagnostic biopsy is often delayed due to the slow progression of disease, patient reluctance to undergo biopsy, technical difficulties, or prolonged treatment of a suspected benign condition. The histology is typically well-differentiated and may appear entirely in situ (Bowen disease). However, as long-standing Bowen disease frequently shows evidence of microinvasion on serial histologic cuts, it is typically treated as aggressively as invasive SCC.

Nail SCC generally has favorable metastatic rates and mortality when compared to nail melanoma. Nail SCC rarely metastasizes, and when it does, it is more common in poorly differentiated tumors and immunosuppressed patients. Neglected lesions may spread contiguously to the adjacent bone.