Canine Oral Melanoma (OM)

Melanoma is the most common oral malignancy in dogs. Surgical removal is the most effective treatment and is often combined with radiation and/or immunotherapy. Despite treatment for this aggressive cancer, many dogs sadly succumb to distant metastasis. Survival time is related to presence of gross (visible) disease at diagnosis, size of tumor, clinical stage, the presence of lymph node metastasis, and caudal location in the oral cavity (in the far back region of the mouth).

Quick Facts about OM

Commonly affected breeds: Scottish Terriers, Cocker Spaniels, Mini Poodles, Chow Chows, and Golden Retrievers

Frequently metastasizes (spreads) to lungs and lymph nodes

Very rare in cats

If untreated, average lifespan after diagnosis is 65 days

Age, tumor size (less than 2 cm is better), and the ability to completely remove the tumor with surgery affects lifespan after diagnosis

Surgery is the most effective treatment with radiation and/or immunotherapy

Despite treatment, the majority of dogs will eventually develop distant metastasis

Amelanotic melanoma (a tumor without a dark color) occurs up to 38% of the time and can make diagnosis more difficult

Immunotherapy (vaccine) in addition to surgery and radiation holds promise at helping to control the disease

Oral melanoma (OM) is a very aggressive oral tumor that frequently metastasizes (spreads) to other parts of the body, most commonly the lymph nodes and lungs. Dogs rarely show signs of disease at home, so this is most often found during routine physical examination by a veterinarian or when an owner sees the tumor in the mouth at home. Symptoms/signs of an oral tumor that might be noted at home include bad breath, bloody oral discharge, oral pain, and change in eating behaviors. OM is often suspected when the tumor is pigmented or dark in color, but up to 38% of oral melanomas are considered amelanotic, meaning that they lack pigment or color. This can often make diagnosis more difficult and special stains might need to be applied to the biopsy sample in order to differentiate this from other types of oral tumors.

Surgery and radiation can control local disease, but most animals with oral melanoma will eventually develop distant metastasis which is why immunotherapy in addition to these treatments holds promise at helping to control this disease. Mortality (death) for dogs treated with aggressive surgery alone varies considerably (150-874 days), but in general, 1 year survival is less than 35%. Oral melanoma responds well to a number of radiation therapy protocols, and this is especially useful after an unsuccessful attempt at complete surgical removal (another way to put this is when margins are not “clean”).

There is a rare type of menaloma that is considered to be “well-differentiated,” meaning that it has a much less aggressive behavior than traditional oral melanoma. This type of melanoma can be cured with local surgical removal.

Diagnosis:

While OM is often asymptomatic (i.e. the patient is showing no signs at home), dogs might show signs such as bad breath and excessive drooling. In more advanced cases, there may be difficulty swallowing, pain, and oral bleeding. Diagnosis of OM is confirmed with either a fine needle aspirate (needle biopsy) or a full biopsy of a larger tissue sample. Often, immunohistochemistry (IHC) is required to fully differentiate OM from other types of oral tumors. For that reason, your veterinarian may call you with the biopsy sample results and ask you if it is okay to perform additional testing on the sample. This is an important step to make sure the proper treatment options are given to you and to give you an appropriate long-term prognosis.
The World Health Organization has a staging system that is widely used for prognosis and treatment decisions in dogs with oral melanoma. The primary size of the tumor at diagnosis appears to be the biggest factor in long-term survival, with tumors less than 2 cm that have not spread to other parts of the body having the best prognosis, and tumors greater than 4 cm with spread having the worst prognosis. You can see a full chart detailing prognostic indicators at https://todaysveterinarypractice.com/diagnosis-and-treatment-of-canine-oral-melanoma/.
Imaging of oral melanoma is necessary to determine both how extensive the local tumor is and if there is any underlying bone invasion, which is very common with this type of tumor. Dental X-rays (radiographs) and CT scans can both provide information, but radiographs (the more common and less expensive option) will not detect bone loss until 40% or more of the dense outer surface of the bone (the cortex) has been destroyed. For that reason, we prefer to rely on our Cone Beam CT scanner (www.animaldentalspecialist.com/cone-beam) for diagnosis and surgical planning as the sensitivity to bone loss is much higher.

In order to tell if melanoma has spread to other parts of the body, a patient will be “staged”, meaning that multiple tests will be performed to find out if other parts of the body show evidence of disease. Common staging tests include blood tests (CBC, Chemistry, U/A), evaluation of the chest/lungs with CT or X-ray, abdominal ultrasound, and CT of the head. We can never guarantee that spread hasn’t occurred, but the absence of obvious disease in the above tests is a positive sign.

Evaluating for spread to lymph nodes is a bit trickier. There are many different interconnected lymph nodes in the head/neck and cancer can spread to them in unpredictable ways. For example, although a tumor might be present on the right side of the mouth, it might drain to the lymph nodes on the left side of the head and neck. While it is still relatively common to simply test or remove the mandibular lymph nodes or all the lymph nodes on the same side as the tumor, this method is likely to miss metastasis in a large number of patients and subject patients to unnecessary surgery. Tests are still be performed to determine the best way to manage possible lymph node involvement with oral melanoma.

Treatment:

Surgery is the most effective treatment for local control of melanoma. We generally aim for 1-2 cm margins in these tumors, but lack of adjacent tissue in the oral cavity can make this difficult. Mandibulectomy or maxillectomy (removal of a portion of the lower or upper jaw) is generally required as we attempt to remove around 2 cm of normal surrounding tissue with the tumor so that we make sure to remove all of the disease (even cells that we can’t see). Removing the tumor without attempting to get those margins of normal tissue (we often call this “debulking”) is generally not recommended because it often results in rapid recurrence.
Radiation may be recommended when surgery doesn’t successfully remove all of the tumor (“incomplete margins”) or for tumors that are in an area where they cannot be fully resected. Survival times after radiation range from 4.5 to 14.7 months, but since there are so many variables with radiation protocols and types, the understanding about increases in survival times is difficult. OM is generally resistant to chemotherapy, so this is often not utilized for this type of oral tumor.

Much research is being put into immunotherapy for melanoma as this hold the most promise for meaningful increases in survival times. Immunotherapy is designed to induce immunity against cancer cells. Studies suggest that the use of the vaccine ONCEPT © increases survival times. Use of ONCEPT in combination with surgery and or radiation should be discussed with an oncologist.

References:

Bergman P. J. (2007). Canine oral melanoma. Clinical techniques in small animal practice, 22(2), 55–60.
Turek, M., LaDue, T., Looper, J., Nagata, K., Shiomitsu, K., Keyerleber, M., Buchholz, J., Gieger, T., & Hetzel, S. (2020). Multimodality treatment including ONCEPT for canine oral melanoma: A retrospective analysis of 131 dogs. Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 61(4), 471–480. https://doi.org/10.1111/vru.12860
Congiusta M, Lawrence J, Rendahl A, Goldschmidt S. Variability in Recommendations for Cervical Lymph Node Pathology for Staging of Canine Oral Neoplasia: A Survey Study. Front Vet Sci. 2020 Aug 13;7:506. doi: 10.3389/fvets.2020.00506. PMID: 32903520; PMCID: PMC7438545