Melanoma is a fairly common, locally invasive and frequently malignant cancer in domestic dogs. There are several forms of melanoma, which typically are classified by their location. These include tumors that tend to localize in the skin (cutaneous melanoma), eyes (ocular melanoma), nail bed (subungual melanoma) and oral cavity (oral melanoma).
Causes of Canine Melanoma
The causes of melanoma in dogs are not known. This type of cancer arises from abnormal, uncontrolled growth of cells that produce melanin, which is a dark, sulfur-containing pigment normally found in hair, skin, eyes and certain nerves. People can develop melanoma from genetic mutations and in part from exposure to ultraviolet light. However, unlike so-called skin cancer in humans, melanoma in dogs does not seem to be exacerbated by excessive sun exposure. In fact, dark brown and black dogs seem predisposed to developing this form of cancer. The fact that some domestic dog breeds are more commonly affected by melanoma suggests that there is a genetic component to the disease.
Canine melanoma cannot realistically be prevented, because veterinary science has yet to discover its underlying cause. Given the suspected genetic contribution to this disease, affected animals probably should not be bred. However, with early detection, the adverse effects of melanoma may be manageable. Dog owners, especially those with predisposed breeds, should familiarize themselves with the symptoms of melanoma and should perform routine, hands-on examinations of their pets to identify any suspicious lumps or bumps at the very earliest opportunity. These should be checked by a veterinarian as soon as they are identified. Currently, there is no vaccine proven to prevent the occurrence of canine melanoma, although there is a vaccine that is licensed to help treat the condition by prolonging survival rates and minimizing clinical signs of the disease.
While surgical removal is the most common treatment for melanoma, the tumors frequently recur in the same or in other areas post-operatively.
Melanoma in companion canines unfortunately is fairly common, especially on the eyelids but also in many other locations. Most dogs with melanoma show no systemic signs of illness – at least not unless and until the disease has metastasized to vital organs.
Symptoms of Melanoma in Dogs
Melanoma typically presents as single or multiple brown or black nodules on areas of darkly-pigmented skin – especially on the eyelids. These tumors also commonly occur on the toes (in the nail bed), on the lips, in the mouth or on the skin of the trunk or limbs. Sometimes, the mass is not pigmented, although this is uncommon. The tumors usually are detected either by an owner or by a veterinarian during a routine examination.
Oral melanoma usually is seen as a raised mass during a dental examination or other veterinary check-up. Oral melanomas almost always are invasive, aggressive, malignant and extremely metastatic. They may be accompanied by:
Noticeable bad breath (halitosis)
Excessive salivation (ptyalism)
Nail bed melanomas tend to be locally invasive but only moderately metastatic. When the toes and/or nail beds are affected, owners may notice:
Licking at the affected area
In dogs, melanomas of the skin are usually but not always benign. Cutaneous (skin) melanoma may appear as:
Raised dark bumps (single or multiple)
Raised skin sores (single or multiple)
Melanomas around the eye and those in the eye itself usually are benign and rarely metastasize. Owners of dogs with ocular melanoma may observe:
Change in the appearance of the eye
Swelling around the eye
Darkening of the iris of the eye
Mass on or near the eye
Mass on or near the eyelid
Twitching of muscles around the eye (blepharospasm)
Impaired vision (bumping into objects in familiar places; difficulty navigating in unfamiliar places)
Dogs at Increased Risk
Both the benign and malignant forms of melanoma occur most commonly in older dogs, usually after 9 years of age, depending on the breed. Certain breeds appear to be predisposed to developing melanoma, including the Doberman Pinscher, Irish Setter, Gordon Setter, Cocker Spaniel, Schnauzer (miniature and giant), Golden Retriever, Miniature Poodle, Boston Terrier and Scottish Terrier. As domestic dogs age, their owners should be especially conscientious about looking for any new lumps or bumps – especially in the mouth, around the eyes, under or around the toenails and on the footpads.
Melanoma usually presents as a lump or bump and typically is fairly obvious to owners and veterinarians. The precise diagnosis of this type of cancer requires microscopic evaluation of cells and/or tissue samples. It can be difficult even for a skilled veterinary pathologist to determine whether melanoma is malignant or benign. Radiographs (X-rays) are commonly used to assess whether the disease has metastasized – especially whether it has spread to the lungs.
Diagnosing Melanoma in Dogs
The first part of any diagnosis is a complete veterinary physical examination and a thorough case history. Normally, the initial data base also will include assessment of blood and urine samples. The results of these tests may reveal other health abnormalities, but unfortunately they are not conclusively diagnostic of melanoma.
A very common procedure used in suspected melanoma cases is taking a fine needle aspirate (FNA) of suspicious skin masses. This procedure involves inserting a small needle into the lump and pulling back on the attached syringe plunger to gather cells from the affected area. The extracted cells are then ejected onto a glass slide and examined under a microscope – a process that is called cytology. The slides can be dyed or stained before this examination to help enhance distinctions between different cell types. Fine needle aspirates are almost painless, much like a vaccine injection or a routine blood draw.
If the results of a FNA are inconclusive, or if they are suggestive of melanoma, the veterinarian probably will recommend taking a biopsy of the suspicious mass. A biopsy involves removing an actual piece of tissue, rather than simply sampling cells. Unlike FNAs, biopsies typically are performed under sedation, and sometimes under general anesthesia. Biopsy samples are sent to specialized pathology laboratories for processing and evaluation through a process called histopathology.
Additional diagnostic tests are also available, including: thoracic radiographs (chest X-rays) to assess whether the cancer has metastasized to the lungs; fine needle aspirates of nearby lymph nodes to determine whether the cancer has infiltrated the lymphatic system; and ultrasonagraphic examination of the abdomen to assess whether other areas of the dog's body have been affected.
The techniques available to help veterinarians diagnose melanoma in dogs are evolving. It can be quite difficult to identify the precise stages of malignant melanoma or to catch the subtle changes in cells or tissues that happen when melanoma transforms from benign to malignant. Because of these difficulties, most veterinarians opt to surgically remove any identified melanoma mass, whether or not it is known to have metastasized or to be malignant.
The primary goal of treating melanoma is to relieve the symptoms that accompany the disorder, thereby restoring the affected dog's quality of life. Good nursing care, surgical removal, chemotherapy, radiation therapy and other medical and supportive techniques are available to treat melanoma in domestic dogs.
Melanomas of the skin usually begin and remain benign, but they can become malignant. If the affected areas of skin are bothersome to the dog, or if a veterinarian feels that a mass is suspicious, it probably should be removed by simple surgical excision. The surgeon will try to take wide surgical margins, to minimize the chance of future regrowth or spreading. This means that she will attempt to remove not only the tumor, but also an area of healthy, normal tissue all the way around and beneath the tumor. The pathologist who examines the mass and attached tissue will be able to determine microscopically whether the entire tumor was successfully removed. In most cases, the results of surgery and the prognosis for the dog are excellent, if the melanoma is benign.
Dogs with malignant melanoma need immediate treatment. Malignant melanoma is extremely aggressive and requires complete surgical removal of the affected area and much of the surrounding tissue (including underlying bone) as well, if possible given the location of the particular mass. In cases of oral melanoma, the entire jaw and maybe other areas of the mouth may need to be removed. Fortunately, reconstructive surgery can help to repair and rebuild those areas. Nearby lymph nodes may also need to be removed. If the cancer is in the nail bed, surgical excision usually includes amputation of the affected digit (toe). If sufficiently wide surgical margins cannot be achieved, the lower part of the leg may need to be amputated.
Surgical removal of melanoma is commonly followed by radiation therapy. In some cases, radiation therapy may be recommended even without surgical treatment. Chemotherapy is also recommended in many melanoma cases – with or without surgery or radiation treatment- because of the highly metastatic nature of many forms of melanoma.
In 2007, the United States Drug Administration (the USDA) conditionally licensed a vaccine for the treatment (not for the prevention) of canine malignant melanoma. This vaccine has been shown to produce immune reactions in dogs, resulting in tumor rejection and prolonged survival times. Studies about this vaccine and other potential treatment protocols for canine melanoma continue to be conducted.
The prognosis for dogs with benign skin tumors removed surgically is very good. Unfortunately, the outlook for dogs with malignant melanoma is guarded to poor. The prognosis is particularly grave for dogs with malignant oral melanoma that returns after initial surgical removal.