Surgical Oncology, Veterinary Emergency & Referral Hospital, VCA Canada, Newmarket, ON, Canada
Oral tumours in dogs and cats are commonly malignant. The approach to the initial work up and treatment are very important for a successful outcome. A lot of oral tumours are discovered during dental prophylaxis. This is because the halitosis caused by the oral tumour may be the catalyst that makes the owner seek a dental. Dental prophylaxis is also the best opportunity to fully examine the oral cavity as many dogs and cats will not allow a full examination when they are awake.
It is important to have a plan to systematically work up an oral mass if one is discovered during dental. When an oral mass is found, a great first step is to take digital radiographs for the patient file. This will help to monitor if there is any growth in the mass and also to help communicate the size, shape and location of the mass to any other veterinarians seeing this pet. The next step is to call the owner to discuss this finding. An incisional biopsy should be recommended to determine the definitive diagnosis. Cytology can also be considered, but if the patient is under anesthesia, an incisional biopsy should be performed to give the best chance of a definitive diagnosis. Whether or not to proceed with the dental prophylaxis should be discussed with the owner. There are pros and cons to this approach. Some owners might prefer to determine the diagnosis before the additional costs of a dental prophylaxis, while others may want to take the opportunity to complete the dental prophylaxis when their pet is under anesthesia. For patients that present for an oral mass, the principles are the same, an incisional biopsy is recommended to achieve a diagnosis. This can often be done under heavy sedation.
The most important step is taking an incisional biopsy to determine the diagnosis. In general, an incisional biopsy, rather than an excisional biopsy recommended. It is tempting to shave the mass off or to attempt to fully remove it. However, this strategy may lead to issues when planning a definitive treatment if this is a malignancy. The oral mucosa has a strong potential to heal quickly, and in cases where an oral mass has been removed by an excisional biopsy, the site of removal may not be evident by the time that the histopathology report is back.
This can be problematic when trying to plan a wide resection of the tumour to include bone or radiation to the site because there are no landmarks of the previous mass. Another great idea is to take digital photographs of the mass when the patient is under anesthetic or sedated. This will help greatly with planning surgery and discussing a plan with clients. Especially in cases where the patient does not allow for oral examination when awake.
Other diagnostic tests to consider when working up an oral mass include dental radiographs, bloodwork, three-view thoracic radiographs for staging and lymph node aspirates. Also, don’t forget to do a rectal exam! The tumour type will dictate the staging that is necessary. The primary differential diagnoses for malignancies include malignant melanoma, fibrosarcoma, osteosarcoma and squamous cell carcinoma. Benign oral masses that are common in dogs include epulides and gingival hyperplasia. Except for gingival hyperplasia, all of these oral masses will require wide or radical excision, likely including bone if they involve gingiva. Lip or tongue masses may be managed with wide excision of soft tissues (lip excision or partial glossectomy). In cats, squamous cell carcinoma is the most common oral tumour, with osteosarcoma another relatively common tumour type.
Part of staging for oral tumours will include a CT scan of the head and thorax for local and distant staging and for surgical planning.
Malignant melanoma is the most common oral tumour in dogs. It commonly metastasizes to regional lymph nodes and lungs, but can also spread to the parenchymal organs in the abdomen or other sites. Larger size, evidence of vascular or lymphatic invasion on histopathology or a mitotic index of >5 (# of mitotic figures per HPF) are associated with shorter survival times. Aggressive surgery is recommended when feasible for tumour control, which often involves the removal of the tumour and >1 cm margins of normal tissue and bone around the tumour. If aggressive surgery is not feasible, a marginal excision combined with radiation or radiation alone can be considered. Hypofractionated radiation is the preferred treatment course for melanoma, which usually involves weekly doses of radiation for 4 weeks. It is extremely well-tolerated and although it does not afford a cure, this tumour type responds well to radiation in 80% of dogs, with MST of 210 days. The MST for melanoma depends on the tumour size, with reported survival times of 630 days, 240 days, and 173 days for tumours that are <2 cm, 2–4 cm, and >4 cm, respectively.
Death is usually due to metastatic disease if a form of local control is employed. Immunotherapy of differing types is being pursued for malignant melanoma in dogs, with encouraging results. A tyrosinase vaccine (ONCEPT) may allow improved survival times over traditional treatments, particularly in dogs with local disease control. One study reported a median survival time of more than 500 days in dogs with stage I–III disease, with a one-year survival rate of >75%. Chemotherapy is generally not recommended for melanoma as it has not been shown to improve survival times.
Squamous cell carcinoma (SCC) is the second most common oral tumour in dogs. These tumours tend to be locally aggressive, with a low metastatic potential. This means that with wide surgical excision, there is the potential to cure a lot of these patients, with a reported one-year survival time of 94% with surgical treatment. Rostral, smaller masses that are more amenable to surgery will have a better prognosis. Tonsillar SCC is a more aggressive form of this disease, with a high metastatic potential. One peculiarity of this disease is that it often metastasizes to the draining lymph nodes and often mandibular lymphadenopathy is the presenting complaint. Combination therapy with surgical excision, radiotherapy and chemotherapy is recommended for this disease, with a reported MST of 180–240 days. NSAIDs have been shown to have a beneficial effect for oral SCC. Piroxicam is often reported as a treatment for this disease. This is controversial, but I tend to recommend a selective NSAID rather than piroxicam due to the improved side effect profile with selective NSAIDS.
Oral fibrosarcomas are locally invasive but with a relatively low metastatic potential (20%). It is very important to keep a specific type of oral FSA on your radar called a high-low FSA. These tumours are biologically high grade and very aggressive locally and histologically low grade. This means that under the microscope, these tumours appear quite bland and may not be diagnosed as a tumour at all. They are sometimes read out as fibroplasia and/or inflammation. If you see this histopathological diagnosis in the maxilla of a medium or large-breed dog, keep you index of suspicion high. These tumours are extremely aggressive locally and it is difficult to achieve clean margins. The best chance of achieving clean margins is early diagnosis. These tumours are also not very radiation responsive, so aggressive surgery is the best chance of a successful outcome. The MST is 18 months with surgical treatment. Death is usually due to local recurrence.
Osteosarcomas are locally invasive and have a metastatic rate of around 50%, usually to the lungs. Wide surgical excision is recommended when possible. Follow up with radiation can be considered if clean margins are not achieved.
Chemotherapy with this disease is somewhat controversial as it does not metastasize in every case and may depend on the individual histopathology characteristics.
Acanthomatous ameloblastomas are locally aggressive and require wide excision with bone, but do not metastasize. If a marginal excision is used for treatment, one source reports a recurrence rate of 91% with a disease-free interval of only 32 days. Wide excision is typically curative.
Dogs do very well with mandibulectomy and maxillectomy. It is important to be in a facility that can provide a blood transfusion and 24-hour care. The mandibular and retropharyngeal lymph nodes are routinely biopsied during surgical excision for staging. This can result in a seroma, but otherwise does not cause any issues. Dogs that have mandibulectomy or maxillectomy require soft or canned food for one month post-operatively and may require some assistance with feedings. I do not routinely place feeding tubes in dogs for these surgeries.
The most common oral tumour in cats is SCC. This can be a devastating disease in cats because it is difficult to achieve clean margins of excision. This disease is not very responsive to radiation alone, with MST reported in the order of 3 months. The current literature suggests that cats do not do well with maxillectomy or mandibulectomy. Although they do not do as well as dogs and the require more aggressive supportive care, successful outcomes are possible with maxillectomies and mandibulectomies in cats. A feeding tube must be placed at the time of surgery and these cats require more nursing care. A recent study by the author reports 8 cats treated with radical mandibulectomy for oral neoplasia. Six of these cats ate on their own within one month. The estimated mean survival time was 651 days, with three cats that lived over one year.
It is critical that primary care veterinarians and specialists work together to achieve an early diagnosis and the best possible outcomes in cases of oral tumours in dogs and cats. Most often, an incisional biopsy is recommended as the first step, followed by local and distant staging. In most cases, wide surgical excision via mandibulectomy or maxillectomy is recommended.