ESTRO 2025 - Abstract Book

S3245

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

motion exceeding 2, 3, 4, and 5 mm were comparable across the systems and increased with treatment time. The impact of intrafraction prostate motion is minimal with standard 5 mm margins and treatments under 8 minutes; however, with tighter margins, motion-induced risk becomes more pronounced even at shorter treatment durations. Conclusion: Both systems effectively measured intrafraction prostate motion, with Clarity providing partial immobilization as the probe’s pressure against the perineum limited longitudinal movement. In contrast, RayPilot may more accurately reflect natural longitudinal prostate motion, though further validation with device-free systems would be beneficial. These findings strenghten the importance of intrafraction monitoring when tighter margins or longer treatments are involved.

Keywords: intrafraction motion, linac-based prostate SBRT

2080

Mini-Oral Quantifying intrafraction motion in prostate SBRT: analysis from a mono-institutional experience with a 4D transperineal ultrasound monitoring system Valeria Faccenda 1 , Denis Panizza 1 , Chiara Ingraito 2 , Chiara Chissotti 3 , Federica Ferrario 3 , Riccardo Ray Colciago 3 , Stefano Arcangeli 3 , Elena De Ponti 1 1 Medical Physics, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy. 2 Physics, University of Milan, Milan, Italy. 3 School of Medicine and Surgery, University of Milan Bicocca, Milan, Italy Purpose/Objective: This study aimed to quantify intrafraction motion using a 4D transperineal ultrasound (TPUS) real-time monitoring system in linac-based prostate SBRT. Material/Methods: One hundred and fifty fractions from fifty patients with localized prostate cancer treated with 30Gy in 3fx since July 2023 were investigated. PTV was obtained by a 3mm isotropic expansion from CTV. Patient setup was achieved through CBCT soft-tissue matching. A TPUS automatic probe fixed to the treatment couch was used for intrafractional prostate monitoring. The system interrupted the beam delivery when the threshold of 2.5mm was exceeded for >5 seconds in any of the three spatial directions. Unless the offset was transient, the patient was repositioned by repeating CBCT or using the coordinates recorded in the system, whether the out-of-tolerance movement occurred in the setup or delivery phase, respectively. Couch-relative shifts of the prostate, from the beginning of the setup to the end of treatment delivery, were analyzed for all fractions to capture the real intrafraction motion as a function of time. Results: Intrafractional TPUS tracking was successfully performed in all fractions. Median [range] sessions duration was 6.2 minutes [4.9–29.3], while the delivery time was less than 2 minutes on average. In 125/150 fractions, no motion exceeding the 2.5mm threshold was detected and no intervention was needed. In 25/150 fractions, at least one motion beyond the threshold was observed. Of these, 5/150 had transient motion with only a beam-hold, 15/150 required patient position correction, and 7/150 involved both transient and non-transient motion. The analysis of couch-relative shifts revealed that the mean (SD) prostate shifts across all data were -0.84 (0.92), 0.91 (0.77), and -3.09 (1.71) in the lateral, longitudinal, and vertical directions, respectively. When limiting the analysis to the first 10 minutes, the mean (SD) shifts were 0.02 (0.07), 0.47 (0.33), and -1.15 (0.74). At 5 minutes, the shifts further reduced to 0.02 (0.03), 0.27 (0.19), and -0.65 (0.38). The prostate motion mainly occurred in the posterior direction. The minimum