S2220
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
studied for the intact prostate, motion characteristics of the prostate bed during SBRT remain poorly defined. This study evaluated intrafraction motion using a 4D transperineal ultrasound (TPUS) system in linac-based post-prostatectomy SBRT and compared it with the intact prostate motion pattern. Material/Methods: Fifty fractions from ten patients enrolled in the POPART trial for biochemical relapse after radical prostatectomy were treated using a TPUS system with an automatic probe fixed to the couch for real-time motion tracking. The PTV was generated with a 5/3 mm expansion from the CTV, delineated per Groupe Francophone de Radiothérapie Urologique guidelines. A gating threshold of 2.5 mm for more than 5 s was applied. The monitored prostate bed volume was defined using the symphysis, bladder, and penile bulb as landmarks. Couch-relative prostate bed shifts from setup to 2-3 minutes after beam-off were analyzed for all fractions and compared with prostate motion data from a previous population treated with the same approach. Motion trend was analyzed using a mixed- effects regression model and Kaplan-Meier analysis estimated the time to exceed 2mm, 3mm, 4mm, and 5mm thresholds. Results: TPUS tracking was completed for all fractions, with the rapid delivery process allowing uninterrupted sessions. Median [range] session duration, including post-treatment beam-off waiting time, was 7.1 minutes [5.2–9.8]. Mean (SD) displacements were 0.13 mm (0.53), -0.18 mm (-0.56), and -0.68 mm (0.94) in the lateral, longitudinal, and vertical directions, respectively. Mean prostate motion was slightly greater: 0.02 (0.79), -0.36 (0.65), and -0.83 (1.03) in the same directions. Prostate bed motion, similar to the intact gland, increased with time and predominantly occurred posteriorly, with motion progression coefficients of -0.14 (prostate bed) vs. -0.20 (prostate). Kaplan-Meier analysis revealed that >5% of fractions exceeded 2mm at 3.5 vs. 1.8 minutes and 3mm at 6.8 vs. 3.8 minutes. The prostate surpassed the 4mm and 5mm thresholds at 6.6 and 8.1 minutes, but prostate bed failure rates never reached 5% for these thresholds.
Figure 2 - Mean absolute errors per fraction Conclusion: XR can reliably position and track spine metastases, safely replacing time-consuming CBCTs. Continuous monitoring allows small margins in spine SBRT, although mobile bones such as ribs may require additional margins. SGRT monitoring helps minimize intra-fraction errors between XR checks. References: Guckenberger, M., Mantel, F., Gerszten, P. C., Flickinger, J. C., Sahgal, A., Létourneau, D., ... & Kersh, R. (2014). Safety and efficacy of stereotactic body radiotherapy as primary treatment for vertebral metastases: a multi-institutional analysis. Radiation oncology, 9(1), 226. Keywords: SBRT, Spine, motion-management Digital Poster Highlight 1374 Preliminary analysis of intrafraction motion in prostate bed SBRT using 4D ultrasound monitoring: subgroup data from the POPART trial (NTC04831970) 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: Stereotactic body radiotherapy (SBRT) to the prostate bed after radical prostatectomy is an emerging approach aiming to improve local control while preserving convenience. However, the reduced target volume and proximity of organs at risk make the technique highly sensitive to intrafraction motion. While real-time motion monitoring has been widely
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