Hypofractionated radiation therapy in the treatment of canine thymoma: Retrospective study of eight cases.

Thymomas are one of the most common tumors of the cranial mediastinum in dogs; however there is limited information available on the use of radiation therapy for treating this neoplasm. Objectives of the current retrospective observational study were to describe outcomes and side effects of a hypofractionated radiation therapy protocol in a group of dogs with confirmed thymoma. A total of eight dogs were included. To generate individualized treatment plans, we designed the planning target volume according to the limits on mean lung dose and the percentage of the total lung volume exceeding 20 Gy (V20). The total administered dose was 48-49 Gy, with one fraction per week for a total of six to seven fractions. After therapy, two dogs achieved complete responses, two achieved partial responses, and the disease remained stable in two. Two dogs died during the radiation therapy protocol and were not classified. The median mean lung dose and V20 were 6.0 Gy (range: 3.1-15.0 Gy) and 12.4% (range: 2.3-27.5%), respectively. The overall response rate was 50.0%, and the median time to response following treatment initiation was 22 days (range: 14-115 days). Acute and late side effects were common in the skin and/or lung and were self-limiting or asymptomatic. The median survival time was not reached (range: 8-1128 days) and the 1 year survival rate was 75.0%. Hypofractionated radiation therapy was well tolerated in this sample of dogs with thymoma and may be considered when owners decline surgical treatment or the tumor is deemed unresectable.