Cat & Dog - Oral Tumours - Melanoma

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.

What is a melanoma?

Oral melanoma (malignant melanoma or melanosarcoma) is a tumour of melanin pigment producing cells (melanocytes). The cancers are rapidly growing and unfortunately are rarely completely cured by surgical removal. Recurrence in the mouth and spread to other parts of the body (metastasis) are common.

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 people, environmental and host factors are important in development of melanoma. Ultra violet irradiation from the sun is the most important environmental factor. Cancers associated with irradiation occur in areas of the body that have not been directly exposed. We do not know if this is the case in dogs and cats but in people both irradiation before birth and radiation hyperthermia treatment have been followed by tumour growth. The cancers behave differently when they occur in different parts of the body, indicating local factors influence their behaviour.

Why has my animal developed this cancer?

Melanocytes are fragile cells and easily damaged. Cancer mutations are type of cell response to damage and, because the more divisions a cell undergoes, the more probable mutation is, cancer is commoner in older animals.

Is this a common tumour?

Melanomas are the commonest malignant oral canine tumour. The gums and inside of the mouth are the most common sites. Tumours are most frequent in animals with a lot of pigment and male dogs may be more predisposed to develop the cancers. Studies on breed prevalence have varied between countries but the average age of affected dogs is 11 years.

Malignant melanomas are rare in the cat.

How will this cancer affect my pet?

These tumours are usually noticed as swellings on the gums that frequently ulcerate and bleed and may become secondarily infected. Many are black in colour. Bad breath is characteristic of melanomas but can occur with other tumours, also with tooth and periodontal gum disease. Other common clinical signs include drooling saliva, difficulty in eating, bleeding, displacement or loss of teeth, facial swelling, pain and swelling of the local lymph nodes (glands).

How is this cancer diagnosed?

Clinically, malignant oral tumours often have a fairly typical appearance. The pigment is not an infallible guide as some are not pigmented (amelanotic) and other types of tumours may also contain pigmented or appear dark.

Radiography (x-rays) may be useful in detecting whether tumours have invaded the bones and to guide surgery. Loss of bone adjacent to the tumour usually means a poorer outlook (prognosis) because malignant gum tumours destroy bone whereas benign ones tend to make the adjacent bone grow.

Cytology, the microscopic examination of a small sample of cells, may be diagnostic in some cases. However definitive diagnosis, prediction of behaviour (prognosis) and a microscopic assessment of whether the tumour has been fully removed rely on microscopic examination of tissue (histopathology). This is done at a specialised laboratory by a veterinary pathologist. The piece of tissue may be a small part of the mass (biopsy) or the whole lump but only examination of the whole lump will indicate whether the cancer has been fully removed. Histopathology also rules out other cancers.

Most of these tumours invade the bone of the jaw. They need wide surgical margins usually including substantial parts of the jaw bone. This type of tissue will need decalcifying so it may take be a few weeks before the final histopathology results are available. The pathologist may also need to remove (bleach) the pigment to check malignancy with greater certainty.

What types of treatment are available?

Surgical removal is the standard method of treatment for all these tumours. The invasive cancers are difficult to remove completely so large pieces of the jaw bone may be removed (hemimaxillectomy or hemimandibulectomy). The complex and extensive surgery is often done at a referral treatment centre.

There has been little progress with other treatments, despite much research on these tumours in people. Melanomas do not respond well to chemotherapy or radiation therapy.  The newest treatment for canine melanoma is a vaccine.  A protein called tyrosinase is present in canine melanoma cells.  The vaccine is made with a foreign version of this protein so the dog's immune system attacks the tumour.  Clinical studies of the vaccine in dogs have demonstrated significantly longer life spans (median survival time of 389 days instead of 60-90 days with presently available therapies) in dogs wich advanced stages of melanoma.  The vaccine is not yet licensed for general use.

Can this cancer disappear without treatment?

Curing infections and healing ulcers will help reduce superficial swelling but not cure the cancer. Very occasionally, spontaneous loss of blood supply to the cancer can make parts of it die but the dead tissue will still need surgical removal. The body's immune system is not effective at making these tumours regress.

How can I nurse my pet?

After surgery, you will probably be provided with an 'Elizabethan collar' to prevent your pet from interfering with the operation site. You may be requested not to examine the surgery but inability to eat or significant swelling or bleeding should be reported to your veterinary surgeon. Your pet may require a special diet. If you require additional advice on post-surgical care, please ask.

How will I know how this cancer will behave?

Histopathology will give your veterinary surgeon the diagnosis that helps to indicate how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence or metastasis (distant spread). The completeness of excision will be assessed and other diagnoses ruled out.

When will I know if the cancer is permanently cured?

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

The outlook for dogs and cats with these tumours is poor. The underlying bone is invaded by over half of these tumours. Approximately 70% of tumours metastasise to regional lymph nodes (glands) and almost the same percentage to distant sites, usually the lungs.

The average post surgical survival time is only about three months and the death rate is almost three quarters of dogs within six months and ninety percent within 24 months. Survival is unrelated to sex, site, rate of growth, histological type, amount of pigment or cancer size.

Tumour stage (how far it has spread) is correlated with survival time. If there is no involvement of the local lymph nodes and no radiographic (x-ray) evidence of lung tumours, survival time is improved by surgery (242 days versus 65 days). Partial removal of the jaw reduces local recurrence of the tumours but does not always prevent the spread elsewhere.

Are there any risks to my family or other pets?

No, these are not infectious tumours and are not transmitted from pet to pet or from pets to people.