Ocular Squamous Cell Carcinoma

Feat not: A variety of therapies are available to treat this common eye tumor.

Most owners are diligent about giving their horses a once-over for scrapes and swellings while grooming. This includes checking the eyes for subtle changes and early indicators of problems. Squamous cell carcinoma (SCC), for instance, is something an owner might notice; it is one of the most common cancerous tumors of the equine eye. Fortunately, with early intervention it's treatable.

Amber Domino Labelle, DVM, MS, Dipl. ACVO, assistant professor and veterinary ophthalmologist at the University of Illinois Veterinary Teaching Hospital, in Urbana, says the most common locations for periocular (around the eye) SCC include the eyelid skin, third eyelid, and the limbus (the junction between the cornea--the clear surface of the eyeball--and the conjunctiva that covers the white of the eye).

"On the eyelid, SCC generally appears as a pink mass, often ulcerated or bloody," she explains. "On the third eyelid or at the limbus it generally appears as a pink proliferative mass that can distort the tissues or create a mass on the surface of the eye. The tumor is generally pink or red in color." The affected eye might squint or emit a yellow or white discharge; this is typically what the owner notices first.

Brendan Mangan, DVM, MS, Dipl. ACVO, clinical assistant professor in comparative ophthalmology at the University of Florida College of Veterinary Medicine, says SCC lesions and the adjacent tissue typically appear more red and inflamed than the surrounding normal tissue.

Causes and Risk Factors

Some horses are more vulnerable to SCC than others. "There are several known risk factors," says Labelle. These have to do with pigmentation around the eye and ultraviolet (UV) light exposure.

"Horses that spend a lot of time in the sun are at higher risk, especially if eyelid skin and conjunctiva are nonpigmented," says Mangan. "Living at higher altitudes and closer to the equator results in more intense and longer periods of sun exposure and UV radiation."

Labelle says some breeds seem more predisposed to developing SCC than others, including Haflingers and some draft breeds. Appaloosas and Paint horses also are prone to SCC, but primarily because of the nonpigmented skin around their eyes.

Mary Lassaline Utter, DVM, PhD, Dipl. ACVO, assistant professor of ophthalmology at the University of Pennsylvania's New Bolton Center, is conducting research with Rebecca Bellone, PhD, associate professor of biology at the University of Tampa, to investigate Haflingers' genetic tendency toward SCC. "This breed appears to be overrepresented among horses with limbal SCC, and they are often diagnosed with SCC at a younger age than other breeds," Utter says. "Affected Haflingers are closely related, suggesting a possible heritable basis for this type of SCC. Our research is currently investigating the association of candidate genes with disease status, which may help clarify both the genetics and the pathophysiology of limbal SCC."

Labelle cites another recent study suggesting that equine papillomavirus might contribute to SCC development. "Infection with this virus may be why some lesions develop," she says. "There is convincing evidence that SCC in other locations, particularly the penis, may be due to equine papillomavirus, but we are not yet sure if this is also the case in lesions of the eye."

Diagnosis and Treatment

Have your vet examine any abnormality in a horse's eye area. He or she might takes samples and order tests. "It may be possible to arrive at a diagnosis of SCC with cells taken from the surface of the lesion, but a more definitive diagnosis requires a biopsy," says Mangan. "A biopsy also allows more effective surgical -planning."

The size, stage, and location of the mass dictate treatment options; the smaller the lesion, the easier it is to treat. Unfortunately, by the time they're noticed, many SCC tumors, especially those located on the third eyelid that is tucked underneath the outer surface, have become quite large. And the longer the lesion is present, the more likely the cancer will invade surrounding tissues, and the more it invades those tissues, the harder it is to treat.

"Lesions on the third eyelid or the eyelid itself can sometimes be treated in the field, without bringing the horse to a hospital facility," Labelle says. "There are many ways to treat these, including topical chemotherapy creams, injectable chemotherapy, freezing the lesion, radiation (which is highly effective), and surgical removal."

Mangan notes that large eyelid masses might require reconstructive surgery due to the amount of tissue removed.

Growths that occur at the limbus, on the eyeball itself, are more likely to be referred to an ophthalmologist for surgical intervention. "These are often treated with surgical excision, followed up with either chemotherapy or cryotherapy (freezing) to make sure there are no stray cancer cells left in the area," Labelle explains. "When we excise those kinds of lesions, we always submit them for biopsy to make sure that we removed the entire tumor (the lab looks for SCC cells close to the edges of the sample)."

Veterinarians might also use a variety of SCC therapies in conjunction with or in place of surgery, including:

Cryotherapy Mangan says veterinarians commonly freeze affected areas to -13°F as an adjunct to surgery. This method can be used on all tissues.

β-irradiation (Sr90) "This is an ideal treatment for corneal lesions, since the radiation travels a very short distance and is -almost completely absorbed by the cornea," says Mangan.

Brachytherapy (Ir192), during which radioactive beads or rods are inserted into the tumor for a period of time, is an effective treatment for eyelid SCC, but it carries with it radiation hazards.

Radiofrequency hyperthermia raises the temperature of the treated tissue to 122°F. Mangan notes that this damages tumor cells but allows normal cells to recover.

Intralesional immunotherapy involves injecting the immunomodulator Bacille Calmette-Guérin into eyelid tumors to treat large SCC.

Intralesional chemotherapy commonly involves using cisplatin and carboplatin with or without surgical debulking.

Topical chemotherapy agents mitomycin C and 5-fluorouracil can be used effectively following surgical mass removal.

Carbon dioxide (CO2) LASER therapy can be used on SCC in all locations and might be useful as a primary procedure or as an adjunct to conventional surgical removal, Mangan explains.

Piroxicam is a non-steroidal anti-inflammatory drug (NSAID) that "may have anti-neoplastic (tumorous) properties, but the effectiveness against SCC in the horse has not been completely validated," says Mangan.

Photodynamic therapy (PDT) is an evolving treatment for eyelid SCC that involves using a drug that photosensitizes tumor cells with a specific wavelength of light. After the practitioner injects the photosensitizing agent, he or she exposes the mass to a specialized light source, such as a laser, that destroys tumor cells and associated blood vessels.

Elizabeth Giuliano, DVM, MS, Dipl. ACVO, an associate professor in the Department of Veterinary Medicine and Surgery at the University of Missouri, has been studying the use of local PDT to treat eyelid SCC. She first introduced the concept of using local PDT for equine eyelid SCC in 2003 and has treated more than 30 affected horses using the therapy.

Giuliano first began using PDT when presented with an SCC case that had already undergone five different treatment modalities to no avail. The cancer was limited to the eyelid, and the horse had retained his vision, so she could not yet bring herself to remove the eye.

"Horses' eyes present a unique challenge because if you can't save the eyelid you can't save the eye," she explains. "The horse is not amenable to eyelid flaps like other species. Eyelid reconstruction surgery for a horse is extremely difficult to perform successfully because their skin is tightly adhering to the underlying bone--unlike that of a dog whose skin is much looser."

When performing PDT Giuliano first debulks the eyelid tumor surgically, then she injects the photoactive agent into the tumor bed, and finally she uses a laser to irradiate the area and kill any residual tumor cells.

"In people, photoactive agents are typically administered intravenously and have been used to treat a variety of conditions including age-related macular degeneration and bladder cancer," she explains. "The volume of drug needed for a horse (if we were to give it systemically) would be too expensive, and we don't know how horses would react. Local PDT is believed to be a much safer approach at this time."

She says one of the reasons PDT is appealing for treating periocular cancers in horses is that it is not toxic to the people handling the drugs, unlike potentially dangerous intralesional chemotherapy agents like cisplatin.

"Additionally, PDT typically does not require multiple treatments," she says. "Therapeutic modalities that require repeat treatments can become very costly. Even if the individual treatment cost is low, if you have to repeat it five or six times in order to be effective, and add the costs associated with trailering the horse to a veterinary facility several times, the total cost is comparable to what you'd spend on one treatment of PDT." The one treatment also means less hassle and stress for the client and the horse.

"For me, the advantages of local PDT are that it is nontoxic, there is no cumulative drug effect, and usually one treatment is sufficient," she concludes. "The disadvantage of PDT is its limited use by veterinary ophthalmologists currently and the initial cost. The drug itself is very expensive."


One of the best ways to prevent potentially cancer-inducing UV light exposure is to keep horses out of intense sunlight. "Anything you can do to minimize exposure will help decrease risk of SCC, especially in areas of the country with a lot of UV light," says Labelle. "Using a fly mask (especially the type with UV blocking material) is often the easiest thing to do if your horse has pink eyelids and you live in the Southwest. In summer when UV light is most intense, you might consider turning that horse out at night and putting him in a stall during the day."

Before the use of fly masks, some owners tried to reduce sun glare by putting dark color around their horses' eyes if they had white face markings. Dark color materials included mascara and theatrical grease paint, but these can be dangerous for the horse. Thus, these old "home remedies" might do more harm than good.

"Also, tattooing the eyelids does not help," Labelle says. "The tattoo ink localizes deeper in the skin than the layer where the tumor occurs, and does not protect the layers where the tumor actually arises."

Take-Home Message

Never neglect an abnormality on your horse's eye, as it can go from something insignificant to serious rather quickly. As with any type of skin cancer, prevention is preferable to treatment, and early treatment assures a better chance for a positive outcome. "The best thing owners can do is look at their horse closely every day and have any abnormality examined by a veterinarian," Labelle advises.


About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.

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