Squamous cell carcinoma

Squamous cell carcinoma is the most common tumor of the horse eye. Squamous cell carcinoma is a type of tumor that can be present on the eyelids, cornea (clear part at the front of the eye), third eyelid (also called the nictitating membrane) or the conjunctiva (the white tissue surrounding the eye) of a horses eye. Environmental factors such as altitude, ultraviolet light and solar radiation as well as breed predisposition are thought to play a role in development of this condition. Certain breeds like Belgians and Haflingers have a genetic predisposition to develop squamous cell carcinoma. Genetic testing is available through www.animalgenetics.us.

Squamous cell carcinoma can be found in one eye or in both eyes at the same time. In some cases the first sign of a problem is thick, yellow ocular discharge. The most common location for this tumor is the third eyelid. On the third eyelid this tumor is often pink or red and can have a rough appearance. Due to the location of the third eyelid, tucked in the lower corner of the eye, this tumor is often hidden from sight until it is quite large.

Another common location for this tumor is the eyelid margin, especially in horses with white or non pigmented skin on the eyelids. On the eyelid, squamous cell carcinoma can look like little scabs, burns or erosions which may bleed intermittently. Squamous cell carcinoma can also be seen at the limbus (where the white part of the eye meets the clear cornea). This tumor is usually slow growing at any of these locations but it in some cases it can spread to local lymph nodes and other parts of the body. For this reason it is better to address this type of tumor sooner rather than later when it is small and more manageable.

Before treatment options are considered it is important that a biopsy is taken to confirm the diagnosis of squamous cell carcinoma. There are other tumors and even parasitic infections that can be mistaken for squamous cell carcinoma. Some of these benign, harmless lesions would not respond to the same therapy used for squamous cell carcinoma.

The treatment options available to tackle squamous cell carcinoma are variable and numerous depending on the size of the tumor and the location. Tattooing of the eyelids in white horses was thought to decrease development of squamous cell carcinoma but more research is needed to determine the efficacy of this therapy.
Chemotherapy injected directly into the eyelid lesion or applied topically to the corneal surface is another treatment modality for squamous cell carcinoma. Some of these chemotherapy agents include 5-fluorouracil, mitomycin C and oral piroxicam.

Interferon is often used in people with eyelid squamous cell carcinoma but it has not been widely used in veterinary ophthalmology. Interferon can be injected into small lesions on a routine basis to promote regression of the tumor. The effectiveness of this therapy in horses is still under investigation.

Surgical excision of a lesion has the potential to be curative if enough surrounding tissue can be removed. In locations on and around the eye obtaining sufficient margins around a tumor can be a challenge so adjunctive therapy is often needed to get the best prognosis. Advancement and rotational grafts, which take tissue from one location and rotate it into the area where the tumor was removed, can be used to fill in defects left when large extensive tumors are removed.
Adjunctive therapies often combined with surgical excision are cryotherapy, hyperthermia, radiation and photodynamic therapy. Cryotherapy uses extreme cold, hyperthermia uses extreme heat and radiation uses intense energy to damage cancer cells. Photodynamic therapy uses drugs in combination with laser light to kill cancer cells. Lastly if the tumor is too large, eye removal may be the only therapy available.

If you are concerned that your horse has squamous cell carcinoma reach out to Dr. Tolar to discuss the best treatment options.