| Sumario: | Simple Summary: Intranasal tumors are common cancers in dogs, frequently causing bleeding, nasal discharge, and facial deformity. Because these tumors are located near critical structures, surgery is often not feasible, making radiotherapy the primary treatment option. This study reviewed medical records of 40 dogs with malignant intranasal tumors treated with cobalt-60 radiotherapy at a veterinary clinic in Brazil. Most dogs had advanced disease at presentation. Radiotherapy resulted in tumor shrinkage or stabilization in the majority of cases, with improvement of clinical signs in most dogs. Side effects were common but manageable, primarily affecting the skin, oral mucosa, and eyes. Dogs with more advanced tumors had shorter survival times, highlighting the importance of early diagnosis and treatment. These findings suggest that radiotherapy provides meaningful clinical benefit with acceptable toxicity and may help guide treatment decisions for dogs with intranasal cancer. Background/Objectives: Intranasal tumors are common malignancies in dogs, characterized by locally aggressive behavior and clinical signs such as epistaxis, nasal discharge, and facial deformity. Radiotherapy (RT) is considered the treatment of choice due to anatomical limitations to surgical resection. This study aimed to evaluate clinical outcomes, toxicity, and prognostic factors in dogs with primary malignant intranasal tumors treated with cobalt-60–based megavoltage radiotherapy. Methods: This retrospective study included 40 dogs with histopathologically confirmed primary malignant intranasal tumors treated between September 2018 and February 2025 at a veterinary radiotherapy clinic in Rio de Janeiro, Brazil. Medical records were reviewed for patient demographics, tumor characteristics, treatment protocols, response, toxicity, and survival outcomes. Tumors were staged using modified Adams criteria based on computed tomography. Definitive-intent protocols (n = 32) delivered 48–54 Gy in 10–13 fractions administered three to five times weekly, while palliative protocols consisted of either four fractions of 8 Gy delivered once weekly or five fractions of 4 Gy delivered daily. Results: Adenocarcinoma was the most common histologic subtype (42.5%), and 82.5% of dogs had stage III–IV disease. The objective response rate was 82.5% (CR: 17.5%; PR: 65.0%), with clinical benefit observed in 92.5% of cases. Acute toxicity was frequent but manageable, primarily affecting skin, oral mucosa, and eyes. Overall median progression-free interval (PFI) and survival time (MST) were 382 days and 430 days, respectively. Stage IV disease was significantly associated with shorter survival when compared to stage I-III (MST 345 vs. 1063 days, respectively; p = 0.016). Treatment response was significantly associated with PFI in univariate analysis (p < 0.05). Conclusions: Radiotherapy provided high response rates and meaningful clinical benefit with acceptable toxicity in dogs with malignant intranasal tumors, highlighting the importance of early diagnosis and treatment. Further prospective studies with standardized protocols are warranted.
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