Cancer in Cats: Melanoma

Cancer in Cats: Melanoma

The Pet Cancer Awareness Month series continues with a look at melanoma in cats.

Melanoma is so named because of its connection with the melanin that produces pigment in the skin, hair, and eyes. If not identified early and treated, it can spread widely to affect the cat internally.

But melanoma doesn't just appear on the skin. It can affect the ears, mouth, toes, and eyes, as well. In fact, melanoma is the most common type of tumor that occurs in a cat's eyes. Here's the rundown on it:

Feline Melanoma

In a healthy cat, melanocytes are the cells responsible for producing melanin, or pigment, in the cat's skin, coat, and eyes.

Abnormal growth of these cells that is not cancerous (benign) is often called melanocytoma. This is the non-spreading form. You will often see these as little black spots on the lips, eyelids, nose, and gums of cats who have orange (red), cream, or silver colored coats. Only if they start to develop further should you be concerned.

When it becomes malignant, melanoma starts to spread (metastasize). Many refer to this form as malignant melanoma. It is rare in cats, but not unheard of. Locally growing at first, melanoma eventually will invade the lymph nodes and spread to other areas of the body. The internal organs most commonly affected are the lungs, but it can spread to any part of the body.

Melanoma accounts for less than 2% of feline tumors altogether, less than 1% of feline oral tumors, and 0.5% of skin (dermal) tumors in cats. Ocular melanomas (those in the eye) are more common than oral or dermal locations. Oral melanoma is much less common in cats than it is in dogs. Oral and ocular melanomas are typically more malignant than are the dermal variety, as well.

Symptoms of Melanoma in Cats

Symptoms vary depending on the location of the melanoma tumor.

In the eye, the tumor will most often appear in the iris of the cat's eye. These are commonly referred to as "iris spots" and cause the iris to thicken or appear irregularly shaped. Your cat's eyes may even start to look a different color. Some melanomas may appear as a dark spot at the point where the white of the eye touches the iris. They can grow on the surface or the interior of the eyeball.

With ocular melanoma, you may first notice the cat's eye appearing red and tearing (weeping) more than normal. It may be painful, and your cat may paw at it as well as cry out or pull away when you try to touch it.

If left untreated, the cat can develop glaucoma in the affected eye. The pupil will become dilated and the eye may bulge. This will eventually cause blindness.

With the dermal variety of melanoma, you may first see little black bumps on your cat's skin, in locations like the ears, nose, back of the head, tail, toes, back, or eyelids. These could be benign melanocytoma growths...or not.

Look for a single, dark-colored growth that is usually raised. Dermal melanoma may also be seen on the scrotum in un-neutered tomcats.

If in the mouth (quite rare in cats), the cat may have bad breath and drooling. There may be some bleeding from the tumor itself. The face may swell and the cat could have a hard time eating.

The tumor may ulcerate and bleed. It may look velvety. The fur around it may fall out as it will be itchy and your cat may lick or scratch it a lot. This can also make the skin around it red and swollen.

Once melanoma has spread to the lungs, the cat may have difficulty breathing.

Cats who are middle-aged (ages 6-14 years) or older have a greater likelihood of showing symptoms of melanoma, especially those affecting the eyes. Cats that are "heavily pigmented" or dark in color, according to some sources, have a higher risk for melanoma. Other sources say that white cats are more prone to it. 

Hairless cats such as the Rex and Sphynx breeds are also prone to melanoma, with no coat to protect their skin from sun exposure. Of course, the growths are also more readily apparent on these cats, so they're easier to catch early.

It's important to remember that not all melanoma tumors are pigmented, and not all dark tumors are melanomas. So if you find a suspicious growth that seems to fit the above symptom descriptions, a trip to your veterinarian is necessary to address the problem early.

Diagnosing Melanoma in Cats

Make notes on the symptoms you have observed and when you noticed them, as well as on your cat's general health and eating habits. This will be important information for your veterinarian to have when making a diagnosis.

Your vet will draw blood and take a urine sample from your cat for analysis. An X-ray of the chest area will show whether any cancer has spread to the lungs. A needle biopsy of the tumor itself may be done. Often a sample is also taken from the nearby lymph nodes, as well. 

If the growth is located on the rear legs, an ultrasound may be performed on the abdomen. 

If the tumor is in the eye, an ophthalmoscope or slit-lamp biomicroscope can often be used to see it. A gonioscope may be used to measure the eye's internal structure for indications that the tumor is spreading. You may be referred to a veterinary ophthalmologist for this. A tissue sample will be required to confirm that the tumor is melanoma by viewing the cells under a microscope. This may be done via needle biopsy, surgical biopsy, or complete removal of the eye, depending on the size and location of the tumor.

There are three different types of melanoma that can affect a cat's eyeball itself:

• Primary conjunctival tumors:  cause pain, redness and weeping.
• Limbal tumors: cause black nodules protruding from where the cornea meets the white of the eye.
• Iris diffuse melanomas (iris spots, iris freckles): cause a thickening and discoloration of the iris.

The tissue samples taken from the suspicious tumor must be sent to a lab for analysis. Included in the pathologist's report will be not only a diagnosis of the type of tumor, but the growth rate of the cancer cells. This is important in determining how far along the cancer has progressed (also known as "staging"). According to the World Health Organization's staging system for tumors, a Stage I melanoma would be smaller than 2 centimeters in diameter. Stage II: 2-4 cm. Stage III: 4 cm or larger. Stage IV: any size tumor with evidence of spreading (metastasis).

Treatment for Melanoma in Cats

As with most tumors, surgical removal is usually the first course of treatment. If the melanoma is on the cat's toe or the bed of the claw, that toe will likely be removed. In an older cat where the disease is progressing slowly, it may be kinder to take a "wait and see" attitude rather than subjecting the cat to radical surgery.

Melanomas in the mouth often require removal of a large section of the jaw to make sure all of the tumor is removed.

Depending on the type, location, and size of melanoma in the eye, laser surgery can be used to destroy a small tumor. This would be the preferable option, as it may be possible to destroy the tumor without the cat needing to lose the entire eye.

If the tumor is on the iris and is identified early, many veterinary ophthalmologists take a conservative approach to observe the tumor's growth on frequent visits. Only if the melanoma is growing or if there is a concern about the cancer spreading will surgery be done. When surgery is used, the entire eye is commonly removed. Surgery will always be chosen if the iris is noticeably changed by the melanoma. No radiation or chemo are typically used with melanomas in the eye.

Chemotherapy may also be used if the tumor could not be completely removed by surgery, although some sources have found melanoma resistant to it so the results have been disappointing. Some vets may recommend radiation therapy in some cases.

Some dogs can benefit from immunotherapy with Ki-67, but this has not been used on cats. Some cat owners choose a homeopathic treatment, which traditional vets view as controversial.

Once the tumor is removed, your cat will typically need to wear a special collar to keep from irritating the surgical site. The incision site needs to be kept clean and dry. Report any changes in this site or delays in healing to your vet. With tumors removed from the eye, there will likely be eye drops or ointments that must be given to fight infection and reduce swelling.

You'll need to keep a diligent watch on your cat after removal of a melanoma, as another could appear. Pay close attention to the site of the surgery, just in case any cancer cells remain there. Tumors affecting the eyes can lead to glaucoma over several years that will eventually force removal of the eye. They require semi-annual checkups with your vet to make sure this is not happening.

Prognosis for Cats with Melanoma

A lot of this depends on the report from the pathologist at the lab when the tumor's cell sample is sent for analysis. Melanoma tends to be more malignant in cats than it is in dogs. And in cats, this type of cancer behaves more like melanoma in humans.

The size of the tumor is one factor in determining a cat's prognosis with melanoma. If caught in Stage I and treated, the cat may live a normal life afterward.

A cat's prognosis is also related to where the melanoma appears. Melanomas in the eye are the most often malignant, but removal of the eye may stop any progression of the disease. Oral malignant melanoma has an average survival rate of around two months. Malignant melanomas on the skin give a cat four to five months after surgical removal.

Melanomas found on the eyelid, eye, scrotum, toe, or in the mouth are more often malignant than in other locations. One estimate puts the percentage of oral melanomas being malignant at 60%, another at about 50%. On the claw bed, 30% to 60% of them are malignant, those figures including dogs as well as cats. Iris spots are said to metastasize about 20% of the time. On the skin, 53% to 71% of tumors are malignant melanoma.

Once the melanoma metastasizes (spreads), the prognosis is more guarded. This can occur without being noticeable on X-rays, and without blood work showing it. Even with radiation therapy used after surgery, a melanoma that has already metastasized will return around 67% of the time.

In any event, once melanoma is diagnosed and treated in your cat, you'll be visiting the vet about every three months for follow-ups. X-rays will likely be taken twice a year to check for any spreading of the cancer.

Preventing Melanoma in Your Cat

Sun exposure is a common cause of melanoma in humans, and it may also be a cause of melanoma on a cat's ears, nose, and other places where the coat is thinner or white. So keeping your cat out of the sun may help prevent it, especially if your cat is among the higher-risk groups. If your cat's favorite thing is to bask in a sunny window all afternoon, put a UV-blocking tint on that window. If your cat is going outdoors, use a pet sunscreen.

But the causes of melanoma are not widely researched in cats. One study reported that experiments had produced melanoma in cats by injecting a particular strain of feline fibrosarcoma virus. There does not appear to be any predisposition to developing melanoma among any particular breeds, nor any strong correlation with age, sex, or coat length.

Veterinary oncologists have been using an enzyme called tyrosinase as a sort of "vaccine" against melanoma in dogs. However, this therapy has not been extended to include cats.

As with all types of cancer, the earlier the tumor can be identified and treated, the better. So keep a close watch on your cat, do frequent physical exams disguised as cuddle sessions, and make note (in writing, including the date) of anything out of the ordinary so you can mention it to your veterinarian.

Sources: "Skin Cancer in Cats: Types and Treatments", WebMD; "Malignant Melanoma in Cats", Dr. Bari Spielman, Pet Place; "Eye Tumors - Melanoma in Cats", VCA Animal Hospitals; "Malignant Melanoma in White Cats", Vet Pet MD; "Iris Melanoma in Cats", Eyevet.info; "Melanomas in Dogs and Cats", Webvet; "Malignant Melanoma", Wendy C. Brooks, DVM, DipABVP, The Pet Health Library; "Skin: melanoma", Dr. Philip K. Nicholls and Dr. Marion O'Leary, Vetstream; "Feline Melanoma: A Comparative Study of Ocular, Oral, and Dermal Neoplasms", A.K. Patnaik and S. Mooney, Veterinary Pathology Online; "Tumor of the Eye in Cats", Pet MD; "Melanoma", Vetbook.