S2166
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
preliminary subgroup of the selected plans. The tolerance level (TL) was defined as the mean PR% minus 2.660 times the moving range, as appropriate for individual–moving-range charts. The action level (AL) was based on the mean and standard deviation across delivered fractions, with a nominal PR% target of 100%. Plans with PR% below AL were flagged as potentially non-robust. Daily variations of PTV and neighboring OAR volumes were used to contextualize fraction-to-fraction fluctuations. Results: Inter-patient variability in plan complexity was moderate and primarily associated with baseline anatomy and initial plan geometry. Metrics such as Tongue-Groove index and ALPO showed dominant between-patient components (>70%), reflecting stable patient-specific features. Conversely, modulation- related descriptors such as the number of low-MU segments displayed larger within-patient variance, indicating sensitivity to daily adaptive re-optimization. Relationship between PCMs and PSQA outcomes revealed strong heteroscedasticity, at least for some predictors (Figure 2). In cases of pronounced anatomical deformation, intra-patient variability exceeded the inter-patient range.
cDocuments/Docs/EMBRACE_II_title_page_and_total_p rotocol_v15_151015.pdf[2] de Jong R, et al. Feasibility of Conebeam CT-based online adaptive radiotherapy for neoadjuvant treatment of rectal cancer. Radiat Oncol. 2021 Jul 23;16(1):136. [3] K. Goudschaal, et al. Clinical implementation of RTT-only CBCT-guided online adaptive focal radiotherapy for bladder cancer. Clinical and Translational Radiation Oncology. Volume 50, 2025,100884, ISSN 2405-6308. Keywords: CBCT-guided oART, GYN cancer, implementation Unraveling the Variability of Plan Complexity Metrics in Pancreatic MR-Guided Adaptive Radiotherapy Matteo Galetto 1 , Alessandro Scaggion 2 , Samuele Cavinato 2 , Andrea Bettinelli 1 , Matteo Nardini 1 , Giuditta Chiloiro 1 , Angela Romano 1 , Marta Paiusco 2 , Luca Boldrini 1,3 , Lorenzo Placidi 1 1 Diagnostic Imaging and Oncological Radiotherapy Department, Fondazione Policlinico Universitario A. Gemelli IRCCS, Roma, Italy. 2 Medical Physics Digital Poster 3851 Department, Veneto Institute of Oncology IOV-IRCCS, Padua, Italy. 3 Dipartimento di Scienze radiologiche ed ematologiche, Università Cattolica del Sacro Cuore, Roma, Italy Purpose/Objective: Integrating complexity indicators during MR-guided online adaptive radiotherapy (oART) provides a safeguard against generating highly modulated or less robust plans when pre-treatment QA is not feasible.Very recent works have demonstrated that plan complexity metrics (PCMs) capture workflow- and patient-specific patterns in oART. However, the relative contribution of inter- and intra-patient variability to overall plan modulation remains unclear. This study systematically investigates the variability of PCMs across adaptive fractions in a homogeneous clinical cohort, with additional consideration of daily volumetric changes in targets and organs at risk (OARs). Material/Methods: 100 patients with locally advanced pancreatic cancer treated with oART on a 0.35 T MR-Linac were retrospectively analyzed. All plans were generated with the same optimizer (A3i) and identical planning strategy, within the clinical workflow shown in Figure 1. Fifteen PCMs were computed for each adapted fraction, and linear mixed-effects models were applied to quantify the between-patient and within-patient components of variance. Statistical process control (SPC) for plan specific quality assurance (PSQA) was applied to PR%(2%,2 mm) values, considering a
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