Malignant Melanoma in Cats

A malignant melanoma tumor tends to grow rapidly, and may spread rapidly to other parts of the body such as the lymph nodes, liver, or lungs. Hence, early detection of any suspicious lumps and prompt treatment, is key to improving the cat's chances of survival.

Malignant melanoma is an aggressive form of cancer, most likely to affect the skin, eyes, or mouth of cats. Fortunately, it is relatively rare, and represents less than 2 percent of all forms of cancer in cats.

"Melanoma" refers to the pigment melanin produced by certain cells in the body. In a melanoma tumor, these pigment cells grow out of control and generate a cancer or tumor. The signs are: 

• A darkly colored patch, spot, lump, or raised area on the skin, gum, or within the eye
• Common sites include the head, such as the lips, gums, tongue, nose, ears, or eye. 
• Other sites include the back, tail, or paws
• Characteristics of a malignant melanoma include a dark color and rapid growth
• Middle aged to older cats are most likely to be affected
• The cat may have a single lump or several
• In the eye, the first sign of a melanoma may be the iris (the colored part of the eye) changing color and darkening.
• The cancer spreads rapidly to the lymph node draining the affected area
• Secondary spread to the liver or lungs is common, causing lack of appetite, breathing difficulties, and weight loss

Unlike in people, malignant melanoma in cats is rarely the result of excessive exposure to the sun. No infective cause has been linked to the development of melanoma and it seems likely that a spontaneous genetic mutation of melanin producing cells is the most likely explanation.

The veterinarian will perform a thorough clinical examination, including checking the cat's irises. As well as assessing any suspicious lumps, they will feel the cat's lymph nodes for any tell-tale signs of enlargement. 

There is a high index of suspicion of cancer in any darkly pigmented lump, especially one that is growing rapidly. Therefore the vet may opt for an 'excisional biopsy'. This involves anesthetizing the patient and surgically removing the lump with wide skin margins. The lump is then sent to a pathology lab for histology (analysis of the cells present), which will give a definitive diagnosis.

In addition, the vet may remove enlarged lymph nodes or harvest a small sample of cells via a fine needle aspirate. This is to check for any secondary spread, and guide the vet as to whether surgery is likely to be curative or not. 

The vet may also wish to radiograph (x-ray) the cat's chest and scan the abdomen, to look for evidence of secondary spread. Again, this helps with decision making with regards to what the future holds and whether therapies such as radiotherapy are appropriate.

The cat's best chance of a full recovery is with early removal of any suspicious lumps, with wide margins. By removing all the cancer cells in one surgery, this reduces the chances of that particular lump seeding off cancerous cells. 

Unfortunately, the location of some melanomas, such as in the eye or mouth, can make surgery complex and highly invasive. It some cases removal of the eye is indicated, which can be done by a first opinion vet. Alternatively, if the mouth is affected, referral to a special surgeon for radical surgery involving removal of part of the jaw may be necessary. 

After surgery, the result of histopathology can guide the clinician as to how clean the margins of removal were, and whether follow up treatments with radiotherapy might be beneficial or not. 

Radiotherapy is undertaken at a few specialist centers, and does require a full general anesthetic for each treatment. 

The cat owner and the veterinarian need to keep a close eye on the cat's quality of life when considering intensive courses of therapy.  

Sadly, the long-term outlook for cats with malignant melanoma is poor. The average survival time for cats with an oral melanoma is just two months, whilst those that had surgery on a skin site lived a further four to five months. For those cats with a melanoma of the eye, around 50 percent went on to develop melanoma-related disease elsewhere in the body.