Tonsillar squamous cell carcinoma (SCC) is a relatively uncommon but aggressive form of cancer in dogs, often posing significant therapeutic challenges. Squamous cell carcinomas arise from the epithelial cells lining the tonsils, a part of the oropharyngeal region. The highly invasive nature of tonsillar SCC in dogs frequently leads to the involvement of locoregional lymph nodes and, in some cases, distant metastasis, further complicating treatment strategies and prognosis.

Tonsillar SCC typically presents with clinical signs such as dysphagia (difficulty swallowing), oral pain, halitosis (bad breath), excessive salivation, and occasionally, visible masses in the oropharynx. Due to the location of the tumors, detection may occur later in the disease process when significant local invasion or metastasis has already occurred. As a result, the disease often presents as advanced or non-resectable, leaving radiation therapy as a primary treatment modality for many affected dogs. Historically, treatments for canine tonsillar SCC have included surgical excision, chemotherapy, and palliative radiation therapy. However, surgery is often not feasible due to the anatomical constraints of the tonsillar region and the advanced stage of the disease at diagnosis. Moreover, chemotherapy alone has demonstrated limited efficacy in achieving long-term control of the disease. This has shifted focus towards the use of more definitive, curative-intent radiation therapy (RT) protocols, especially for non-resectable tumors.

In the study of ten dogs with unilateral tonsillar SCC, definitive-intent radiation therapy was utilized as the primary treatment. The RT protocol employed an accelerated hypofractionated schedule, with dogs receiving ten consecutive daily doses of 4.5 to 4.8 Gy each, delivered using either volumetric modulated arc therapy (VMAT) or three-dimensional conformal radiation therapy (3D CRT). Both VMAT and 3D CRT are advanced techniques that allow for precise targeting of the tumor while minimizing radiation exposure to surrounding healthy tissues, reducing the likelihood of severe side effects.

The study revealed promising results. All dogs achieved a complete response to the radiation therapy within 4-8 weeks post-treatment. This is a remarkable finding, highlighting the potential efficacy of radiation therapy in achieving tumor control in tonsillar SCC. Furthermore, the observed side effects were manageable and mostly mild. Acute radiation toxicity, according to VRTOG (Veterinary Radiation Therapy Oncology Group) toxicity criteria, was documented as grade 1 in 30% of cases and grade 2 in 70%. The most common side effects included desquamation of the skin and mucositis, but no higher-grade toxicities were observed. These side effects were managed with NSAIDs, and 50% of the dogs were also prescribed tyrosine kinase inhibitors (TKIs) after radiation therapy.

One of the critical considerations in radiation therapy for any cancer is the long-term outcome, both in terms of tumor control and quality of life. This study demonstrated a median overall survival time of 1055 days (95% CI: 655-1455 days) for the treated dogs, which is a significant achievement for such an aggressive tumor type. Only two dogs experienced disease progression post-RT: one with local recurrence after 261 days, and one with pulmonary metastasis 88 days post-treatment, without local progression. Additionally, 30% of the dogs died from causes unrelated to their tumors, with a median follow-up time of 1143 days.

Importantly, 50% of the dogs in the study were still alive at the time of reporting, with a median follow-up time of 1250 days (range: 66-1536 days). This outcome highlights the potential for prolonged survival and quality of life in dogs treated with this radiation therapy protocol. Late toxicities were minimal and primarily included depigmentation of the oral mucosa, a cosmetic issue that did not affect the dogs’ well-being. The absence of significant late toxicity is another positive aspect of this treatment approach, indicating that the radiation dose and schedule used were well- tolerated over the long term. This retrospective study provides valuable preliminary data on the use of definitive-intent radiation therapy for tonsillar SCC in dogs. The results suggest that radiation therapy using an accelerated hypofractionated protocol is both safe and effective, with a high rate of complete response and a low incidence of severe side effects. The findings also highlight the potential for this approach to achieve prolonged tumor control, offering hope for dogs with non- resectable tonsillar SCC. In conclusion, tonsillar squamous cell carcinoma in dogs remains a challenging cancer to treat, but advances in radiation therapy, particularly with definitive-intent protocols, offer a promising avenue for achieving long-term disease control. Further research and larger studies are needed to confirm these findings and optimize treatment protocols for canine patients with this aggressive tumor type. References available upon request.