CANINE MELANOMA          


Melanocytes are the pigment containing cells in the skin. They can form both benign (melanoma) and malignant (malignant melanoma) tumours in the skin and elsewhere in the body. These account for over 5% of skin tumours in dogs and are most in male dogs and those over 7 years of age. The amount of pigment in these tumours can vary very much from virtually nil to a very dark staining mass. They are very uncommon in the cat.

The only way of confirming whether a particular mass is benign or malignant is by biopsy and histopathology but a guide is that those under 20mm in diameter are less likely (but no always) to be benign whilst those that are larger are more likely to be malignant. The malignant melanomas are more likely to be ulcerated, raised and crusty, whilst the benign are more likely to be smooth. Those developing at so-called mucocutaneous junctions are more likely to be malignant. The degree of pigmentation is not a guide to malignancy.

Any pigmented skin lesion should be treated with respect and, as with the human, and any noticeable increase in size, any crustiness, ulceration of raising from the skin surface should immediately be investigated. Malignant melanomas will spread initially to the local drainage lymph node and then to other parts of the body with the lung and muscles often being early targets. We have seen several cases presented with metastases in the heart muscle.

Once a malignant melanoma has metastasised there is very little treatment that can be offered at present and the outlook is very poor. Some developments using inteferons and cytotoxic drugs are proving very successful in the human but these have not currently been used in the domestic species partly as a result of the cost and more generally because of the adverse effects that tend to occur. The key to success is early diagnosis, aggressive surgical removal (resection) and careful monitoring. Without any doubt the most important of these is early diagnosis.

Melanoma Tumor In A Dog

Melanoma tumors in dogs, more than most cancers, demand immediate attention.  As a group,melanomadunn1 melanomas can be either benign or malignant.  Early recognition of melanomas can lead to more successful attempts at removal and identification of the grade or stage of cancer.  The risk of metastasis for benign forms of melanoma is not very high but these can be locally invasive.  Malignant melanomas can metastasize (spread) to any area of the body especially the lymph nodes and lungs and present very challenging and dangerous prospects for the dog.  Cats seem much less susceptible to melanoma tumors than dogs.

Some dog breeds are more at risk for melanomas such as those below:
Airedales Boston Terrier Boxer Chihuahua Chow Chow Cocker Spaniel Doberman Golden Retriever
Irish Setter Miniature Schnauzer Scottish Terrier Springer Spaniel

Benign cutaneous melanomas of dogs are usually seen as round, firm, raised, darkly pigmented masses from one-quarter to 2 inches in diameter.  They occur most often on the head, digits or back.

In the dog, presence of malignant melanoma may be first discovered in the lungs where diffuse pulmonary infiltration of tumors will be displayed throughout the lung tissue on a radiograph (x-ray).  Lymph node swelling or enlargement may be a clinical sign of malignant spread of a melanoma.  Some melanomas do not display the characteristic darkly pigmented color of most melanomas.  The pigment called melanin is a hallmark of these tumors and usually is present in large amounts in melanomas.

A definitive diagnosis is made via microscopic analysis (histopathology evaluation by a Specialist in Veterinary Pathology) of a small section of the growth.  This is called a "biopsy" of the tumor.  The examining pathologist usually will grade the specimen according to how actively the cells are replicating.  This gives an approximation of how likely the growth is to invade and spread.  If an entire growth is removed, the pathologist can report on the tissue's grade as well as any evidence that parts of the tumor may not have been thoroughly excised by the surgeon.

Treatment of melanomas is best provided by surgical excision of the tumor and nearby surrounding tissue.  Localized tumors may be completely removed and the patient cured.  However, if a malignant melanoma has had the opportunity to spread to distant areas of the body, long term survival of the dog is not likely.  Chemotherapy has been performed with marginal success; complete remissions of metastatic melanoma cases are rare.  Fortunately most cutaneous (skin) melanomas are benign, but individual growths need to be evaluated as unique and unpredictable since any given melanoma may become malignant.

Case Presentation:
A Golden Retriever was presented for routine vaccinations.  The attending veterinarian, as part of the pre-vaccination physical exam, noticed an abnormal, darkly pigmented, raised tissue mass at the lateral edge or the dog's right corneal-scleral junction.  The suspicious mass was creating a slight deviation in the smooth surface of the cornea and seemed to be invading both the sclera (white area of the eye ball) and the cornea.  Because the veterinarian suspected the mass was a melanoma, referral to a specialist in Veterinary Ophthalmology was done.  Dr. Sam Vainisi of the Animal Eye Clinic in Denmark, Wisconsin, evaluated the four-year-old Golden Retriever and suggested that surgery be done.  Using a CO2 laser the growth was excised.  Because of the depth and diameter of the growth, as well as the unusual location, Dr. Vainisi performed a frozen tissue, cornel-scleral graft with healthy tissue from the clinic's eye bank to fill in the defect. The tissue graft was carefully sutured into the surgical site.  Topical and oral antibiotics and an anti-inflammatory medication were used after the surgery and healing of the surgical site was uneventful.  The photos below display the melanoma prior to the surgery and six months after.  Annie, the patient, is healthy and active and is expected to have no visual impairment as a consequence of the tumor.  Thanks to the specialist's careful evaluation and surgical excision of this melanoma, Annie is expected to have no further problems with the eye.

(click on an image to see close-up view)
Two views of a dark, raised mass of six month's duration at the corneoscleral junction in a Golden Retriever
Two views of the healed surgical site, six months after surgical excision and with tissue transposition

If you discover a darkly pigmented, raised, thickened growth anywhere on your dog, be sure to have your veterinarian evaluate it.  Keep in mind that pigmented (black) areas of the skin are common in dogs and cats, especially in the tongue, gum and eyelid tissues... and these darkened areas may be completely normal for that individual.  However, if any darkly pigmented areas are actually raised above the normal surface or seem thickened or ulcerated or inflamed, an exam is indicated.

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Canine Melanoma
Malignant Melanoma in Dogs

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The above information is simply informational. It's intent is not to replace the advice of a veterinarian nor to assist you in making a diagnosis of your pet. Please consult with your own veterinarian for confirmation of any diagnosis. Your pets life may depend on it.