S2129
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
this group. This study aims to analyze the protocol’s performance and clinical outcomes of patients treated within it. Material/Methods: From 2019 to 2022, we prospectively enrolled 49 inoperable NSCLC patients referred to concurrent chemoradiotherapy. Of these, 46 received IMRT with prescribed dose 60-70 Gy, 2 Gy/fraction. We registered a daily CBCT to the planning CT using bone match and assessed anatomical changes with the traffic lights: yellow (observed changes, no action), orange (physicist review) and red (treatment withheld pending oncologist evaluation). We recorded atelectasis, pleural effusion, infiltrative changes, baseline shift, tumor growth or shrinkage. Using a cross-sectional design, we obtained repeated CTs (rCT) during week 1 and 3, registered them to the planning CT and evaluated dose distributions. We re-delineated target volumes, recalculated plans, and estimated CTV D98% with and without ART, defined as corrections from bone to tumor match or replanning. We defined insufficient target coverage as CTV D98% <95% of prescribed dose. We retrieved clinical follow-up data, and assessed time to locoregional failure, distant failure and overall survival (OS). Results: Forty-five patients had complete traffic light registrations for adaptive analyses. Thirty-eight had at least one rCT for dosimetric analyses. Thirty-eight with disease stage II-III were considered locally advanced (excluding stage I and single metastatic stage IV) and included in survival analyses. Baseline shift (93.3%) and tumor shrinkage (40.0%) were most frequent anatomical changes, corrected by replanning in 13.3% of patients and adjustment to tumor match in 37.8%. We acquired 73 rCTs. Without ART, 6 of 38 patients (15.8%) would have received insufficient CTV coverage at one evaluated time point (Fig.1). Twenty-seven patients were PD-L1 positive, and 26 received immunotherapy. Median OS was 43.5 months (95% CI: 32.9-54.1) (Fig. 2). Median time to locoregional failure was 27.4 months (95% CI: 0-61.9). Median estimated time to distant failure was 30.7 months (95% CI not estimated due to censoring). Conclusion: The adaptive protocol recognized relevant anatomical changes, directly improving CTV coverage. Clinical outcomes among locally advanced patients were consistent with international results.
Figure 1. D98% to the CTV was significantly lower without ART compared to ART (Related-Samples Wilcoxon Signed-Rank Test, p = 0.031).
Figure 2. Median OS of 43.5 months (95% CI: 32.9-54.1) for stage II-III patients (Kaplan-Meier). Keywords: Traffic light protocol, tumor match, replanning
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Fast on-line adaptive MR-guided radiotherapy treatment of high-risk prostate cancer patients. Emma K.K. ten Hoor, Thomas Willigenburg, Gijs H. Bol, Eline N. de Groot - van Breugel, Piter R. Huisma, Alexis N.T.J. Kotte, Bas W. Raaymakers, Jochem R.N. van der Voort van Zyp, Cornel Zachiu, Hans C.J. de Boer Radiation Oncology Department, UMC Utrecht, Utrecht, Netherlands Purpose/Objective Radiotherapy (RT) treatment of high-risk prostate cancer (PCa) patients is usually performed on the 1.5T Unity MR-linac (Elekta AB) in our institute, using 20 treatment fractions. These patients are treated using an adapt-to-position (ATP) workflow, where the pre-
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