Learning objectives: Develop an approach to the diagnosis and staging of oral tumours in dogs and cats. Understand the principles of surgery for oral tumours, and indications for other treatments such as radiation therapy, chemotherapy or immunotherapy either as adjuncts to surgery or as sole treatments.

The most common oral tumours in dogs are melanoma, squamous cell carcinoma (SCC), and fibrosarcoma (FSA), though many other tumours can occur in the oral cavity including the tongue. In cats, 60-70% of oral tumours are squamous cell carcinoma with fibrosarcoma being the second most common. General principles of staging and diagnosis apply regardless of tumour type. Although cytology may be diagnostic, given the difficulty of FNA in a conscious patient, sedation/anaesthesia and incisional biopsy for histopathology is generally most appropriate. All the common tumours in dogs and cats have some risk of metastasis, so staging is indicated prior to surgery. CT is generally preferred to assess the primary tumour, local lymph nodes (LN) and lungs all at the same time. Abdominal imaging could be considered, especially for canine oral melanoma. LN palpation is not sensitive or specific for metastasis and so cytology/ histopathology is recommended. However, identification of the ‘sentinel’ lymph node based on anatomy alone can be challenging. Metastasis to both ipsilateral and contralateral submandibular and retropharyngeal lymph nodes can be seen in dogs with malignancies of the head, including the oral cavity.

For tumours without distant metastasis, surgery (primary tumour ± LN excision) is recommended where possible. However, oral tumours may be extensive at the time of diagnosis, especially those located more caudally in the mouth, precluding excision.

Surgery

General surgical oncologic principles apply and oral tumours should be removed via wide en bloc surgical resection. For oral tumours associated with the maxilla or hard palate, this will be achieved by maxillectomy which can be partial or a complete hemimaxillectomy. Closure of the resultant oronasal defect is by a local mucosal flap raised from the ipsilateral gingiva. Most dogs have god to excellent functional outcomes and minimal cosmetic changes. Mandibular tumours require mandibulectomy (rostral, segmental or hemi). Post operatively, some animals experience ‘mandibular drift’ where the mandibular teeth do not align with the maxillary teeth. Cats can undergo mandibulectomy or maxillectomy but often require a longer period of adaption after surgery and oesophageal feeding tubes are required to provide nutrition in the post-operative period.

Removal of the draining lymph nodes is recommended in cases of oral tumours to provide accurate staging information. A recent description of a technique to remove all three draining lymph nodes through a single ventral cervical access incision has been published.

Lingual tumours can be treated by partial or subtotal glossectomy. The tongue is very vascular so appropriate haemostatis is required during surgery. If major glossectomy is required (e.g., for large or caudally located tumours), then the owners should be aware of the management and morbidity concerns before undertaking such a procedure.

Tonsillar tumours are most commonly SCC. They can be removed by tonsillectomy which is made easier with the use of a vessel sealing device. Surgery is considered palliative in these cases as tonsillar SCC is a highly metastatic and aggressive disease in dogs.

Following surgery, adjunctive therapy should be considered from two aspects - the need for additional local control due to narrow or incomplete histologic margins and the need for systemic therapy due to risk of metastasis. From a local control standpoint, assuming an aggressive first surgery has been performed, as discussed above, more extensive surgery is not generally possible in the oral cavity. Local recurrence can occur despite apparently complete excision (approx. 15–20% in canine oral tumours in general), but is very common following incomplete excision (>60%), and recurrent disease negatively impacts survival time. In the specific cases of feline oral SCC and canine oral FSA, local recurrence may be more common than other oral malignancies. In general, local recurrence is less common for mandibular tumours than maxillary tumours, likely due to the ability to obtain wider margins. Local recurrence is also greater for larger tumours, again likely due to incomplete excision. Where additional local control is indicated, radiation therapy (RT) is often very effective. With aggressive local therapy, outcomes for SCC and FSA in dogs can be very good, with median survival times of greater than 1.5–2 years (SCC is generally better than FSA). Acute side effects from RT in the oral cavity are common but typically manageable with aggressive analgesia, and generally resolve within a few weeks. Feeding tubes may be required in some patients.

Adjuvant systemic therapy after local therapy is generally not indicated for canine oral SCC and FSA as they do not typically cause distant metastasis (varies with the study). If lymph node metastasis is documented, adjuvant chemotherapy may be warranted. In contrast, oral melanoma in dogs is considered to have a high risk of metastasis. The local disease (primary tumour ± LN) can often be effectively controlled with surgery and/or RT, but development of metastasis is common. Systemic therapies including chemotherapy (carboplatin, dacarbazine, temozolomide) and immunotherapy (Oncept and other vaccines) have not shown a statistically significant improvement in survival over local therapies alone. Small tumours (<2 cm) may have fair to good outcomes with aggressive local therapy alone (median 1.5–2 years compared to 6–8 months for larger tumours).

For tumours where surgical excision is not possible, radiation therapy is generally the preferred option. Canine oral melanoma tends to be very radiation responsive. Hypofractionated-/palliative-type protocols are generally recommended for melanoma. As discussed above, although the local tumour can be effectively controlled, metastasis is common. There is some information about RT as sole treatment for canine SCC, with median control time of approximately 1 year reported with definitive-type protocols, though this may be better in smaller tumours. Canine FSA is generally thought to be less radiation responsive, though median control of 10 months is reported in one study. Palliative RT can be considered for any painful oral tumour.

Chemotherapy could be considered for macroscopic oral tumours if surgery and radiation are not feasible, or for metastatic tumours, however evidence of efficacy is sparse. Canine oral melanoma may respond to platinum agents, and dacarbazine and its related drug temozolomide have shown some efficacy in melanoma in humans. The Oncept melanoma vaccine may also have some benefit in some dogs, with occasional responses reported in macroscopic disease. There may be a potential benefit to combining Oncept with metronomic chemotherapy either in the adjuvant setting or with macroscopic melanoma. For canine oral SCC, responses are reported to carboplatin and piroxicam in some dogs, toceranib (± NSAID ± cyclophosphamide), and even piroxicam alone. Canine oral FSA is likely to be relatively chemoresistant given responses of macroscopic soft tissue sarcomas in general, though some response to doxorubicin or to metronomic chemotherapy may be seen.

There are three specific companion animal oral tumours that bear separate consideration.

• Feline oral SCC - these are often locally extensive at diagnosis, and if not surgical are very resistant to treatment, with median survival times of a few months. Even with surgery, recurrence is common, though for small tumours that can be treated with mandibulectomy and adjuvant radiation, survival times may be improved. RT alone does not appear to be effective in most cases. There is one small study evaluating systemic bleomycin chemotherapy along with radiation and surgery where outcomes may have been better, and toceranib (± NSAIDs ± metronomic chemotherapy) may be of benefit in some cats. The use of NSAIDs is recommended based on COX expression in feline SCC for potential anti-tumour effects as well as for analgesia.
• Canine tonsillar SCC - unlike non-tonsillar oral SCC (with the possible exception of lingual SCC), tonsillar SCC is highly metastatic and aggressive in dogs. Often the diagnosis is made from a metastatic lymph node which is externally visible, rather than the primary tumour in the tonsil. Survival times are generally short (months), even with treatment, though combinations of chemotherapy (usually carboplatin) and radiation therapy may achieve reasonable palliation. However, in those where the disease is diagnosed early (no metastasis, one tonsil affected), prolonged survival may be possible with treatment.
• Canine hi-low (histologically low-grade biologically high grade) FSA - a subset of canine oral FSA will appear non-aggressive (low-grade sarcoma or even fibroma or reactive fibrous tissue) and yet have very aggressive behaviour with rapid local growth and extensive tissue invasion. This syndrome seems to be more common in the maxilla of large breed dogs. If amenable to aggressive therapy, outcomes can still be favourable.

References

1.  Vail DM, Withrow SJ, eds. Withrow and McEwen’s Small Animal Clinical Oncology (5th edition) W.B. Saunders, Philadelphia, 2012 is recommended as a general resource