S2855
RTT - RTT education, training, and advanced practice
ESTRO 2026
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
Poster Discussion 1676
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 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 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
For the 50 analyzed fractions, organ-at-risk (OAR) sparing was comparable between the reference plan cost function and the step-by-step approach, with differences of ±2% in volume indices and ±1 Gy in dose metrics—favoring either method depending on the case. These variations were negligible across full treatment courses.In contrast, target volume coverage consistently favored the step-by-step method: 41 fractions showed superior coverage, 8 were equivalent (within 1% difference), and only 1 favored the reference plan cost function. The average gain in V100% for the planning target volume (PTV) was 7.73±7.36%.Planning time averaged 8±5 minutes for prostate SBRT fractions and 15±10 minutes for other sites. This included verification of layers and electron density mapping, plan optimization, acquisition of the Comprehensive Motion Management (CMM) template, cine sequence launch, and multidisciplinary review involving the radiation therapist, medical physicist, and radiation oncologist. This represents an increase of approximately 2 minutes compared to the reference plan cost function approach. Conclusion: Target coverage (V100%) was significantly improved using the structured step-by-step approach (ANOVA p = 0.003). Organ-level post-hoc contrasts demonstrated 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
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