Canine Oral Malignant Melanoma

206

Canine Oral Malignant Melanoma

Key Points

Oral malignant melanoma (OMM) is the most common malignant oral tumor in the dog
Overrepresented breeds include Chow Chows, Golden Retrievers, Poodles, and Cocker Spaniels

Melanoma- tumor of melanocytes
About 1/3 of OMM are amelanotic and may not contain any melanin
Diagnostic challenge
- Morphologically, OMM can look like neoplasms of mesenchymal (sarcoma), epithelial (carcinoma), or round cell origin
- May be difficult to diagnose if they are amelanotic
The majority of OMM are metastatic (up to 80%)
- Frequently metastasize to local lymph nodes and then the lungs
Besides the oral cavity, other common places for melanoma to occur include:
- Cutaneous (more commonly benign)
- In nail beds (about 50% are malignant and 50% are benign)

Biopsy is needed to definitively diagnose OMM
- Can distinguish from common oral tumors (i.e. squamous cell carcinoma or fibrosarcoma)
Once a diagnosis is obtained, staging should include:
- Minimum database
- Regional lymph node aspirates
- Thoracic radiographs
Cross sectional imaging with CT or MRI are often helpful for assessing bone involvement and surgical planning

Treatment of choice for the primary tumor is surgical excision with wide margins, which usually necessitates removal of underlying bone (partial mandibulectomy or maxillectomy)
Radiation therapy is useful in shrinking the tumor or temporarily slowing its growth if surgical excision is not an option or if there is high chance of local recurrence
Adjuvant systemic therapy in the form of chemotherapy or immunotherapy are additional treatment options which may help to slow the progression of, or to shrink metastatic disease
- The platinum agents, carboplatin and cisplatin, have shown mild efficacy in shrinking gross OMM lesions temporarily

Melanoma is often an immunogenic tumor and many forms of immunotherapy have been attempted
Currently, a xenogeneic human DNA vaccine is commercially available for use in dogs with OMM
The vaccine is to be used for treatment of OMM after it has developed, NOT to prevent the disease from developing as with traditional vaccines
How the vaccines works:
- Injected DNA encodes for a human protein called tyrosinase, which is found in melanocytes and melanoma cells
- Since tyrosinase is a human protein, the canine immune system recognizes it as a foreign antigen and mounts an immune response by forming antibodies against it
- Similarities between human and canine tyrosinase cause the antibodies to cross react and bind with canine tyrosinase as well, creating an immune response to the melanoma cells
Early reports of the vaccine showed some promise but its efficacy remains to be proven and requires further investigation

Variables with prognostic significance include:
- Tumor size: tumors <2cm have a median survival time of about 17 months, >2cm have a median survival of about 5.5 months
- Clinical stage
- Complete margins obtained with the first surgery
- Tumor location: dogs with tumors in the rostral oral cavity have longer median survivals than those in the caudal oral cavity