What is a melanoma of the eye?
A melanoma of the eye is a type of cancer that develops from the uncontrolled growth of melanocytes, a type of cell found throughout the body in many tissues (e.g., skin, eye, inner ear, bones, and heart). In the eye, they can be found in the iris (the part of the eye that gives the eye its color and controls the size of the pupil), as well as beneath the retina (the thin layer of tissue that lines the back of the eye). Melanocytes produce melanin, a pigment that gives skin, hair, and eyes their color.
There are two kinds of ocular melanomas in dogs: uveal melanomas and limbal melanomas.
Uvealmelanomas are the most common eye tumors in dogs. These melanomas grow from the tissues that make up the uvea (the iris, ciliary body, and choroid). Most uveal tumors arise from the iris or ciliary body (part of the wall of the eye that makes the fluid that fills the eye). About 80% of uveal melanomas are benign (non-cancerous). The rate of metastasis (spread to other areas of the body) is less than 5%.
Limbal (sometimes called epibulbar) melanomas are less common. These tumors develop from the melanocytes found at the limbus, the border between the cornea (transparent front part of the eye) and the sclera (white part of the eye). Limbal melanomas are non-cancerous (benign).
What causes this cancer?
The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. With limbal melanomas, there is evidence that they are at least partly heritable, with Golden retrievers, Labrador retrievers, and German shepherd dogs predisposed to developing primary ocular melanomas. Breeds that have greater skin pigmentation may be predisposed.
In all breeds (even crossbreeds), the incidence of ocular melanoma is higher in middle-aged to older (7-10 years) dogs, with an average age of 9 years. In the case of limbal melanomas, dogs in the range of 2–4 years have also been diagnosed with the condition. There are no environmental factors known to cause ocular melanomas in dogs.
What are the signs of these types of tumors?
These tumors, whether benign or malignant, change the appearance of your dog’s eye.
With melanoma of the iris, you may see one or more roughly circular brown or black spots (lesions) on the iris. They may be flat or raised, and may grow over time to become discrete, raised pigmented masses. With a ciliary body melanoma, you may see a dark mass in the eye that protrudes through the pupil.
As the tumor grows, it may change the shape of the pupil or cause the pupil to get bigger (dilate). It may also cause hyphema (a pooling of blood in the middle layer of the eye); or a condition called uveitis (inflammation of the middle layer of the eye) that makes the eye appear cloudy; or glaucoma (increased pressure within the eyeball) that can cause the eye to bulge and lead to blindness. Both uveitis and glaucoma are very painful. Signs of pain with glaucoma may include squinting, head shaking, whining, placing the paws on the head, lethargy, and slow movements.
With limbal melanoma, you may see a distinct, raised, dark-colored mass arising from the edge of the limbus along the white of the eye (sclera). They often invade the cornea and can cause inflammation of the cornea, causing the cornea to become opaque or cloudy. It is common for them to grow outward from the surface of the eye, which can cause conjunctivitis (inflammation of the surface of the eye) and excessive tearing.
With both uveal and limbal melanomas, your dog may rub or scratch the affected eye. This could lead to an eye infection or corneal ulceration (an open sore on the cornea), which can cause redness, tearing, discharge, and squinting or closed eyes. Corneal ulcers usually cause intense pain.
How is this cancer diagnosed?
This cancer is diagnosed in large part by the clinical signs and the appearance of the tumor.
Your veterinarian will perform a full physical examination on your dog, followed by a complete ophthalmic examination. The ophthalmic examination will include inspecting the interior structures of the eyes using an ophthalmoscope and measuring the pressures within the eye using a tonometer.
Other diagnostic procedures may be performed, usually by a veterinary ophthalmologist. X-rays and ultrasound may also be helpful to determine the size of tumor and extent of spread within the eye. Ultrasound is also helpful to differentiate tumors from cysts, which are benign and of no concern.
Since 20% of uveal melanomas are malignant and can spread to other areas of the body, your veterinarian may recommend staging. This is the process of determining the extent to which a cancer has grown and spread. Staging may include bloodwork, urinalysis, chest and abdominal X-rays or ultrasound, and fine needle aspiration (FNA) of the nearby lymph nodes.
How does this cancer typically progress?
Without treatment, uveal melanomas continue to grow, although they tend to grow slowly. Even though most uveal melanomas are benign, they are invasive and will expand to involve other intraocular structures over time. They will usually cause uveitis, glaucoma, and vision loss. An untreated benign melanoma may even grow to the point that it ruptures the eye and invades the eye socket. Spread (metastasis) to the liver and lungs can occur but is not common.
Most limbal melanomas grow slowly, but if left untreated, may invade the cornea and cause secondary keratitis (inflammation of the cornea) or invade other eye structures, leading to other problems. Evidence of metastasis with limbal melanomas has not been reported.
Both uveal melanomas and limbal melanomas can seriously compromise the structure, function, and health of the eye.
What are the treatments for this type of tumor?
The treatment of choice for uveal melanoma will vary according to the initial appearance of the cancer, how it progresses over time, and the breed and age of the dog. Lesions are usually monitored with regular examinations to determine if they grow. If growth is noted, the masses may be removed with partial iridectomy (the surgical removal of part of the iris) or laser surgery, with the aim of retaining the eye and vision.
If the lesions substantially grow or spread within the eye, the pupil changes shape, or there is evidence of hyphema, uveitis, or glaucoma, enucleation (surgical removal of the eye) is recommended. Enucleation is always advised in cases of fast-growing and locally invasive melanomas.
Histopathology (examination of tissue with a microscope by a pathologist) should always be done in cases of eye removal. This will provide a definitive diagnosis, indicate whether the tumor has metastasized, and allow you and your veterinarian to form a long-term care plan. If there is evidence of metastasis, your veterinarian may recommend referral to a veterinary oncologist for melanoma vaccination. The vaccine is designed to induce an immune response by the dog’s body against the cancerous melanoma tissue. As well, your vet will recommend periodic imaging (e.g., X-rays or ultrasound) to determine the presence of metastatic disease. There is no evidence that chemotherapy is effective in treating ocular melanoma.
As limbal melanomas are benign and grow slowly, they are usually monitored. If the tumor grows rapidly or starts to grow into other eye structures, surgery is required. Surgery may be done to remove the tumor and may include removal of part of the cornea or sclera. Surgery may be combined with other treatments, including cryotherapy (use of extreme cold to destroy tissues), laser therapy, or radiation therapy. If the tumor regrows and penetrates the eye, enucleation is needed. Enucleation is curative and generally recommended for serious and painful tumor-related complications.
Is there anything else I should know?
It is difficult to think about removing your dog’s eye, even with the diagnosis of cancer, but enucleation can both prevent pain and save your dog’s life. Most dogs quickly adjust to the change in visual capacity and function. The overall prognosis for ocular melanoma is good, as most of these tumors are benign and there is minimal risk of metastasis.