Squamous cell carcinoma of the hoof wall in a stallion.

Squamous cell carcinoma of the hoof wall, with resultant invasion of the right hind distal phalanx, was identified in a 15-year-old Thoroughbred stallion. The clinical features included a chronic grade 2/5 right hind limb lameness and a sessile dorsal hoof wall mass that was not sensitive to palpation. Radiography revealed a well-circumscribed circular lucency within the distal phalanx, beneath the clinically noticed hoof wall mass. These features were considered to be characteristic of a hoof wall keratoma. Surgical intervention was done 10 months later. The histologic diagnosis at the time of surgery was squamous cell carcinoma. Eight months after surgery, progressive tumor invasion of the distal phalanx resulted in a pathologic articular fracture. This case highlights the need for accurate histologic diagnosis of equine hoof wall masses to differentiate between benign and malignant conditions.