Melanoma is relatively common in dogs, accounting for 3% of all neoplasms and up to 7% of all malignant tumors. They are usually found in older dogs.
Canine oral melanomas are virtually always considered malignant tumors, whereas more than 95% of cutaneous melanocytic lesions are benign. Many are locally aggressive and metastasize early, leading to many cases being diagnosed later in the disease progression. Malignant melanoma also have a propensity for migrating along anatomical structures such as nerves (neurotropism) and skin appendages and therefore, diagnosis can be ascertained relatively late in the disease.
The metabolic difference between benign and malignant melanocytes appears to correlate well with COX-2 expression, with benign melanocytomas appearing to be less productive of this inflammatory mediator compared with melanomas, suggesting that possible chronic inflammation may be an initiator in the transformation from benign to malignant status.
Diagnosis is usually based on visualization of a deeply-pigmented non-pruritic lesion in the skin, mucocutaneous junctions or eye, however, histological analysis is essential for diagnosis. The histological characteristics of malignant melanomas include metastasis, high mitotic index, nuclear atypia, increased cellular size/volume, the presence of deep inflammation, and intralesional necrosis.
Malignant melanomas must be distinguished both visually and histologically from benign melanocytomas, which are a pigmented macule, papule or plaque or pedunculated mass that are usually just an incidental finding but may alarm some clients and clinicians.
Malignant melanomas appear different to melanocytomas, and have a raised, swollen and ulcerated appearance that is variably pigmented, often with underlying skin, nail or bone destruction.
Micro RNA assays have been developed as both diagnostic and prognostic tests for canine melanoma.
Treatment is usually a combination of wide-margin excision of the localized tumour, with digit amputation or rim mandibulectomy (with oral melanomas).
In concert with surgical excision, a combination of radiation therapy and/or chemotherapy is usually implemented to augment patient longevity.
Unfortunately, response rates to chemotherapy in dogs with advanced melanoma range from 8% to 28% with little evidence that treatment improves survival.
Recently, a human tyrosinase DNA vaccine has been developed which shows promise against canine oral melanoma.
- Flick River
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