ESTRO 2025 - Abstract Book

S3201

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

ESTRO 2025

Conclusion: Open-face masks, when combined with SGRT, offer comparable stability to traditional closed facemasks in H&N radiotherapy. The findings demonstrate that intrafraction motion, as measured by both CBCT and SGRT, remains within clinically acceptable limits across all mask types. The use of SGRT allowed for the detection of transient deviations and rotational differences not detected using CBCT alone. These results support the broader adoption of open-face masks with SGRT to potentially enhance patient comfort without sacrificing the precision of radiotherapy delivery.

Keywords: OPEN Trial, Faceless orfits, SGRT, Intrafraction

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Digital Poster Is the interplay effect always relevant for free-breathing proton therapy of lung and esophageal cancer patients? Giorgio Cartechini, Esther Kneepkens, Gloria Vilches-Freixas, Mirko Unipan, Indra Lubken, Marije Everaerts Velders, Ilaria Rinaldi Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands Purpose/Objective: This study evaluates the interplay effect on lung and esophageal cancer patients treated in free breathing proton therapy without volumetric repainting. Material/Methods: We analyzed a cohort of 10 patients (seven lung and three esophageal cancer patients) treated with pencil beam scanning. Multi-field optimization was applied, and all plans were 3D robustly optimized on ITV with a 5 mm setup uncertainty, and a 3% range uncertainty. Treatments were conducted in free breathing without volumetric repainting. The Mevion S250i system passively degrades the pristine energy resulting in relatively large spot sizes [1]. To assess the interplay effect, we integrated the delivery time data from machine log-files with the patient-specific breathing traces and the 4DCT images and recalculated the dose delivered across the eight phases of the 4DCT. We used FRED, a GPU-based Monte Carlo dose engine, clinically validated for our beam line [2]. We employed the deformable registration algorithm in RayStation 12A to warp doses from each phase to a reference phase. Setup or range uncertainties were not considered. We calculated the inter-fractional D98 for the clinical target volume (CTV) for both lung and esophageal patients. Additionally, we assessed the mitigation effect of fractionation by accumulating the dose distribution over treatment fractions. Results: The interplay effect for lung and esophageal cancer patients is found to be negligible. Patient P5 showed the largest deviations from the prescribed D98 up to 8% (Figure1). The inter-fractional D98 variations are caused by the presence of hot and cold spots in the target due to organ motion during the delivery. An example of these effects is shown for P5 in Figure 1. The interplay effect is mitigated by fractionation. The relative difference between the accumulated D98 and the nominal plan D98 is within 2% for all patients after 3 fractions, except for P5, where D98 converged after 6 fractions (Figure2).