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Melanomas in Dogs
Veterinary & Aquatic Services Department, Drs. Foster & Smith
Cancer and Tumors
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Melanomas in DogsMalignant melanoma is a common cancer in dogs. Malignant melanoma is an abnormal growth that involves the pigment-producing cells called melanocytes. These cells are located throughout the body, wherever tissues are pigmented (colored). Malignant melanomas are more common in the dog than in the cat. Oral melanoma is the most common oral malignancy in dogs.

Which dogs are at the most risk for developing a malignant melanoma?

Ultraviolet light appears to predispose humans to malignant melanoma. There does not appear to be this same close association in dogs. In dogs, malignant melanomas appear to be caused by a combination of genetics and environmental factors. Breeds at risk include but are not limited to:

  • Scottish Terrier
  • Airedale
  • Boston Terrier
  • Cocker Spaniel
  • Springer Spaniel
  • Doberman Pinscher

What are the symptoms of malignant melanoma in a dog?

Malignant melanomas can occur almost anywhere on the surface of the body. They are generally classified as oral, or non-oral.

Oral malignant melanomas occur on the:
  • Gingiva (gums)
  • Lips
  • Tongue
  • Hard palate
Non-oral malignant melanomas occur at other locations including the:
  • Skin
  • Nail bed
  • Foot pad
  • Eye
The appearance of a malignant melanoma can be variable. It usually is a solitary tumor that is 1/4 inch to several inches in diameter. It is often a dark color (grey, brown or black) but may be nonpigmented, as well. It is usually hairless and may become ulcerated.

Terms Related to this Discussion:
Cancer - A general term frequently used to describe any of various types of malignant neoplasms, most of which invade surrounding tissues, may metastasize (see below) to several sites, and are likely to recur after attempted removal and to cause death of the patient unless adequately treated.

Neoplasm - An abnormal tissue whose cells grow more rapidly than normal and accumulate. Closely related to a tumor.

Tumor - An abnormal growth of tissue resulting from uncontrolled multiplication of cells and serving no normal function in the body. Closely related to a neoplasm.

Malignant - Resistant to treatment; occurring in severe form, and frequently fatal.

Benign - Denoting the mild character of an illness or the nonmalignant character of a neoplasm.

Metastatic - The movement of a disease from one part of the body to another. In cancer, the appearance of neoplasms in parts of the body remote from the site of the primary tumor.

Chemotherapy - Treatment of disease by means of chemical substances or drugs.

Radiation Therapy or radiotherapy - The use of high-energy radiation from x-rays, gamma rays, neutrons, protons, and other sources to kill cancer cells and shrink tumors. Radiation may come from a machine outside the body (external radiation therapy), or it may come from radioactive material placed in the body near cancer cells (internal radiation therapy).

How is malignant melanoma diagnosed?

A biopsy, in which a portion of the tumor is examined under a microscope, is necessary to diagnose a malignant melanoma. In addition to making the diagnosis, it is important to "stage" the tumor, which is a way of describing how serious it is.

How do we size and stage a malignant melanoma?

Accurately staging melanomas requires:

Oral melanomas
For oral melanomas in the dog, the veterinary profession has adopted the World Health Organization's (WHO) staging system:
  • Stage I - tumor is less than 2 cm in diameter
  • Stage II - tumor is 2-4 cm in diameter
  • Stage III - tumor is greater than 4 cm or any tumor that has spread to the lymph nodes
  • Stage IV - includes any tumor with evidence of distant metastasis
Non-oral melanomas
The staging system for non-oral hematomas in dogs is as follows:
  • T1-tumor - tumor is less than 2 cm and is superficial
  • T2-tumor - tumor is 2-5 cm and has minimum invasion (has not spread below the skin)
  • T3-tumor - tumor is greater than 5 cm or has invaded the subcutaneous tissues (tissues below the skin)
  • T4 - tumor has invaded deeper into the tissues or bone

What is the treatment of malignant melanomas in dogs?

For melanomas without distant metastatic involvement, surgery is recommended to remove the tumor. The extent of the surgery depends on the melanoma's location. Melanomas in an area of haired skin generally require a simple lumpectomy. All other sites require a much larger area around the tumor be removed as well as the tumor itself. Accurate staging of the tumor will define whether or not additional measures such as radiation will be needed. The success of chemotherapy has been disappointing, though it has been tried in some cases. If the biopsy shows that tumor cells were present on the edges of what was removed, a second surgery to remove more of the tissue may be performed, if possible.

There is a vaccine that targets melanomas. The melanoma vaccine uses genetically engineered DNA (deoxyribonucleic acid) to stimulate the dog's immune system. This vaccine is indicated for the treatment of dogs with either stage II or stage III oral melanoma for which the positive lymph nodes have been surgically removed or irradiated. Through the vaccine, antibodies are produced that attack a specific site on the melanoma's cells leading to their destruction. The vaccine is only available through veterinary oncologists (veterinarians specializing in cancer diagnosis and treatment).

What is the prognosis for a dog with malignant melanoma?

The prognosis of an animal with a melanoma depends on several factors including the anatomical site and the stage of the tumor. In general, tumors located on hair-covered skin are most often benign. Tumors located at or near a muco-cutaneous junction (where the skin meets the inside of the body, such as near the gums or the anus) are most often malignant and metastatic. Tumors arising from the animal's nail bed are almost always highly malignant. The higher the stage of the tumor, the more serious the prognosis.

Summary

Melanomas can be benign or malignant. They can remain localized or metastasize and invade other parts of the body. They can be treated by complete surgical removal or, if the entire tumor cannot be removed, then radiation therapy may be added to the treatment. The development of the melanoma vaccine is a major step forward in the battle against canine malignant melanoma (and possibly other cancers). As with other illnesses, early detection usually leads to a more favorable outcome. If you suspect that your dog has a tumor of this kind, please seek veterinary care as soon as possible.

 
References and Further Reading

Janet L. Peterson, C. Guillermo Couto. Tumors of the Skin and Subcutaneous Tissues. In Birchard, SJ; Sherding, RG (eds.) Saunders Manual of Small Animal Practice 2nd Edition. W.B. Saunders Co. Philadelphia, PA; 2000: 233-234.

Kenneth M. Rassnick. Tumors of the Skin. In Ettinger, SJ; Feldman, EC (eds.) Textbook of Internal Medicine, Diseases of the Dog and Cat 6th Edition. Elsevier Inc. Saint Louis, MO; 2005: 747.

Philip J. Bergman. Malignant Melanoma. In Kirk's, John D. Bonagura, David C. Twedt (eds.) Current Veterinary Therapy XIV. Saunders Elsevier Inc. Saint Louis, MO; 378-382.


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