A Retrospective Evaluation of a Cyclical, Hypofractionated Radiotherapy Protocol ('Quad-Shot') for the Treatment of Canine Nasal Tumours in 81 Dogs.

This retrospective study evaluated a cyclical, hypofractionated palliative-intent radiotherapy protocol ('quad-shot', QS) in 81 dogs with sinonasal tumours treated between 2011 and 2023. The protocol consisted of a 'cycle' of four fractions of 3.25-4.0 Gy delivered over 48-72 h, repeated every 3-4 weeks, up to three cycles (maximum cumulative dose, 48 Gy). Most were treated with 3D conformal radiation therapy and a small number with a clinical setup. Carcinomas accounted for 78% of cases and tumours were modified Adams stage 1 (n = 5; 6%), 2 (n = 8; 10%), 3 (n = 29; 36%), 4 (n = 33; 41%) or unknown in n = 6 (7%). Ninety percent of patients received three full cycles to a total of 39-48 Gy, and 77% showed clinical improvement at presentation for Cycle 2 and 90% at presentation for Cycle 3. Median progression-free interval (PFI) was 207 days (95% CI: 124-290), and median overall survival time (OST) was 296 days (95% CI: 177-415). One-, two-, and 3-year survival rates were 40.6%, 17.2%, and 9.4%, respectively. Acute toxicity was generally mild, with conjunctivitis (33%), mucositis (7%), and radiodermatitis (6%) being the most frequent. Severe late toxicity was infrequent, but toxicities were considered likely under-reported. In the multivariable analysis, three QS cycles (vs. 1 or 2 cycles only) was a positive prognostic factor. The QS protocol resulted in improvements in nasal clinical signs, with survival outcomes comparable to other palliative radiation protocols. Toxicity was acceptable, despite the poor conformality of the radiation therapy in this population.