Melanoma is a cancer that happens as a result of the unregulated proliferation of melanocytes, a pigment-carrying cell that resides in mammalian skin and mucous membranes. Older dogs are especially prone to this cancer, particularly to its highly aggressive oral form. In fact, melanoma of the mouth accounts for almost 40% of all oral tumors seen in dogs.
Melanoma is considered rare in the cat but can nonetheless occur either in the skin or in the eye.
Melanoma tumors can behave in all kinds of ways. Some are extremely aggressive, not only because they can be highly locally invasive, but also because they have a tendency to spread readily to distant tissues (metastasis). Though difficult to predict, the location of the tumor, the thickness or depth of the primary tumor, and the properties of the cancerous cells themselves are all used to forecast the behavior of melanoma tumors.
Though melanomas of the skin tend to be benign and readily curable, highly aggressive malignant melanomas can still arise anywhere on the skin. More often, however, the eyes, foot pads, nail beds, and where the skin meets the mucous membranes (on the lip, for example) are the sites where aggressive tumors tend to occur. As in the case of oral tumors, these can be locally invasive and metastasize readily to local lymph nodes and eventually to distant organs, such as the lungs and liver.
Unfortunately, the prognosis for most malignant melanoma patients is poor unless aggressive treatment is elected.Some genetic predisposition is assumed in the case of canine melanoma. Unlike the human form, ultraviolet light is not considered a factor in this disease's development in dogs.
Symptoms and Identification
Melanoma's presentation depends on the site it affects:In the case of oral melanoma, owners usually first notice symptoms such as bad breath and abnormal chewing behavior. Because these tumors can invade the underlying bone, deformity, pain, bleeding, and tooth loss are often eventually observed. Bleeding from the mouth, poor appetite, or weight loss may also become evident.
In other locations, such as the footpads or toes, owners may notice pain, bleeding, or limping, if not the mass itself.A tumor that's black in color is most typical of melanoma; however, a full 17% of melanomas are non-pigmented ("amelanotic") and will not appear characteristically black.
The following tests are strongly recommended in the course of melanoma diagnosis:
Complete blood count (CBC)
Blood biochemistry panel
Fine needle aspirate of local lymph nodes
Fine needle aspirate of the tumor
Biopsy of the tumor
Like so many other cancers, the disease can only be diagnosed definitively by retrieving a specimen from the affected tissues (biopsy) and evaluating it through histopathology (microscopic analysis of the tissues by a board-certified pathologist). This may or may not require anesthesia or sedation.
Histopathologic evaluation of the tumor is essential in helping predict the behavior of the cancer. Pathologists will make a variety of determinations to help guide treatment.
If treatment is to be considered, chest X-rays and fine needle aspiration of the nearby lymph nodes are also essential. Abdominal ultrasound may also be recommended. These tests help determine the extent of the disease's progression and help outline a patient's individualized treatment plan.
Because it's difficult to fully appreciate the extent of an oral tumor's margins, CT scans are typically recommended before surgery or irradiation of an oral melanoma mass. This helps guide the surgeon's surgical margins and the radiation oncologist's target area, respectively.
Analysis of the bone marrow may also be undertaken to stage the disease's progress and help direct treatment.
Oral melanoma has been reported to affect smaller breeds like Cocker Spaniels and Miniature Poodles, but one study found that melanoma or the tongue itself seemed to affect larger dogs preferentially -- Chow Chows and other breeds with heavy tongue pigmentation, in particular. For all forms of melanoma, the following breeds are considered predisposed:
No genetic predisposition has been identified in cats.
Melanoma is typically treated via surgical resection. Though tumors of the skin, nail bed and footpads are often excised by general practitioners, oral melanoma is generally considered the realm of the board-certified surgeon. As notoriously invasive as they are, clean margins are usually obtained only via aggressive, highly specialized surgical techniques.
Nonetheless, dogs with aggressive melanomas should not rely on surgical treatment alone. The rate of metastasis is far too high. In fact, dogs treated surgically for very small oral tumors (without any evidence of distant spread at the time of surgery) live only about 17 months. But if those tumors were over 2 cm in diameter, dogs only lived 5.5 months after surgery.
These chilling statistics explain why melanoma treatment often involves the use of systemic chemotherapy drugs, too. Though historically less helpful than chemotherapy in the case of other cancers (such as lymphoma, for example), it's nonetheless recommended as a tool to help delay metastasis.
Radiation therapy is another common tool in the treatment of melanomas, especially when clean margins are not obtainable through surgical resection. In fact, radiation is now sometimes replacing surgical resection altogether. The success rates for this approach, however, depend to a significant extent on the location of the melanoma.
The most promising therapy for melanoma of all involves the recent development of a vaccine that specifically targets its abnormal tumor cells. The vaccine does this by tricking the patient's immune system into recognizing malignant melanoma cells so it can destroy them like they would any other foreign invader. Though the benefits of this technology are still largely undefined, this tool is now widely available through board-certified veterinary internists and oncologists throughout the US.
It is strongly recommended that owners who hope to gain the most comfortable longevity on behalf of their pets consult a board-certified oncologist to receive the fullest understanding of the broad range of their pet's treatment options.
As with so many other aggressive cancers, treatment should be undertaken swiftly as malignant melanoma tumors have a tendency to spread rapidly.
As with many cancers that require specialized surgery, advanced cancer treatment modalities, and an oncologist's consultation for best outcomes, the cost of this disease can prove prohibitive for many pet owners.
Pets who suffer from the aggressive forms of melanoma may experience treatment costs that extend well beyond the $10,000 mark, especially should owners elect to explore all available treatment options, including the novel melanoma vaccine.
The cost of definitive diagnosis is typically around $500 or less, but increases precipitously should patient's owner elect to have the tumor investigated and staged to explore treatment options.
Chemotherapy: The cost of chemotherapy in these cases varies according to the patient's size, but $1,000 to $3,000 is considered typical.
Surgery: The cost of surgery itself will depend on the surgeon's degree of specialization along with the anatomical location of the tumor, but $2,000 to $5,000 is typical of aggressive oral surgery involving the bone. At around $500 to $1,000, toe amputations are usually far less expensive.
Radiation: Radiation therapy in oral cases tends to cost anywhere between $5,000 and $10,000.
Vaccine: The vaccine itself costs approximately $1,000 to $1,500 per dose, with a minimum of four initial doses recommended and later one booster vaccine every six months.
It cannot be overstated that the cost of diagnosis and treatment depends on many factors including geographic location (cost of living), standard of care (lower vs. higher standards of veterinary care), whether specialty hospitals are employed (higher quality equipment, certified personnel, and board-certification for veterinarians specialized in surgery, internal medicine, oncology, and/or radiation oncology).
Unfortunately, many owners elect not to treat pets affected by melanoma's more aggressive forms due to the extreme expenses often associated with this disease.
As with most cancers, there is no known approach to melanoma prevention.
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