ESTRO 2025 - Abstract Book

S3319

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

ESTRO 2025

Conclusion: IMPT BH treatments, combined with SS and NHFT, are feasible for treating mediastinal lymphoma.This method results in stable breath-holds, acceptable treatment times, and a simplified, fast off-line workflow in case of plan adaptation.

Keywords: breath-hold, proton therapy, lymphoma

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Digital Poster Breathing phase targeted treatment planning in preparation of FLASH proton therapy for lung cancer Simone N Visser 1,2 , Krista C.J. van Doorn-Wink 1,2 , Coen R.N. Rasch 1,2 , Steven J.M. Habraken 1,2 1 Radiotherapy, Leiden University Medical Center, Leiden, Netherlands. 2 Radiotherapy, Holland Proton Therapy Center, Delft, Netherlands Purpose/Objective: Ultra-High Dose Rate (UHDR) irradiation, or FLASH radiotherapy, holds the promise of a reduction of radiation toxicity through the ‘FLASH effect’. In the treatment of moving targets, FLASH comes with the additional advantage of freezing intra-fraction motion. With small target and robustness margins, timing is crucial for UHDR delivery. However, it remains unknown which breathing phase is optimal in terms of dosimetric endpoints, complication probabilities, and target location. Here, this issue is addressed in a retrospective planning study to identify the optimal breathing phase(s) for UHDR proton irradiation of lung cancer. Material/Methods: This study includes data from twenty consecutive patients with a model-based indication for proton therapy based on normal tissue complication probability (NTCP) models for radiation pneumonitis, dysphagia, and two-year treatment-related mortality. Patients received a total dose of 60-66 Gy in 24-30 fractions. The datasets included 4D CT scans with clinical target and organ-at-risk (OAR) delineations and beam angle setups. To reduce planning variation, ITV-based surrogate clinical plans based on the average intensity projection (AIP) were automatically generated. These plans served as a baseline for comparison. Additionally, ten CTV-based breathing-phase plans per