Malignant Melanoma Canine & Feline
Most people have heard of malignant melanoma and know that it is a tumor involving the pigmented cells of the skin. Melanoma is a classical skin cancer of humans and in humans it is associated with sun exposure. In pets, the patient is usually canine (though the iris melanoma of the feline eye is also a well-described condition). Recently new treatments have come to light and while research continues, there is a great deal of new information to review.
The melanoma is a tumor of pigmented cells called melanocytes.
Benign Versus Malignant Melanomas
It turns out that the behavior of a melanoma is highly dependent on the region of the body where it develops. Most areas of skin grow benign versions of the melanoma that are called melanocytomas. These tumors typically do not spread and do not behave in a malignant manner. Since some haired skin melanomas are definitely malignant in behavior, it is important to have all tumors analyzed by a pathologist. There is some suspicion that haired skin melanomas developing within 1 cm of a mucosal margin (like the mouth) behave more malignantly than one would expect based on what is seen under the microscope.
There are several areas that grow particularly invasive melanomas, which are not only locally destructive but spread cancer to the lung, liver and other areas. The areas where melanoma development is particularly threatening include:
- The mouth
- The toe or toenail bed
- The eye (in cats)
Melanoma of the Mouth
In the mouth, the size of the tumor is extremely important in considering prognosis. Veterinary medicine has adopted the World Health Organization staging system where Stage I disease is represented by a tumor less than 2 cm (just less than 1 inch) in diameter, Stage II is represented by tumors 2 – 4 cm in diameter, and Stage III tumors are 4 cm or larger or any tumor with local lymph node involvement. Stage IV disease includes any tumor with evidence of distant spread.
Median survival times for oral melanoma have been reported as:
Stage I: approximately one year
Stage II: approximately 6 months (with surgery)
Stage III: approximately 3 months (with surgery)
Melanoma of the Digit (Toe)
Melanoma development of the toe or toenail bed seems to be a particular problem for black dogs. The tumor is particularly destructive to the bone and quite painful. Further, this form of melanoma is aggressive with 30-40% already having spread at the time of diagnosis. If the toe can be amputated before there is evidence of spread to local lymph nodes, median survival time is approximately one year.
Melanoma of the Feline Eye
The eye is the most common site of malignant melanoma in cats. The iris (colored portion of the eye) is the most common area of involvement though the pigmented vascular tissues in the back of the eye can also be involved. Iris melanoma appears as a bump on the surface of the iris with or without distortion of the pupil; a tumor in the back of the eye may not be readily apparent. In either case, glaucoma or blindness can result. The best treatment is to remove the eye (enucleation) which, in most cases, prevents tumor spread.
As one might expect, treatment begins with surgery. The growth is removed, biopsied, and determined to be a melanoma. Depending on the staging criteria determined, a more extensive surgery may be required in an attempt to cure the disease or at least postpone recurrence. In a cat with an ocular melanoma, removing the eye is often curative.
If the tumor cannot be completely removed or it has spread to local lymph nodes but not beyond, then radiation therapy becomes important. Remission rates are reported as high as 69% though ultimately recurrence becomes an issue. For more details on this type of treatment, it is necessary to consult a veterinary oncologist.
One might suspect chemotherapy would be helpful in more distantly spread forms of melanoma since drugs are able to travel anywhere in the body that the circulatory system travels. Unfortunately, chemotherapy has been a disappointment and malignant melanoma is felt to be resistant.
The Melanoma Vaccine
An enzyme called tyrosinase is crucial to the melanocyte’s ability to produce melanin (pigment). By using human tyrosinase as a stimulator, the patient’s immune system can be tricked into attacking the melanoma cells that contain the patient’s natural tyrosinase. The vaccine is available only through veterinary oncology specialists. It is given in four single doses at 2 week intervals. Booster shots are given every 6 months and are best used for dogs with oral melanoma without node involvement. Life expectancy has been extended to over one year in many cases.
For more information on the new melanoma vaccine, visit:
To Locate a Veterinary Oncologist
No oncologist in your area? On-line consultation and chemotherapy drugs are available to any veterinarian through www.oncurapartners.com
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