The Definitive-Intent 10-Fraction Radiotherapy Protocol ('the Zurich Protocol') Yields Lasting Tumour Control and Survival in Dogs with Intracranial Meningiomas

Moderately hypofractionated definitive-intent radiotherapy protocols can be used for canine intracranial meningiomas to shorten overall treatment time while maintaining tumour control and limiting normal tissue toxicity. This retrospective study evaluated oncologic outcomes, toxicity, and radiation dose-volume-related prognostic factors in dogs with imaging-based intracranial meningiomas treated with a 10-fraction definitive-intent protocol. Medical records from 2015 to 2025 were reviewed. All dogs received 10 daily fractions of 4 Gy delivered with a homogeneous dose prescription, risk-adapted heterogeneous dose distributions, or an 11% simultaneously integrated boost (SIB) to the gross tumour volume. One hundred and ten dogs were included, with a median follow-up of 498 days. Median time to progression was 873 days (95% CI: 552-1195), median overall survival after first radiotherapy was 756 days (95% CI: 474-1039), and median DSS was 1078 days (95% CI: 915-1242). Time to progression differed among treatment subgroups (p = 0.024). Larger volumes of normal brain receiving ≥ 32 Gy (> 11.5 cm3) were associated with shorter overall (p < 0.001) and disease-specific survival (p = 0.002). Use of an SIB was associated with shorter disease-specific survival (HR 2.47; p = 0.012), and post-radiotherapy seizures were associated with reduced time to progression (HR 2.01; p = 0.026). Tumour location was not identified as a prognostic factor. Acute or early delayed neurologic toxicity occurred in 10% of dogs and was generally transient and generally corticosteroid responsive. In conclusion, the definitive-intent, moderately hypofractionated 10-fraction radiotherapy protocol achieves long lasting tumour control and survival outcomes in dogs with intracranial meningiomas, provided that evidence-based brain dose-volume constraints are respected.