SCC: Not So Easy on the Eyes

SCC: Not So Easy on the Eyes

Squamous cell carcinoma can involve several parts of the eye, including the lower eyelid.

Photo: Chelsey Miller, DVM

Squamous cell carcinoma (SCC) is the most common neoplasm, or cancer, of the equine eye. In horses, an ocular SCC often appears raised, pink, rough, and irregular. Veterinarians describe classic SCC as having a distinctive “pink cauliflower” or “cobblestone” appearance. It can involve the cornea (the eye’s transparent outer coat), the junction between the cornea and the conjunctiva (the mucous membrane lining the eyelids and covering the white surface [sclera] of the eyball), the third eyelid, and the upper and lower eyelids. SCC is an epithelial (membranous) tumor and tends to be locally aggressive; however, metastasis (spread) is possible. Limbal SCC, which affects the coreal limbus, found at the border of the cornea and the sclera, is most likely to be associated with eventual loss of the globe, or eyeball. Third-eyelid SCC is most likely to be associated with terminal disease, because tumor cells generally migrate into the orbit (eye socket), invade bone, or involve the brain. 

Incidence of these neoplasms is related to increased age, poor skin pigmentation (pink skin around the eye), and increased ultraviolet light exposure. Appaloosas, Paints, Haflingers, and draft horse breeds (especially Belgians, Shires, and Clydesdales) are at an increased risk for developing SCC. These breeds’ characteristic coat and skin -coloring—they are often chestnuts or other coat colors with pink skin around the eyes—likely contributes to SCC’s prevalence among these horses. This does tend to be a cancer of older horses, but horses of any age can be affected. Ultraviolet light causes mutations in a tumor suppressor gene, which is why it is implicated in this condition. 

Tumors involving the corneolimbal or limbal region often go unnoticed, especially in their early stages. Approximately 30% of ocular SCCs are corneolimbal in origin, most commonly involving the lateral limbus (outside the cornea) and extending to the adjacent conjunctiva and cornea. Early lesions may present as a red eye due to corneal or conjunctival hyperemia (redness). Veterinarians often diagnose these precancerous lesions via microscopic exam of a tissue sample, whereas more progressive lesions have the telltale white-to-pink appearance with raised cobblestone architecture. Less commonly, SCC invades the cornea’s deeper layers, resulting in a diffuse white to pink corneal haze. Veterinarians make definitive diagnoses using biopsy results, though limbal SCC’s unique look helps them distinguish it from other diseases. 

Treatment depends on SCC location and size, extent of invasion, vision status, the horse’s purpose, treatment equipment availability, and the owner’s financial constraints. Options for treating corneolimbal SCC frequently involve surgical removal combined with a second therapeutic modality. Veterinarians select a particular complementary therapy based, again, on the equipment available and expense to the client, along with the need for general anesthesia versus sedation. They have several options to choose from to most effectively and efficiently tailor treatment to each case. -Adjunctive therapy options include cryotherapy (freezing), radiofrequency hyperthermia, β-irradiation (Sr90), CO2 laser ablation, or topical chemotherapy with mitomycin C (MMC). Long-term successful treatment or nonrecurrence rates range from 45% to 100% with the various therapies; however, veterinarians have found it necessary to use some form of adjunctive therapy following surgical excision to decrease tumor recurrence.

The chemotherapeutic antibiotic mitomycin C (MMC) is one of the more common, newer SCC therapies, as it’s a safe modality that practitioners can administer in the field. It targets rapidly dividing SCC cells by stopping cell replication, resulting in generalized cytotoxity (cell death) and reduced scar formation. Side effects associated with administering topical MMC are relatively uncommon but include mild redness and irritation, local allergic reactions, and corneal and tear duct irritation. To reduce complications, veterinarians recommend only administering MMC when no corneal ulcers are present. Therefore, if surgery has been performed, they typically delay MMC treatment until surgical wounds have healed. 

Veterinarians have employed various protocols for using MMC as an antineoplastic agent. The most common approach is twice daily administration for one week, followed by one week without treatment, repeated for two to four rounds, based on the horse’s SCC and response. The combination of reported clinical efficacy, few side effects, and ease of at-home administration makes topical administration of MMC a useful therapeutic option for equine corneolimbal SCC.

About the Author

Chelsey Miller, DVM

Chelsey Miller, DVM, works at Iron Will Veterinary Services, in Burlington, North Carolina. Her veterinary interests include ophthalmology, neonatal medicine, prepurchase examinations, and sport horse medicine and lameness.

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