Cat & Dog - Melanocytic Tumours

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. If not already performed your veterinary surgeon may suggest certain tests to help confirm or eliminate the 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 of tumours in animals is improving all the time.

We understand that this can be a very worrying time and we apologise for the need to use some technical language. If you do not understand anything, please do not hesitate to ask.

Melanocyte biology

Melanocytes 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 of 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. 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'?

This is a tumour of pigment producing melanocytic cells. In humans the classification of melanocytic tumours 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 tumour. It may be described as compound or simple meaning with or without tumour melanocytes in the overlying epidermis (skin layer) and with other subclassifications indicating its microscopic appearance.

The malignant tumour 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 tumours are benign but those on mucocutaneous junctions (such as nail beds, and lips) are often malignant. The exception to this rule is eyelid melanocytic tumours, 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 seemingly the culmination of a series of circumstances which 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 cancerous changes (carcinogenesis). In dogs and cats, some melanocytic tumours develop in the areas of skin most exposed to sunlight but overall these tumours are most frequent in more heavily pigmented animals.

Why has my animal developed this cancer?

Although some of these tumours 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 tumour?

Melanomas are common in dogs. One common site is the nail bed. The peak incidence for benign tumours is between five and eleven years of age and for malignant cancers, nine and thirteen years. Melanocytic tumours 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 animal?

Tumours of the haired skin are usually noted as local areas of increased pigmentation although not all have this pigment. Most are an obvious lump that may ulcerate, bleed or have physical effects on the surrounding structures. Some tumours have smaller 'satellite' tumours around them, particularly in cats. The tumour may not be so obvious 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 tumours may spread through the body and the clinical effects caused will depend on the tissues and organs where the tumour cells seed new tumours.

How is this cancer diagnosed?

Clinically, this pigmented tumour can be suspected from appearance but some are not pigmented. Also some other types of tumours may also be pigmented or look dark. Other types of tumours commonly occur in predilection sites for melanocytic tumours such as the nail bed.

Accurate diagnosis therefore relies upon microscopic examination of a sample of the suspect tissue (histopathology). This is done at a specialised laboratory by a veterinary pathologist. For grading the severity and outlook for the tumour's likely progress, the whole lump is usually needed by the pathologist. Pigmented lesions are diagnosable by cytology (the microscopic examination of cell samples) or a needle biopsy from the lump, but without grading and staging by histopathology, cytology is inadequate for prognosis (prediction of future behaviour).

Histopathology reports typically indicate the type of tumour and mitotic rate (an estimate of rate of division of the tumour cells, which is an important predictor of outcome) and whether the tumour has been completely removed. These findings help forecast how the cancer is likely to behave but occasionally melanocytic tumours 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 little different (Ki-67 97% predictive compared with 91% for histopathology) so these techniques are not used routinely.

What types of treatment are available?

The commonest treatment for a melanocytic tumour 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 interferons has not improved survival in humans. Current research is focussed on combining immunotherapy (cytokine and gene therapy) with other standardised treatment. The newest treatment for canine melanoma is a vaccine.  A protein called tyrosinase is present in canine meloma cells.  The vaccine is made with a foreign version of this protein so the dog's imune system attacks the tumour.  Clinical studies of the vaccine in dogs have demonstrated longer life spans in dogs with melanoma.  The vaccine is not yet licensed for general use.

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 tumour to regress.

How can I nurse my pet?

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

After surgery, the operation site similarly needs to be kept clean and your pet should not be allowed to interfere with the site. Any loss of sutures (stitches) or significant swelling or bleeding should be reported to your veterinary surgeon. If you require additional advice on post-surgical care, please ask.

How will I know if the cancer is cured?

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

Histopathology will provide the 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 tumour 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. There are a few tumours that are difficult to predict behaviourally.

Dogs with skin tumours that have a low rate of cell division, have a 90% chance of surviving more than 2 years after diagnosis; but this reduces to 25% if the rate of cell division (mitotic index) is high. Only 10% of the first group have a tumour-related death; but 45% of dogs in the second group will die from tumour related causes. More nail-base melanocytic tumours have an unfavourable outcome than tumours occurring at other sites.

We have less information about recurrence of tumours and survival times in cats.

Are there any risks to my family or other pets?

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