What is a melanoma?
A melanoma is a cancer of the melanocytes, the pigmented cells which are responsible for the dark colour of the skin and mucosae. Most melanomas arise from the skin and oral cavity, and can be either benign or malignant. Most of the cutaneous ones tend to be benign (with the exception of digital, subungual and footpad locations), whereas most of the oral ones tend to be malignant. Melanomas are the most common oral tumours in dogs. Malignant melanomas can metastasise to the regional lymph nodes and more distant sites such as the lung, and less commonly other organs. Melanomas are less common in cats, and are generally cutaneous.
How can I tell if my dog has a melanoma?
Melanomas generally appear as dark pigmented nodules. They can be very invasive in the mouth, and grow up to a large infiltrating and ulcerated mass. They can occasionally lose their pigment and appear have a fleshy colour.
What causes melanomas?
In humans, it is well established that melanomas may arise due to mutations induced by UV-light. This is unlikely to be the case in dogs, and there is currently no well-established risk factors in pets.
How are melanomas diagnosed?
A fine-needle aspirate is often sufficient to support the initial diagnosis of melanoma. Melanomas may be more difficult to diagnose in some cases, especially when they have lost their pigment. In these cases, the diagnosis can be established with a biopsy and some specific markers (immunohistochemistry).
Other tests can be performed to stage melanomas and assess if it has spread. These may include lymph node fine-needle aspirates, lymph node sentinel lymph node mapping, lymph node ultrasound, thoracic radiographs or CT scan.
How are melanomas treated?
The treatment of choice of melanomas is not well established. Complete excision of the primary tumour appears to be the most important. Oral melanomas often require some bone excision (mandibulectomy, maxillectomy) to achieve complete margins. Regional lymph nodes may also be removed but the possible therapeutic benefit of removing metastatic lymph nodes is not well established with this cancer, as it may spread directly to more distant sites. As some malignant melanomas have a high risk of spread, a course of adjuvant chemotherapy after surgery is generally offered, but there is currently no clear evidence of benefit. A commercial melanoma vaccine (Oncept®) is currently available. The possible benefit of this immunotherapy is also controversial, but it is typically not associated with clinically relevant side-effects. At AURA, we may also be able to make an autologous vaccine from a fresh tumour sample using an APAVAC® kit, but the potential benefit of this type of vaccine is currently unknown in melanoma cases.
If surgical excision is not possible or declined, radiation therapy can also be prescribed as melanomas can respond relatively well to radiation. Metastatic cases can be treated with systemic treatments, including chemotherapy. Other medical options and tailored treatment could also be discussed with our medical oncology team.
What is the prognosis for melanoma?
Benign melanomas carry a good prognosis and complete excision is generally curative. In a recent retrospective study, about 22% of dogs with malignant cutaneous melanomas developed metastases. Digital melanomas may be more aggressive as only 10-15% of dogs are still alive at 2-years following their diagnosis. The prognosis of oral melanomas is more variable and largely depends on the stage (size of tumour, lymph node and distant metastases), location in the oral cavity and treatment prescribed. Dogs with stage 1 oral melanomas (tumour ≤ 2cm) have reported median survival times of 1.5-3 years depending on the studies, whereas dogs with stage 3 (tumour > 4cm or lymph node metastasis) have median survival times of 3-6 months.
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