S2234
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Gastroenterology, Université Libre de Bruxelles, Route de Lennik 808, 1070 Brussels, Belgium
Dosimetric evaluation confirmed that the segment- specific margin strategy maintained adequate target coverage while reducing OAR doses by 6.5-24.4% compared to conventional uniform margins.
Purpose/Objective: Pancreatic tumors exhibit considerable intrafraction motion due to respiration and gastrointestinal activity, challenging precise dose delivery in stereotactic body radiotherapy (SBRT). The Elekta Unity MR-Linac integrates high soft-tissue contrast magnetic resonance imaging (MRI) with a Comprehensive Motion Management (CMM) system, enabling real- time 3D tracking and automatic beam gating. This study retrospectively characterizes intrafraction motion and treatment efficiency in pancreatic cancer (PC) patients treated with MRI-guided stereotactic radiotherapy (MRIgSBRT) under abdominal compression. Material/Methods: Sixteen PC patients (78 fractions) treated between September 2024 and October 2025 were analyzed. The gross tumor volume (GTV), including tumor–vessel interfaces, was delineated daily on MRI without additional respiratory margins, and a uniform 3-mm planning target volume (PTV) margin was applied. The CMM gating envelope was coincident with the PTV. The Volumetric Overlap Criterion (VOICE) quantified the percentage of PTV volume within the 3D gating boundaries; beam delivery was interrupted when VOICE dropped below 95%. Baseline shifts were applied using CMM Intrafraction Drift Correction when a clear target displacement was observed. Time- stamped 3D motion vectors and beam-on intervals extracted from audit logs were used to compute total monitoring time, beam-on duration and ratio, maximum 3D displacement amplitude, and intrafraction variability (standard deviation). Descriptive statistics were calculated per patient and per fraction, and cumulative VOICE distributions were analyzed to assess target coverage stability. Results: A total of 78 fractions were analyzed. The mean monitoring duration was 13.8 min (range 6.2–24.1), with a meanbeam-on time of 12.2 min (range 5.3– 21.5), resulting in a meanbeam-on ratio of 82%. Baseline shifts occurred in 14% of fractions (11/78), indicating infrequent but clinically relevant drift corrections at some point during the treatment. The mean maximum 3D motion amplitude was 9 mm (range 6–14 mm), and the meanintrafraction variability was 2.5 mm (SD), demonstrating effective motion control. VOICE analysis showed that the target remained within the gating envelope for approximately 95% of the beam-on time, confirming stable coverage during treatment.
Conclusion: Compared to conventional uniform margin expansion, the tumor location-based segmental approach is a more precise radiotherapy strategy. This approach ensures sufficient target coverage while reducing radiation dose to OARs, establishing it as a promising pathway toward organ and function preservation in rectal cancer patients. References: [1] Valentini V, Gambacorta M A, Cellini F, et al. The INTERACT Trial: Long-term results of a randomised trial on preoperative capecitabine-based radiochemotherapy intensified by concomitant boost or oxaliplatin, for cT2 (distal)-cT3 rectal cancer[J]. Radiother Oncol, 2019,134:110-118.[2] Eijkelenkamp H, Boekhoff M R, Verweij M E, et al. Planning target volume margin assessment for online adaptive MR- guided dose-escalation in rectal cancer on a 1.5 T MR- Linac[J]. Radiother Oncol, 2021,162:150-155.[3] Kronborg C J S, Christensen P, Pedersen B G, et al. Anorectal function and radiation dose to pelvic floor muscles after primary treatment for anal cancer[J]. Radiother Oncol, 2021,157:141-146. Keywords: Margin expansion, dosimetric validation, MR-Linac Digital Poster 2693 Intrafraction motion and gating efficiency in pancreatic MR-guided SBRT (MRIgSBRT). Paulo Ferreira 1 , Zelda Paquier 1 , Younes Jourani 1 , Sara Poeta 1 , Ana Margarida Costa 2 , João Perdigão 2 , Leonor Saavedra 2 , Nicolas Jullian 2 , Robbe Van den Begin 2 , Akos Gulyban 1 , Christelle Bouchart 2,3 1 Medical Physics Department, Institut Jules Bordet, Hopital Universitaire de Bruxelles, Université Libre de Bruxelles, Brussels, Belgium. 2 Radiation Oncology Department, Institut Jules Bordet, Hopital Universitaire de Bruxelles, Université Libre de Bruxelles, Brussels, Belgium. 3 Laboratory of Experimental
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