S2222
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
longitudinal axis at the 2 mm threshold (log-rank test, P=0.006). The failure rates at 3–10 minutes for this threshold ranged from 7.3% to 52.4% (system A) and 4.2% to 22.4% (system B). A substantial percentage of fractions (>5%) exceeded the 2 mm motion threshold at 1.8 minutes, the 3 mm threshold at 3.8 minutes, the 4 mm threshold at 6.6 minutes, and the 5 mm threshold at 8.1 minutes.
Guided Adaptive Radiation Therapy in One Fraction (SMART ONE): A Multicenter, Single-Arm, Phase 2 Trial. Int J Radiat Onc Bio Phys 2025;122:957-967 Keywords: MR-guidance, single-fraction, plan quality
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Analysis of Intrafraction Prostate Motion and Implications for Margin Selection in Linac-Based SBRT Denis Panizza 1,2 , Valeria Faccenda 1,2 , Federica Ferrario 3 , Giulia Rossano 2 , Lorenzo De Sanctis 2 , Riccardo Ray Colciago 2 , Elena De Ponti 1,2 , Stefano Arcangeli 2,3 1 Medical Physics, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy. 2 School of Medicine and Surgery, University of Milan Bicocca, Milan, Italy. 3 Radiation Oncology, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy Purpose/Objective: Intrafraction motion poses a major challenge in ultrahypofractionated prostate SBRT, where high doses and fewer sessions amplify the impact of positional shifts. Its management varies across centers due to differences in technology, protocols, and resources, and standardized clinical guidelines are still lacking despite widespread recognition of its importance. This study aims to provide insights into real-time monitoring and margin selection by quantifying intrafraction prostate motion using two distinct monitoring systems – system A (electromagnetic transmitter-based) and system B (4D transperineal ultrasound-based) – and evaluate their impact on motion patterns in Linac-based SBRT. Material/Methods: Intrafraction motion data were collected from 92 treatment fractions using system A (47 patients) and 100 fractions using system B (30 patients). All patients underwent SBRT (1–5 fractions) with strict bowel and bladder preparation. Motion was recorded at 1 Hz (system A) and 2 Hz (system B) from CBCT acquisition to treatment completion, without any zeroing due to patient repositioning. Motion trend was analyzed using a mixed-effects regression model, and Kaplan- Meier failure analysis estimated the time to exceed 2 mm, 3 mm, 4 mm, and 5 mm motion thresholds. Results: Median monitoring time was 13 minutes for system A (range: 5.9–35.6) and 6.4 minutes for system B (range: 4.9–29.3). Motion increased over time in all directions (P<0.001), predominantly in the posterior direction (time coefficient = -0.20, P<0.001). A significant longitudinal motion divergence was observed between systems (P<0.001). Kaplan-Meier failure curves were comparable, except for a significant difference in the
Conclusion: Both systems exhibited predominant posterior motion, with system B reducing small longitudinal prostate fluctuations, likely due to probe pressure against the perineum. For fast Linac-based SBRT, intrafraction motion can be managed with 4-5 mm margins without continuous monitoring. However, if 2- 3 mm margins are used, real-time monitoring becomes essential. Keywords: EM Transmitter, Ultrasound, Margin Selection Comparative Assessment of kV Imaging Dose for Intrafraction Prostate Fiducial Monitoring Using Three Clinical Systems Paul RETIF 1,2 , Abdourahamane DJIBO SIDIKOU 1 , Motchy SALEH 1 , Romain LETELLIER 1 , Anwar AL SALAH 1 , Estelle PFLETSCHINGER 1 , Fabian TAESCH 1 , Emilie VERRECCHIA-RAMOS 1 , Xavier MICHEL 3 1 Medical Physics Unit, CHR Metz-Thionville, Metz, France. 2 CRAN, Université de Lorraine, Nancy, France. 3 Radiation Therapy Department, CHR Metz-Thionville, Metz, France Digital Poster 1630 Purpose/Objective: To compare the imaging dose associated with intrafraction fiducial-based prostate localization in
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