S2165
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Keywords: Synthetic-CT, adaptive, pelvis male
workflow included CBCT acquisition, AI-based auto- contouring, target and organ-of-interest evaluation, plan adaptation and review, pre-treatment CBCT verification, treatment delivery, and post-treatment CBCT for target coverage verification [2,3]. Workflow evaluation was based on retrospective session logs documenting (1) timing of process steps and (2) oART events such as corrections and unplanned interventions. Results: In total, 492 adaptive sessions were analysed. Median in-room time was 32 minutes (IQR 28-38) (Figure 1). Manual correction of AI-generated contours was the most time consuming step: “influencer” structures were adjusted in 91.9% of sessions, and target volumes in 43.7% (Figure 2). The non-adaptive plan was selected only once, and all plans met CTV coverage requirements (mean D98% of 1.79 ± 0.10 Gy). Learning curve effects were not yet quantifiable in this cohort and require further study.
Mini-Oral 3833
Two-Year Experience with CBCT-Guided Online Adaptive Radiotherapy for Gynaecological Cancer Danique L.J. Barten 1,2 , Karin N. Goudschaal 1,2 , Karel A. Hinnen 1,2 , Rianne A.J. de Jong 3,1 , Jan Wiersma 1,2 , Niek van Wieringen 1,2 , Angelique R.W. van Vlaenderen 4,2 , Sandy Loopeker 4,2 , Zdenko van Kesteren 1,2 , Arjan Bel 1,2 , Bradley R. Pieters 1,2 , Jorrit Visser 1,2 , Colette B. van den Broek 1,2 1 Radiation Oncology, Amsterdam University Medical Center, University of Amsterdam, Amsterdam, Netherlands. 2 Cancer Therapy, Treatment and Quality of Life, Cancer Center Amsterdam, Amsterdam, Netherlands. 3 Radiation Oncology, Leiden University Medical Center, Leiden, Netherlands. 4 Radiation Oncology, Amsterdam University Medical Center, Vrije Universiteit Amsterdam, Amsterdam, Netherlands Purpose/Objective: Managing inter- and intrafraction motion remains a key challenge in gynaecological cancer radiotherapy. CBCT-guided online adaptive radiotherapy (oART) allows daily plan adaptation to patient anatomy, enabling tighter margins and reduced dose to healthy tissue while maintaining robust target coverage. This study aimed to determine the required treatment timeslots for CBCT-guided oART for gynaecological malignancies using an RTT-only workflow, and to identify workflow bottlenecks. Material/Methods: Twenty patients with cervical or primary endometrial cancer were treated with CBCT-guided oART (Ethos platform, Varian Medical Systems, a Siemens Healthineers company) between October 2023 and August 2025, following the EMBRACE II protocol [1] for pre-treatment preparation, target and organ delineation, and dose prescription (45 Gy in 25 fractions, and four patients with a simultaneous integrated boost (SIB) to pathologic lymph nodes (55.0–57.5 Gy)).A radiation oncologist and a medical physicists expert supervised all treatment sessions for the first patient and the first five sessions for subsequent patients, after which an RTT-only workflow was adopted. RTTs could consult a supervisor as needed, supported by a traffic-light protocol for ad hoc multidisciplinary backup. All RTT-only sessions underwent offline QA. Team members were experienced in pelvic oART (bladder and rectum) and received additional gynaecology-specific training [2,3].Gynaecological oART was introduced in phases, progressing from low-risk single-isocenter cases to more complex two-isocenter setups and SIBs as experience and QA confidence grew. The daily
Conclusion: CBCT-guided oART for gynaecological cancer is feasible using an RTT-only workflow. Time slots of 30– 40 minutes are recommended, depending on patient complexity (e.g., SIB vs. no SIB). Further standardisation and faster evaluation/acceptance of influencer structures, as well as flexible time slots, are needed for optimisation. References: [1] Pötter R, et al.EMBRACE II: The GEC-ESTRO phase II study on MRI-guided adaptive brachytherapy in locally advanced cervical cancer (v1.5). Aarhus University Hospital; 2015. Available from: https://www.embracestudy.dk/UserUpload/Publi
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