Tolerability and Efficacy Following Definitive‐Intent Fractionated Radiotherapy for Canine Pituitary Tumors

Conventionally fractionated (CFRT) radiotherapy (20 × 2.5 Gy) is commonly used in clinical practice for canine pituitary tumors (PTs), yet published data on long-term outcomes and imaging response remain limited. Medical records were retrospectively reviewed for dogs diagnosed with PTs that received definitive-intent radiation therapy (RT) with a planned total dose of 50 Gy in 20 fractions between 2014 and 2022. Prognostic factors, including age, sex, weight, the presence of neurological signs, pituitary-dependent endocrine abnormalities, pituitary size and ratio to brain, and tumor response according to RECIST, were evaluated for association with outcome measures using Cox proportional hazard models. Twenty-nine dogs were included in this multi-institutional study. Follow-up imaging was available for 14 dogs (48.2%); nine (64.3%) achieved partial response, and five (35.7%) had stable disease, including sustained tumor reduction exceeding 3 years in four dogs. Median progression-free survival was 523 days (95% CI, 57-2469 days), and the median all-cause survival time was 669 days (95% CI, 58-2469 days). Estimated 1-, 2-, and 3-year survival rates were 65%, 48%, and 24%, respectively. Hyperadrenocorticism was significantly associated with reduced all-cause survival (p = 0.02). Three dogs were alive at the time of analysis. Acute Grade 1 brain toxicity occurred in four dogs. Hypoadrenocorticism developed in two dogs at 67 and 232 days after RT initiation. One dog developed a presumptive radiation-induced trigeminal nerve sheath tumor 1724 days after RT, suspected to represent late toxicity. CFRT RT, delivered at 50 Gy in 2.5 Gy fractions, provides durable tumor control with few adverse events. Monitoring for post-RT hypoadrenocorticism is warranted.