Skin - Melanocytic Tumors

By Joan Rest, BVSc, PhD, MRCPath, MRCVS


December 9, 2008

These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer". Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding about tumors and their treatment in animals is improving all the time.

We understand that this can be a very worrying time. If you have any questions please do not hesitate to ask us.

What are melanocytes?

skin__melanocytic_tumors-1Melanocytes are cells that produce a pigment called melanin. They are found in many parts of the body where there is pigment, particularly skin, hair and eyes.

Melanogenesis (formation of melanin by melanocytes) is a complex process controlled by genetic, hormonal and ultraviolet (UV) light. A sun tan is the result sunlight (UVA and UVB) stimulating our melanocytes. The immediate response to sunlight is not protective. It redistributes melanin and fades in minutes to days. The delayed response is protective but takes 3-4 days to develop. Melanocytes also have functions in thermal insulation and salt balance to protect animals from sunstroke.

Melanocytes are very sensitive to many types of damage and "wear out" so they lose their ability to reproduce themselves. In humans, after ten hair scalp growth cycles, melanocytes fail to respond to signals to move into new hairs so by 50 years of age, 50% of the population are 50% 'grey'-haired.

What is a 'melanoma'?

"A tumor of pigment producing melanocytes."

This is a tumor of pigment producing melanocytes. In humans, the classification of melanocytic tumors is very complex. The names are confusing and used in different ways by human and veterinary pathologists.

Melanocytoma (dermal melanoma, benign melanoma) is a benign tumor. It may be described as compound or simple, meaning with or without tumor melanocytes in the overlying epidermis (see diagram above); there are other subclassifications which indicate its microscopic appearance.

A malignant tumor may also be called melanoma but can also be more clearly identified by the terms malignant melanoma or melanosarcoma. Melanocytic hyperplasias (non-cancerous cell overgrowths) are benign and may be called 'lentigo' or 'lentigo simplex'.

On the hairy parts of the skin of dogs and cats, most melanocytic tumors are benign but those on mucocutaneous junctions (such as nail beds and lips) are often malignant. The exception to this rule is eyelid melanocytic tumors, which are usually benign.

What do we know about the cause?

The reason why a particular pet may develop this, or any cancer, is not straightforward. Cancer is often the culmination of a series of circumstances that come together for the unfortunate individual.

In humans, environmental and host (individual person) factors are important in development of melanoma. UV irradiation is the most important environmental factor. UVB causes direct damage to molecules (pyrimidine dimers) in the genetic blueprint (DNA) of cells and UVA causes oxidative damage to cell chemical reactions. This damage leads to poor cell repair and sometimes leads to cancerous changes (carcinogenesis). In dogs and cats, some melanocytic tumors develop in the areas of skin most exposed to sunlight but overall these tumors are most frequent in more heavily pigmented animals.

Why has my pet developed this cancer?

"Genetic make up of your dog or cat or heavy natural pigmentation is most likely to be the main predisposing factor."

Although some of these tumors develop in the areas of skin most exposed to sunlight, the genetic make up of your dog or cat or heavy natural pigmentation is most likely to be the main predisposing factor.

Is this a common tumor?

Melanomas are common in dogs. One common site is the nail bed. The peak incidence for benign tumors is between five and eleven years of age and for malignant cancers, nine and thirteen years. Melanocytic tumors are uncommon in cats. When they occur in cats between the ages of four and thirteen they are usually benign; in elderly cats they may be malignant (invasive and spreading).

How will this cancer affect my pet?

Tumors of the haired skin are usually noted as local areas of increased pigmentation although some melanocytic tumors are non-pigmented. Most are an obvious lump that may ulcerate, bleed or have physical effects on the surrounding structures. Some tumors have smaller satellite tumors around them, particularly in cats. The tumors may not be so obvious if they develop in a site such as the nail bed, where the cancer grows inwards. Pain may be the most obvious sign at such a site. Malignant tumors may spread through the body and the clinical effects caused will depend on the tissues and organs where the tumor cells seed new tumors.

How is this cancer diagnosed? 

Clinically, if your veterinarian observes a pigmented growth, a melanocytic tumor will be suspected. However, some of these tumors are not pigmented, and some other types of tumors may be pigmented or look dark. Other types of tumors commonly occur in predilection sites for melanocytic tumors such as the nail bed.

Therefore, accurate diagnosis relies upon microscopic examination of suspect tissue (histopathology). Your veterinarian will submit the tissue samples to a specialized diagnostic laboratory, where a veterinary pathologist will examine them. In order to evaluate the tumor completely, the pathologist must examine the whole lump. Although pigmented lesions are diagnosable by cytology (the microscopic examination of cell samples or a needle biopsy from the lump), cytology is inadequate for prognosis (prediction of future behavior) because the samples are inappropriate for grading and staging the tumor.

Histopathology reports typically indicate the type of tumor and mitotic rate (an estimate of rate of division of the tumor cells, which is an important predictor of outcome) and whether the tumor has been completely removed. These findings help forecast how the cancer is likely to behave, but occasionally melanocytic tumors that appear benign on histopathological appearance turn out to be malignant. This occurs in some 10% of "benign" melanomas - so there needs to be a mildly guarded prognosis on all of them. The pathologist may need to remove (bleach) the pigment to check malignancy with greater certainty.

There are various research techniques used to indicate prognosis more accurately, such as the Ki-67 proliferative index. However, the predictive value is often only slightly better (Ki-67 is 97% predictive compared with 91% for histopathology) so these techniques are not used routinely.

What types of treatment are available?

"The most common treatment for a melanocytic tumor is surgical removal of the lump."

The most common treatment for a melanocytic tumor is surgical removal of the lump. As in humans, there has been little progress with other treatments. Responses to chemotherapy and radiation therapy are poor. Immunotherapy with interferon has not improved survival in humans. Current research is focussed on combining immunotherapy (cytokine and gene therapy) with other standardized treatment. Vaccines are in clinical trials in humans.

Can this cancer disappear without treatment?

This cancer very rarely disappears without treatment. Very occasionally, spontaneous loss of blood supply to the cancer can make it die but the dead tissue will still need surgical removal. The body's immune system is not effective in causing this type of tumor to regress.

How can I nurse my pet? 

skin__melanocytic_tumors-4Preventing your pet from rubbing, scratching, licking or biting the tumor will reduce itching, inflammation, ulceration, infection and bleeding. Any ulcerated area needs to be kept clean.

After surgery, the operation site needs to be kept clean and your pet should not be allowed to interfere with the site. Report any loss of sutures or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask.

How will I know if the cancer is cured?

'Cured' has to be a guarded term in dealing with any cancer.

The histopathology report will provide a diagnosis, which helps to indicate how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence at the same or close site (if the tumor is removed), or metastasis (distant spread).

In many cases, the diagnosis and prognosis indicate there can be a complete cure. Sadly, there are some cases where the diagnosis and prognosis indicate that surgical removal will only give remission and the cancer will recur or spread. A few melanocytic tumors are difficult to predict behaviorally.

Dogs with skin tumors that have a low rate of cell division have a 90% chance of surviving more than 2 years after diagnosis; this reduces to 25% if the rate of cell division (mitotic index) is high. Only 10% of the first group have a tumor related death; but 45% of dogs in the second group will die from tumor related causes. Melanocytic tumors that develop on the nail bed have a less favorable outcome than tumors occurring at other sites.

There is little information about recurrence of tumors and survival times in cats.

Are there any risks to my family or other pets?

No, this is not an infectious tumor and it is not transmitted from pet to pet or from pet to people.

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