Finding and treating oral melanoma, squamous cell carcinoma, and fibrosarcoma in dogs
The oral cavity is a common site for malignant tumors, accounting for 5% to 7% of all canine cancers.1 The most common oral malignancies in dogs are melanoma (30% to 40%), squamous cell carcinoma (17% to 25%), and fibrosarcoma (8% to 25%),1,2 although the frequency of occurrence varies depending on whether tonsillar squamous cell carcinoma is included in the squamous cell carcinoma category. In three studies analyzing 893 tumors either submitted to a laboratory or entered into a database, squamous cell carcinoma was most prevalent (41%), followed by melanoma (37%) and fibrosarcoma (22%).3-5 These studies included tonsillar squamous cell carcinoma (161 cases). If this category was excluded, melanoma would be most prevalent (46%), with squamous cell carcinoma (28%) and fibrosarcoma (26%) making up the remaining tumors.3-5
Less common malignant oral tumors include osteosarcoma, mast cell tumor, hemangiosarcoma, lymphoma, plasma cell tumor, and multilobular tumor of bone (previously called multilobular osteochondrosarcoma). Other disease processes such as benign tumors, gingival hyperplasia, and infectious conditions can occur within the oral cavity and must be considered as differential diagnoses. Benign tumors such as ossifying and fibromatous epulides and the locally aggressive yet nonmetastatic acanthomatous epulis are not discussed in this article.
Canine malignant oral tumors can occur in any breed, but certain breeds appear to be overrepresented depending on the tumor's histologic type and location within the mouth. In two studies, melanomas were overrepresented in smaller breeds such as cocker spaniels and miniature poodles,6,7 while in another study lingual melanomas were more common in large-breed dogs.8 Yet other studies show dogs of all sizes being affected by melanoma at any oral location.9-14 Dogs with a heavily pigmented oral mucosa, such as chow chows, have an increased risk of developing melanoma.7-10
Squamous cell carcinoma and fibrosarcoma are more commonly seen in large-breed dogs.1,3,13 While no breed predisposition is noted for gingival squamous cell carcinoma, lingual squamous cell carcinoma may be more common in poodles, Labrador retrievers, and Samoyeds.8,12 Fibrosarcomas also have not been associated with a particular breed, except for the histologically benign, biologically malignant variant (see below) for which golden retrievers are overrepresented.15
The median age reported for melanoma development (11 or 12 years) is slightly greater than that for squamous cell carcinoma (8 to 10 years) or fibrosarcoma (7 to 9 years).3-5,7,9,10,16 One exception is papillary squamous cell carcinoma, which is seen in dogs < 1 year old and has a better prognosis than squamous cell carcinoma does in mature dogs.17,18
Sex predilection has been variably reported for oral tumors, with older studies reporting a male predominance3-5,7 and more recent studies showing no predilection.8-13,16,19,20