S2912
RTT- RTT operational practice and workflow innovations
ESTRO 2026
they exceeded the corresponding dose-guidance level. Results: The workflow generates daily CBCT dose calculations in 5min18s±43s. The median dose variations are within ± Δ 3%, except for D0.03cc esophagus and left brachial plexus, which are within ± Δ 5%. The brachial plexi showed the highest variability (IQR:14.8%,15.2%). Dose differences before and after adaptation were not significant, except for esophageal D0.03cc, where post- adaptation dose differences were higher (Fig.1). After adaptation, interfraction dose variability decreased significantly for targets and most organs (Levene, FDR- adjusted p<0.05). Five fractions showed ≥Δ 5% underdosage in CTVp, eight in CTVn, though no accumulated CTV underdosage occurred over patients. The esophagus exceeded dose-guidance levels in 81% of fractions, but only 32% of fractions surpassed the Δ 5% threshold. For the spinal canal, 21% of fractions exceeded Δ 5%, and 5% exceeded Δ 10%. Three patients had accumulated D0.03cc above clinical limits. The number of patients exhibiting | ≥Δ 3%| dose trends remained comparable before (N=18) versus after adaptation (N=19) with mainly trends in esophagus and heart. Higher dose difference trends (| ≥Δ 10%|) occurred less frequently in patients after adaptation (N=12 vs N=8). Trend durations were consistent, spanning 4-6 fractions (Fig.2).
that this benefit was most salient in the context of liver cases ( Δ V100% = +11.1%, p = 0.006), with non- significant, yet favorable, trends in pancreas, prostate, adrenal, and kidney cases. By empowering radiation therapists to lead the adaptive planning process, this strategy supports efficient, high-quality treatment delivery with minimal additional training. These findings highlight the evolving role of radiation therapists in MR-guided radiotherapy and suggest opportunities to expand their scope of practice, optimizing human resource utilization in this cost- intensive workflow. Keywords: Adaptative, MR-Linac, Dosimetry Automated CBCT-based dose calculations reveal dose stability improvements after plan adaptation in lung radiotherapy Dylan Callens 1,2 , Jan Verstraete 2 , Patrick Berkovic 1,2 , Maarten Lambrecht 1,2 , Wouter Crijns 1,2 1 Laboratory of Experimental Radiotherapy, KU Leuven, Leuven, Belgium. 2 Department of Radiation Oncology, UZ Leuven, Leuven, Belgium Purpose/Objective: Daily CBCT-based dose calculations provide insights into delivered dose distributions[1,2], but their use is limited by manual workload. To overcome this, we automate CBCT-based dose calculations to obtain daily and accumulated doses. Using CBCT-based dose calculations, we evaluate variability and trends in target and organ doses during treatment of locally- advanced non-small-cell lung cancer and assess how routine plan adaptations impact these variations. Material/Methods: Nineteen patients from the prospective CBCT-based adaptive lung study ECLAIR (KUL s68182;NCT pending) were analyzed. All received fractionated chemoradiotherapy with mid-treatment plan adaptation (N=514 fractions). A Level-3 automated workflow was developed in MIM Software Inc. (v7.4, Cleveland, OH, USA)[3], performing CBCT contouring with deformable propagation for organs, rigid propagation for targets, followed by MonteCarlo dose Poster Discussion 1676 calculation (SureCalc®, 2.5mm-grid, default HU– density calibration). Interfraction dose variability was quantified by evaluating daily dose-guidance levels relative to the clinical pre-treatment/mid-treatment plan, and at thresholds ±3% Δ , ±5% Δ , ±10% Δ [4,5]. Dose trends were defined as ≥ 3 consecutive fractions with dose-guidance differences above the thresholds, with absolute dose differences below 1Gy excluded unless
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