Canine oral malignant melanoma
Is it time to talk about it as a chronic disease?
Malignant melanoma (OMM) is the most common nonodontogenic oral tumour in dogs. Clinical signs may vary greatly; the tumour is not necessarily pigmented (black). Pathohistological diagnostics may be complicated as a tumour may present as amelanotic variant and/or as epithelioid-cell OMM, spindle-cell OMM or mixed-cell OMM. Therefore, immunohistochemistry is often needed to diagnose OMM. OMM is locally invasive, with 50% of tumours being associated with surrounding bone invasion. Metastases are also common: in 74% of cases, OMM metastasise in regional lymph nodes and in up to 92% of cases in the lungs. When conducting a diagnostic work up of a patient with OMM, it is, therefore, of utmost importance to perform a proper staging of the disease: to diagnose the local extent of the tumour (CT of the head pre- and post-contrast is needed) as well as regional and distant metastases. To diagnose regional and distal metastases, CT of the neck (regional lymph nodes) and lungs, or even whole-body CT, and fine needle aspiration of regional lymph nodes is recommended. Palpation of lymph nodes is not appropriate to diagnose metastases, as the method is inaccurate; 40% of palpably normal lymph nodes contain metastases.
Prognosis for animals, especially if the tumour arises from dentate areas, is guarded due to early and common metastases. Dogs with small OMM (smaller than 2 mm in diameter) located rostrally and without metastases, have the best prognosis with a median survival time of 18 months (without treatment). The longest survival times are associated with radical tumour resection (tumour with associated 1 cm of healthy tissues as determined by CT); studies report survival to be between 206 and 730 days; this large variability is associated with the stage of the disease at the start of the treatment. It has also been reported, that even incomplete tumour resection (dirty margins) increases survival time. Surgical treatment can be complemented or even substituted with other treatment options, most commonly with radiation therapy and/or, more recently, with immunotherapy and/or electrochemotherapy and/or gene-electrotherapy. Radiation therapy is indicated in cases in which the size and/or location of OMM prevents complete surgical removal without significantly impairing function and/or cosmesis or when the client refuses surgical procedure. Radiation therapy is also indicated, when surgery has resulted in incomplete removal (dirty margins) or when there are regional metastases. The outcome of radiation therapy depends on the radiation protocol, but most commonly hypo-fractionated radiation protocols are recommended.
If you have noted any problems with your animal, please consult your veterinarian.
Text by Ana Nemec