ESTRO 2025 - Abstract Book

S3188

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Material/Methods: An Ethos 2.0 emulator was used to create initial reference plans on ten patient-specific FBCTs for simulated spine targets, typical of emergent palliative treatments. Generalized physician intents were used to create all plans. Reference plans were also created using FBCTs of i) a library patient with clinically acceptable contours and ii) a water-equivalent phantom with placeholder contours. Adaptive sessions were simulated with each patient’s HyperSight CBCT using the three different starting points. Resulting adaptive plans were compared for plan quality, using Gamma analysis and Wilcoxon rank tests, to determine the significance of patient-specific information prior to the start of treatment. Results: The library patient and phantom reference plans did not generate adaptive plans that differed significantly from the standard workflow for all clinical constraints for target coverage and organ at risk sparing (p<0.2). Gamma comparison between the three adaptive plans for each patient (3%/3 mm) demonstrated overall similarity of dose distributions (pass rate >95%), for all but two cases. All plans passed gamma comparison when only the high dose region was analyzed. Failures occurred mainly in low-dose regions, highlighting difference in fluence used to achieve the same clinical goals. Conclusion: This study confirmed the feasibility of a procedure for treatment of spine and bone metastases that does not rely on previously acquired patient-specific imaging, contours, or plan. Reference-free direct-to-treatment workflows are possible for the investigated disease site and can condense a multi-step process to a single location with dedicated resources.