Abstract
Oral malignant melanoma (OMM) is the most common neoplasm of the canine oral cavity. It is characterized by its aggressive local disease as well as its high rate of lymphatic invasion and distant metastasis. OMM carries a poor prognosis, with most patients succumbing to the disease due to progression of the neoplasm. Histopathologically, OMM is characterized by significant nuclear atypia, a mitotic index of greater than 4/10 hpf, and evidence of vascular invasion or metastasis. Clinically, these lesions can become locally invasive, causing lysis of bones and severe inflammation of the surrounding soft tissue. With time, these lesions can spread to the regional lymph node and to the lungs and other organs. Prognosis can vary depending on the size of the primary tumor, regional node involvement, and distant metastatic disease; however, multiple studies report a relatively short median survival time ranging from less than 4 months to 8 months. Histologically well- differentiated melanocytic neoplasms (HWDMN) are a variant of OMM and sometimes referred to as canine oral melanocytic neoplasms of low malignant potential. Unlike OMM, patients with HWDMN have longer survival times. Histopathologically, HWDMNs have well-differentiated melanocytes with a low mitotic index of 3 or less per 10 hpf and minimal nuclear atypia. HWDMNs have better prognosis with a mean survival time of up to 34 months. This article is a comparative review of OMM and its less aggressive counterpart.
Highlights
Canine oral malignant melanoma (OMM) is an aggressive cancer of the oral cavity in dogs that accounts for 11.5% [1] to 17.1% [2] of all oral tumors and is the most common malignant oral tumor, making up between 33% [3]–35.8% [1] of all malignant oral tumors
A retrospective study reported that 32% of dogs with melanocytic neoplasm were alive at follow-up or died of causes unrelated to the neoplasm and concluded that not all melanocytic oral neoplasm should be considered highly malignant [20]
A variant of OMM, known as histologically well-differentiated melanocytic neoplasm (HWDMN), with a low mitotic index (MI < 4 per 10 hpf), has a more favorable prognosis compared to traditional OMM [21]
Summary
Canine oral malignant melanoma (OMM) is an aggressive cancer of the oral cavity in dogs that accounts for 11.5% [1] to 17.1% [2] of all oral tumors and is the most common malignant oral tumor, making up between 33% [3]–35.8% [1] of all malignant oral tumors. The survival times following palliative radiation therapy (RT) varies from 5.3 to 11.9 months [10,14,15,16,17], with a longer survival time of 11.9 months reported with concurrent cisplatin or carboplatin administration along with radiation therapy [17] Systemic treatments such as immunotherapy and chemotherapy have been studied; chemotherapy for OMM has not been promising and immunotherapy, mainly the Oncept® vaccine, shows varying results [8,9,18,19]. The purpose of this review is to shed light on the lesser-known variant and to provide better understanding of this disease compared to the more common aggressive variant of malignant melanoma in the oral cavity of dogs
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