Surgery is the most effective local treatment for melanomas. Small tumours (<2 cm in diameter) that are not attached to the underlying tissue layers, are well defined and grow slowly tend to be benign and easily excisable. Large, ill-defined, rapidly growing tumours (> 2 cm) with ulceration can make surgical excision challenging.
Because melanomas are so invasive and can spread quickly, wide margins need to be calculated when the tumour is removed. With melanomas that occur on the gums, it is possible that this involves the removal of part of the upper or lower jaw. With melanomas in the cheeks, lips or tongue, bone removal is often not necessary. In general, a margin of 1 to 3 cm should be calculated depending on the degree of the tumour (ideally 1 layer of tissue). If only incomplete margins (space around the tumour) can be obtained, the combination of surgery with radiotherapy or additional treatments such as chemotherapy and immunotherapy should be considered - although the efficacy of chemotherapy and immunotherapy against metastases is still under investigation.
For dogs that undergo a partial removal of the upper or lower jaw, quality of life is usually very good, and most dogs resume eating within three days of surgery.
Melanomas are considered relatively insensitive to radiation therapy (although much depends on the radiation therapy protocol). However, radiation therapy is considered an effective therapy for the local control of oral melanoma with minimal side effects in dogs. Usually, the tumour of the oral cavity and the local lymph nodes (mandibular and retropharyngeal) are treated. Typically, a margin of 2 cm is applied around the tumour or the surgical incision site.
- Curative radiotherapy (with healing intent):
- Malignant melanomas generally respond well to dose fractionation (with a total of 6 fractions either once per week for 6 weeks or twice per week for 3 weeks).
Response to treatment occurs fairly quickly (tumour size decreases within weeks of first treatment).
A complete or partial reduction of the tumour is possible, although it is often temporary. The occurrence of relapse depends on the protocol used. When cancer cells are still microscopically present, 26% of the treated dogs relapse. When macroscopic cancer tissue is still present, 45% of the treated dogs relapse. In tumours treated with radiotherapy smaller than 5 cm3, chances of response and survival time were higher than for larger tumours.
The reported most common survival time for radiotherapy range from 4.5 to 14.7 months.
The prognosis improves as the tumour is located more at the front of the mouth, there is no radiographic evidence of bone damage and no microscopically detectable cancer cells are present.
The side effects of radiotherapy depend on various factors such as the total dose used, the dose / fraction and the volume of tissue in the irradiation field.
Rapidly occurring side effects can include swelling of the mucous membranes, as well as mild inflammation with possible pain, bleeding, ulceration and tissue death of the gums, tongue and cheek mucosa. At the level of the skin, the skin may feel dry and a skin thickening and baldness may occur in that place. These side effects usually recover within 1-2 weeks of treatment. Side effects can also occur at a later time: in 1/10 dogs a certain amount of bone tissue death has been observed in the long term and tumours can also paradoxically develop through radiation.
- Palliative radiotherapy:
- This is a form of radiotherapy that is not intended to cure, but to slow down the disease. This form of treatment is mainly used in metastatic or advanced oral malignant melanoma.
Chemotherapy may be appropriate in dogs with malignant melanoma because of the high risk of metastasis (up to 75%). This chemotherapy can be administered directly into the tumour or intravenously. Until now, chemotherapy has not been shown to be very effective in melanomas.
- Intralesional chemotherapy (directly in the tumour):
- The information about these treatment options is rather anecdotal. Implants that release the chemotherapeutic agent cisplatin are placed in the tumour. With this treatment, up to 70% of treated dogs may experience temporary tumour reduction, but no cure. One can also inject cisplatin into the tumour and then apply electrodes to the tumour. These provide a small shock that temporarily causes pores in the cancer cells. Through these pores, the chemotherapeutic agent penetrates into the cancer cells which die. The latter treatment is not yet standard treatment in veterinary medicine and cannot be offered on a regular basis due to the need for a special device called an electroporator (see non-exhaustive list on electrochemotherapy). Overall, electrochemotherapy seems to be a reasonable option in dogs who are not surgical candidates. The response rate is similar to that of radiotherapy.
- Systemic chemotherapy:
- The best clinical effects against oral malignant melanomas have so far been obtained with chemotherapeutics such as carboplatin and cisplatin. However, chemotherapy has not been shown to be very effective in melanomas so far.
Immunotherapy is a promising additional treatment for dogs with oral malignant melanomas. Until recently, the most popular supportive / adjunct treatment was chemotherapy. However, melanomas are not very sensitive to chemotherapy, which means that immunotherapy is gradually becoming more and more prominent as an alternative method to stop the division of cancer cells.
At the moment, there are few immunotherapeutic treatments available in Europe.
- Melanoma expresses the pigmentation enzyme tyrosinase and its expression increases as the tumour grows. Therefore, treatments that target tyrosinase will also allow targeting of melanomas. Oncept® is a therapeutic cancer vaccine that produces a foreign (human) tyrosinase. This vaccine teaches the dog's immune system to better recognize melanoma cells and attack them.
A study compared 58 prospectively enrolled dogs with surgically resected stage II and III oral melanomas treated with adjuvant Oncept® to a historical control group of 53 dogs. Dogs treated with Oncept® had a significantly better outcome: the median survival time for dogs in the historical group was 324 days whereas the median survival time could not be determined in the vaccinated group as only 26% of the dogs died as a result of their disease. However, caution should be exercised when interpreting these results because, due to increasingly earlier detection and better treatment methods, dogs have a longer survival time today than in the past. This vaccine could be interesting as an additional treatment in a minimal residual disease setting (when as little cancer tissue as possible is present (e.g. after surgery)) and this approach is still under investigation.
- Pet Biocell
- is a German company that produces personalized cancer vaccines. To date, there are no large studies available that demonstrate this treatment option is effective.
- Elias Animal Health
- is an American company that produces cancer vaccines and collects white blood cells after administration of the cancer vaccine. When the white blood cells that specifically recognize the cancer cells are cultured in large numbers, they are returned to the dog to more efficiently kill the cancer cells. For the time being, there are no trial results available for melanoma, only for bone tumours (14 dogs enrolled of which 50% of dogs survived greater than one year and 36% survived multiple years).