ESTRO 2025 - Abstract Book

S3176

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Conclusion: The passive adaptive strategy helps to safely deliver SABR treatment for pancreatic cancer and minimizes treatment cancellations. The challenges in image matching due to obscuring artefacts from bowel gas and changes in the position of GI tract which increase overlaps with the PTV, could be partially mitigated by taking the patient off the couch and repeating CBCT. The availability of the backup plan removes the need for treatment gaps. This technique may be used for CT-based SABR where online adaptation is not readily available.

Keywords: Adaptive, Pancreas, SABR

3447

Digital Poster Can Online Adaptive Proton Therapy Improve Delivery Speed and Maintain Treatment Quality with Unconventional Beam Arrangements? Evangelia Choulilitsa 1,2 , Katarzyna Czerska 1 , Nicola Bizzocchi 1 , Jan Magdalenic 1 , Barbara Bachtiary 1 , Damien Charles Weber 1 , Antony John Lomax 1,2 , Francesca Albertini 1 1 Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland. 2 Department of Physics, ETH, Zurich, Switzerland Purpose/Objective: Daily reoptimization in online adaptive proton therapy offers dosimetric advantages over non-adaptive methods. The clinical implementation of daily full reoptimization (DAPT) at PSI [1] opened opportunities to enhance treatment efficiency and quality. We compared a non-adaptive workflow with two online adaptive workflows—one standard and one with unconventional beam arrangements—to determine whether DAPT could allow for reduced beam numbers to decrease delivery times while maintaining treatment quality. Material/Methods: Five previously treated H&N patients, with acquired daily CBCTs, were planned with a simultaneously integrated boost (CTV1=54.1 GyRBE, CTV2=59.4 GyRBE, CTV3=69.9 GyRBE). Synthetic CTs were created by deforming the planning CT to each CBCT. Targets and OARs were deformably propagated on daily images. Three treatment plans were generated for each patient: i)Non-adapted (NA PTV ): clinically used beam arrangement (4-6 fields), optimized to CTV-to-PTV 4mm margin, ii)DAPT CTV : same beam arrangement and robustly optimized to 3% range on CTV-to-PTV 2mm margin, iii)DAPT UNC : an unconventional beam arrangement (2-3 fields), robustly optimized to 3% range on CTV-to-PTV 2mm margin (Figure1). Online adaption was simulated for DAPT CTV and DAPT UNC by performing full daily plan reoptimization on each synCT [2]. A non-adaptive workflow was simulated for the NA PTV plan by recalculating on each synCT, and by performing offline replanning on the same fractions if performed during the actual treatment.