Finding and treating oral melanoma, squamous cell carcinoma, and fibrosarcoma in dogs - Veterinary Medicine
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Finding and treating oral melanoma, squamous cell carcinoma, and fibrosarcoma in dogs
The three malignancies most likely to occur in dogs' mouths can have devastating local or metastatic effects if not identified and treated quickly. Review how surgery—in conjunction with radiation, chemotherapy, or vaccination—may increase your patients' survival times.


VETERINARY MEDICINE


Thoracic imaging

Three-view thoracic radiographs are part of the staging process to assess for distant metastasis. At the time of oral tumor diagnosis, these radiographic findings are usually normal. One study revealed that only 13.6% of dogs with malignant oral tumors had radiographically detectable pulmonary metastasis at the time of diagnosis.33 Another study of 361 dogs with a variety of oral tumors reported radiographic evidence of pulmonary metastasis at the time of diagnosis in eight of 59 (14%) dogs with melanoma, four of 40 (10%) dogs with fibrosarcoma, two of 24 (8%) dogs with tonsillar squamous cell carcinoma, and one of 35 (3%) dogs with nontonsillar squamous cell carcinoma.3

Despite this low yield, thoracic radiography is easy, affordable, and strongly recommended since the presence of pulmonary nodules greatly changes a patient's prognosis and treatment plan. One reason for the low yield may be the limitations of radiographs to detect an early miliary pattern of lung metastasis. In one study, six dogs with melanoma without evidence of metastasis on thoracic radiographs had evidence of miliary lung nodules at necropsy performed one to 21 days later, which suggests the disease was likely present but not identified on radiographs.3

A CT scan would be a more sensitive way to detect these early metastatic lesions. However, the cost and availability of CT needs to be balanced with the likelihood of finding metastatic disease, and, thus, it may be more indicated for tumors with an ultimately high pulmonary metastatic rate such as melanoma. As the availability of CT increases, thoracic CT may become a more routine part of staging for cancer metastasis in general.

Cytology or biopsy of the tumor

If lymph node aspirates do not provide a definitive diagnosis and thoracic radiographic findings are normal, cytology or, preferably, tissue biopsy of the oral mass can be performed to determine the tumor type. Knowing the tumor type can help owners make treatment decisions based on the tumor's biologic behavior and likely prognosis.

At times, especially with round cell tumors such as melanomas and plasma cell tumors, cytologic examination of the mass can provide valuable diagnostic information. Unfortunately, cytologic diagnosis can be challenging because of secondary inflammation found in conjunction with oral lesions as well as high degrees of cellular anaplasia in some tumors; thus, tissue biopsy with histology is required in most cases.


4. Proper technique for a wedge biopsy of a left caudal maxillary mass. Note that the section being biopsied is away from the ulcerated and necrotic center of the tumor. Histologic examination revealed a melanoma.
Biopsies should be performed from within the oral cavity and not through the overlying skin, if possible, to spare unaffected normal tissues that may be needed for surgical reconstruction after tumor excision. A wedge biopsy with a scalpel blade (Figure 4) will generally yield a good sample of a tumor in the oral cavity, but be sure to avoid areas of ulceration or suspected necrosis, and try to get a deeper rather than more superficial section. Alternatively, a 4- to 8-mm Keyes punch or Tru-Cut biopsy needle may provide a good sample. For firm masses extensively involving a jaw bone, a Jamshidi biopsy needle can be used. Before the sample is placed in formalin, make impression smears for possible cytology submission, which may help in difficult–to-diagnose cases. Be sure to keep all cytology slides away from formalin, or the fumes will affect the sample.

Poorly pigmented, or amelanotic, melanomas are an example of a tumor that can be challenging to diagnose. Cytology can help in some of these cases, and special stains can be used on histologic samples. The immunohistochemical markers Melan A, vimentin, and S-100 protein can help confirm the diagnosis of melanoma in amelanotic variants as well as in poorly differentiated tumors.34 Additionally, since some rare melanomas do not behave malignantly, multiple features have been evaluated to help determine how aggressively oral melanoma will behave. Two such features, mitotic index and nuclear atypia, have been determined to be the most helpful in correlating with clinical outcome.24,25

Abdominal ultrasonography

To further evaluate a patient with an oral mass, abdominal ultrasonography may be recommended. The likelihood of identifying metastatic spread of an oral tumor into the abdomen is low. Oral malignant melanoma can spread to the liver in rare circumstances but this is unlikely if regional lymph nodes and lungs are disease-free.3 The true value of abdominal ultrasonography is as a screening procedure for occult problems that are concurrent with the oral tumor. Evaluating internal organs for abnormalities unrelated to the primary tumor in an older dog before it undergoes aggressive and expensive therapies, such as surgery or radiation, is advisable.

Skull and dental radiography

Skull radiography can be performed to help evaluate neoplastic extent, but because of limitations in the information provided by these radiographs, they have been largely replaced by more advanced imaging modalities. More than half of the tumors arising from or near the gingiva will have evidence of bony lysis on radiographs. Remember that 30% to 50% of the bone must be destroyed before radiographic evidence is noted, so plain films will often underestimate the degree of bony involvement.33,35

Dental radiographs can be performed and are useful for evaluating rostral mandibular tumors but prove more difficult to interpret for more caudally extensive tumors and tumors of the maxilla.


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