Corneal Squamous Cell Carcinoma Arising From a Conjunctival Graft Scar in a Horse.

Objective: To describe the case details of an equine corneal squamous cell carcinoma (SCC) arising from the central scar of a previous conjunctival pedicle graft.

Animal studied: An 18-year-old Thoroughbred gelding, showing a progressive increase in diameter, depth, opacity, and pigmentation of a conjunctival pedicle graft scar of the left eye, 34 months after surgery.

Procedure(s): Complete ophthalmic examination, standard histologic and immunohistochemical evaluation of an incisional biopsy to formulate a definitive diagnosis. Carbon dioxide (CO₂) laser photoablation combined with topical mitomycin C treatment.

Results: Initial examination with slit-lamp biomicroscopic examination revealed a raised, pale pink and peripherally pigmented, ovoid corneal mass (25 × 20 mm, raised 5 mm) with a smooth to cobblestone contour, paraxial ventral at 6 o'clock of the left eye. Corneal neovascularization extended from the ventral limbus to the graft scar. The remaining ophthalmic examination was unremarkable. Histologic and immunohistochemical evaluation of an incisional biopsy were supportive of a diagnosis of corneal SCC. Two rounds of CO₂ laser photoablation, 10 weeks apart, combined with topical mitomycin C chemotherapy, without surgical debulking, was successful in resolving the neoplastic lesion. The horse was euthanized 4 years later for unrelated reasons, with no evidence of SCC recurrence grossly or histologically.

Conclusion: To the authors' knowledge, this is the first report in a horse of a SCC developing at the site of corneal scarring after a conjunctival pedicle graft.