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Diagnosis of Melanoma: Yes, it's scary and the prognosis is sobering, but today's treatments are extending the lives of many dogs.

When you hear the word "melanoma," sun exposure is probably your first thought. That makes sense, since it's a widely known cause in people. But, at this time, canine melanoma doesn't appear to be related to ultraviolet (UV) light. In fact, canine melanoma often occurs in areas that don't get a lot of sun: the toes, eyelids, and inside the mouth. Melanoma occurs when melanocytes--cells in the epidermal layer of the skin that produce pigment--lose their normal growth controls and grow unchecked.

The term "melanoma" refers to a group of cancers and benign tumors that develop from these melanocytes. The cells can be found in hair roots and are associated with the ducts of oil and sweat glands. Melanocytes are also found in various structures of the eye. The cells get their name from the pigment, which is called melanin.

Benign or Malignant

In the past, all melanocytic tumors were termed melanomas. Today, benign tumors are called melanocytomas and malignant ones melanomas. Melanocytic masses tend to act differently (from a cancer point of view) depending on where they are located. The three most common sites of melanomas are in the mouth, on the feet, and in the eye. In general, masses that develop in haired areas tend to be benign while those growing along mucocutaneous junctions are more likely to be malignant.

Staging of cancers is important to determine prognosis and helps in developing a treatment plan. Staging usually requires a histopathologic evaluation of the growth. One thing the pathologist will look at is mitotic index. This is the rate at which the cancer cells are multiplying. A high mitotic index reflects a more aggressive cancer. While a high mitotic index is generally not a good sign, rapidly dividing cancer cells are sometimes more susceptible to treatment such as chemotherapy and radiation. Staging may also look at factors such as evidence of metastasis.

Toe Tumors

Toe cancers may show up initially as a bump along the toe. The lump may or may not be pigmented. Sometimes the cancer first will be noted as an ulcerative sore. Dogs may or may not be lame.

About 16 percent of all digital (toe) cancers are melanomas. Squamous cell carcinomas are the most common toe cancer with melanomas a strong second. Because these cancers have different behaviors and prognosis, it is important to have histopathology done even if surgery is done to remove the entire affected toe.

The most aggressive toe melanomas are in the nail bed. About 50 percent of them metastasize (spread to other parts of the body). Even if metastases are not noted when the toe is removed, the cancer may spread microscopically. Dogs with these cancers have a median survival time of one year. Treatment is ideally surgery; follow-up radiation or chemotherapy also may be done.

Melanomas not involving the nail bed are more likely to be benign. For these tumors, surgical removal alone may be curative. Benign tumors tend to show up around five to 11 years of age with malignant tumors appearing a bit older--about nine to 13 years of age. Age should not be considered the deciding factor on whether a growth is malignant, however.

Eye Melanomas

A common site for a melanocytoma is on the eyelid. These are usually pigmented, though color may range from black to brown or red. Breeds considered at possible higher risk include Viszlas, Miniature Schnauzers, Airedale Terriers, Doberman Pinschers, and Chesapeake Bay Retrievers. Surgery is generally curative for these growths.

Other tumors may grow on the inner structures of the eye. Iris growths are generally benign but growths on the choroid and retina may eventually lead to visual deficits. Diagnosis here is done via specialized examination of the eyes done by a veterinary ophthalmologist. Surgery may be done with laser or the eye may be rechecked for changes.

Female dogs, and German Shepherd Dogs in particular, are predisposed to a melanotic growth along the limbus of the eye. The limbus is the border between the cornea (clear covering of the eye) and the sclera (white of the eye). Surgical treatment is rarely required as these tend to be benign growths, but laser could be used for removal if needed.

Oral Melanomas

Oral melanomas found in any and all parts of the mouth constitute the most aggressive canine melanomas. It is estimated that 30 to 40 percent of all oral tumors are melanomas. They are usually pigmented but not always. These cancers tend to spread very quickly--metastasizing to any part of the head and neck as well as local lymph nodes and the bones of the jaw. Distant spread to the liver, lungs, and kidneys may also occur.

Clinical signs that owners may notice are bad breath, increased drooling, blood in any saliva, and trouble eating. Loose teeth may be observed or you may notice your dog chewing in an unusual way. Your dog may appear to have trouble closing his mouth. An oral tumor may also be picked up during a routine physical or dentistry or during home dental care. The earlier this type of cancer is detected, the better the chance of controlling it and limiting or preventing any spread.

From there, the plan for diagnosis and treatment is fairly orderly. Dr. Yike Bing, DVM, resident in oncology at the Cornell University College of Veterinary Medicine Small Animal Hospital, describes the ideal diagnostic plan. "After an owner or their veterinarian notices an oral mass, diagnosis usually starts with incisional biopsy, which is typically preferred over excisional biopsy. Histopathology hopefully will yield a diagnosis. Staging would be the next step for oral melanoma, including CBC (complete blood count), chemistry, UA (urinalysis), three-view thoracic radiographs, FNA (fine needle aspirate), and cytology of both mandibular lymph nodes, abdominal ultrasound, and CT scan of the head and neck." Many of these steps are looking for signs of metastasis and local spread of the cancer to plan how extensive surgical treatment will need to be.


Once your dog's cancer has been verified and staged--including a mitotic index to determine how aggressive it is, plus evaluating any identifiable spread--a treatment plan is developed. Dr. Bing notes, "Treatment options for canine oral malignant melanoma consist of surgery, radiation therapy, and immunotherapy, as chemotherapy hasn't been shown to be particularly effective for this type of cancer. Often a combination of these is needed to maximize survival time and quality of life. Which combination is best depends on the individual patient's overall health, the size and location of the tumor, and evidence of metastasis. In general, surgery and/or radiation therapy is recommended for local control whereas immunotherapy is recommended to delay the onset of metastasis."

Surgery is almost always part of the treatment plan. A thorough and extensive surgery will often significantly improve survival time. "Typically, the oral surgeon will remove a portion of normal tissue around the mass to maximize the chance of removing all the cancerous cells. A CT scan will help determine if such a surgery is feasible, as well as the margins required. Often a portion of the jaw will need to be removed, which the CT scan would also help determine," says Dr. Bing.

"For patients that require partial removal of the jaw, this may seem daunting, but the majority of dogs will have a favorable outcome including being able to play with toys, eat, drink, and have an acceptable cosmetic appearance," states Dr. Bing. Owners should expect that dogs may take some time to figure out how to carefully eat or drink after extensive surgery, but dogs are quick learners and will generally be back to normal activities quickly.

"Radiation therapy is another way to gain local control of the tumor (i.e. treat the oral tumor only; it doesn't address metastasis). It can be used in two ways; post operatively if the tumor could not be removed completely or if surgery is not feasible due to tumor size and/or location. Melanomas tend to be sensitive to radiation therapy. In the latter scenario when surgery is not feasible, palliative radiation therapy is recommended," suggests Dr. Bing. The goal then is to maintain quality of life for your dog.

Did You Know?

An incisional biopsy takes part of the tissue being studied, such as a mass in your dog's mouth. An excisional biopsy removes (hopefully) the entire mass.

An incisional biopsy is preferred with oral masses. If an aggressive cancer is identified, the surgeon will take wide margins around the mass, rather than just removing the obvious mass. It can be difficult to go back in and remove compromised tissue after an excisional biopsy has been done.

Melanoma Vaccine

A new tool for fighting oral melanomas in dogs is a vaccine designed to attack the tumor cells. It does this via DNA encoding for the gene for the protein tyrosinase found in human and canine melanocytes. The vaccine uses the human DNA coding. The science behind this is that when given to a dog, the dog's own immune system will react against the foreign human tyrosinase protein. Since human and canine tyrosinase are similar, the dog's immune response will be stimulated to attack the tyrosinase-containing melanoma cells.

Currently, the vaccine use is limited to veterinary oncologists and the treatment of dogs with Stage II or Stage III oral melanoma. It has USDA approval but has not undergone the full set of testing that the FDA requires. While it appears to be quite safe, further studies need to be done to establish just how effective it is. Initial studies suggest it can lengthen survival times. One report said from less than six months after initial diagnosis to 20 months or longer.

The protocol for the vaccine is to give it every two weeks for four treatments and then "boosters" are administered every six months thereafter. Side effects include irritation at the injection site and changes in coat and skin color. Some dogs respond well but others may not respond at all to the vaccine. It is important to note that this is not a vaccine given as a "preventive" but rather is included as part of the therapeutic plan for a dog with oral melanoma. Surgery and/or radiation are still the backbone treatments for this cancer.
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Title Annotation:DISEASE
Publication:Dog Watch
Date:Oct 1, 2018
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