A three year old female spayed English Mastiff presented with a four month history of unilateral nasal discharge with intermittent epistaxis. Computed tomography (CT) revealed a large, space occupying mass obliterating the ethmoturbinates with involvement of the frontal sinuses. Rhinoscopic nasal biopsies were obtained: a low grade, lymphocytic, T-cell lymphoma was diagnosed. Lymphoma is one of the most common neoplasms in the dog and is considered a systemic disease, often involving the lymph nodes, liver, spleen, and bone marrow. The patient exhibited an atypical presentation, more commonly seen in the cat, with evidence of solitary, extranodal lymphoma in the nasal cavity. Chemotherapy is typically the treatment of choice in canine lymphoma, but due to the local nature of this dog’s disease, radiation therapy is a viable option. Because lymphoma is considered a systemic disease, treatment of localized disease alone may not be entirely efficacious, thus chemotherapy is also recommended. The owner opted for radiation therapy without chemotherapy. A 3-dimensional conformal radiation therapy plan was developed based on CT images. A total dose of 45 Gray (Gy) was delivered to the nasal cavity and sinuses in fifteen fractions of 3 Gy each. A cone-down field was used to concentrate the dose in the region of the tumor and spare normal tissue as determined by a second CT scan obtained after fraction 10. Serial imaging demonstrated a reduction in tumor size and clinical signs. The patient endured few acute reactions to the radiotherapy and she continues to thrive at home almost six months after therapy.