ESTRO 2025 - Abstract Book

S3187

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Conclusion: The combination of the manually adjustable RPS Head platform with an SGRT system enables accurate correction of head rotations during treatment setup, achieving high reproducibility (maximum 0.04° as one standard deviation). This procedure allows for a quicker positioning compared to the IGRT workflow with the prospect of saving imaging dose from repeated CBCTs until the final position is reached. Comprehensive system calibration is essential to align the isocentre with the megavoltage beam and prevent systematic errors. System calibrations need to be thoroughly performed, setting the isocentre to match the megavoltage beam, as otherwise systematic errors can be introduced.

Keywords: positioning, rotations, surface guidance

References: [1] J Appl Clin Med Phys. 2024;25:e14172

4328

Digital Poster Direct-to-treatment adaptive radiotherapy: Live planning for spine metastases using HyperSight CBCT K. Maiti McGrath 1 , Robert Lee MacDonald 1,2,3 , James L Robar 1,2,3 , Amanda Cherpak 1,2,3 1 Physics and Atmospheric Science, Dalhousie University, Halifax, Canada. 2 Radiation Oncology, QE2 Cancer Centre, Nova Scotia Health, Halifax, Canada. 3 Radiation Oncology, Dalhousie University, Halifax, Canada Purpose/Objective: CBCT-based online adaptive radiation therapy is carried out using a synthetic CT, sCT, created through deformable registration between the patient-specific fan-beam CT, FBCT, and daily CBCT. Varian Ethos 2.0 allows for plan calculation directly on HyperSight CBCT and uses AI-informed tools for daily contouring without the use of a priori information. This breaks an important link between daily adaptive sessions and initial reference plan preparation. This work explores adaptive radiation therapy for spine metastases without prior patient-specific imaging or treatment planning. We hypothesize that adaptive plans can be created when patient-specific positioning and anatomy is incorporated only once the patient has arrived at the treatment unit.