ESTRO 2025 - Abstract Book

S3306

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Conclusion: Reducing the CTV-PTV margin to 3 mm in rectal cancer ART is clinically feasible. This approach maintains target coverage while significantly reducing PTV volume, potentially minimizing radiation exposure to OARs. Minimal intrafraction motion and efficient treatment times further support the adoption of narrower margins. These findings highlight the potential for ART to enhance precision and reduce toxicity in rectal cancer radiotherapy.

Keywords: Adaptative radiotherapy, margin reduction, Rectal

3363

Proffered Paper The influence of daily imaging and target margin reduction on secondary cancer risk in image-guided and adaptive radiotherapy Andreas Smolders 1,2 , Katarzyna Czerska 1 , Zarko Celicanin 1 , Tony Lomax 1,2 , Francesca Albertini 1 1 Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland. 2 Department of Physics, ETH Zurich, Zurich, Switzerland Purpose/Objective: Image-guided and adaptive radiotherapy often rely on daily CBCT, or for proton therapy in-room CT imaging, increasing radiation dose and the associated risk of radiation-induced cancer. Image guidance and adaptation also reduce setup uncertainty however, allowing to reduce robustness margins which may compensate this additional risk. This work developed a framework to investigate the required margin reduction to offset the secondary cancer risk from additional imaging dose, and applied it to proton therapy for head-and-neck cancer. Material/Methods: For five patients with sinonasal or nasopharyngeal tumors with lymph node involvement, voxel-wise CT and CBCT imaging doses were estimated with Gate Monte-Carlo simulations, calibrated with air and PMMA phantom measurements. The total dose of several treatment protocols with various imaging types and frequencies was calculated by summing the planning CT dose, daily and weekly CBCT or CT dose, and proton therapy dose, robustly optimized with setup robustness between 0 and 4 mm, hereafter simply referred to as setup margins . These were compared to a reference protocol with 4 mm setup margin without daily imaging. For each patient, protocol, and