ESTRO 2025 - Abstract Book

S3244

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

ESTRO 2025

Conclusion: While reduced, the derived 8mm margin is larger than some recommended in the literature (Yen 2022). Further work could examine anisotropic margins, with greater reduction in the inferior direction, due to the trend of superior CTV1 displacement. Overall, the reduction of the uterine margin, from 15mm to 8mm, was shown to be feasible without compromising target coverage, even in cases of large intra-fractional uterine displacement.

Keywords: Adaptive, radiotherapy, gynaecology

References: Shelley CE, Bolt MA, Hollingdale R, Chadwick SJ, Barnard AP, Rashid M, Reinlo SC, Fazel N, Thorpe CR, Stewart AJ, South CP, Adams EJ. (2023) “ Implementing cone-beam computed tomography-guided online adaptive radiotherapy in cervical cancer ”. Clin Transl Radiat Oncol. 2023 Feb 14;40:100596 Yen, A, Choi B, Inam, E, Yeh, A, Lin, MH, Park, C, Hrycushko B, Nwachukwu C, Albuquerque K. (2022) “Spare the Bowel, Don’t Spoil the Target: Optimal Margin Assessment for Online Cone-Beam Adaptive Radiation Therapy (OnC-ART) of the Cervix”. Pract. Radiat. Oncol, in press.

2075

Digital Poster Intrafraction prostate motion in linac-based prostate SBRT: can the choice of monitoring system affect the motion patterns? Denis Panizza 1,2 , Valeria Faccenda 1 , Chiara Chissotti 2 , Federica Ferrario 2 , Riccardo Ray Colciago 2 , Stefano Arcangeli 2,3 , Elena De Ponti 1 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: To quantify intrafraction prostate motion using two different continuous monitoring systems – RayPilot (electromagnetic transmitter-based) and Clarity (4D transperineal ultrasound-based)– and determine whether the choice of the device affects the motion patterns. Material/Methods: Data were collected from 93 treatment fractions using RayPilot (47 patients) and 100 fractions using Clarity (30 patients). All patients received SBRT regimen (1-5 fractions) and underwent a strict bowel and bladder preparation regimen. Prostate motion was recorded at 1 Hz for RayPilot and 2 Hz for Clarity from the start of CBCT to the treatment completion, without any zeroing due to patient repositioning. Median monitoring session durations and average motion metrics were calculated for each system. The probability of prostate displacement over time was also analyzed. Results: Median monitoring session time for Clarity was 6.4 minutes (range: 4.9–29.3). RayPilot sessions were longer (median duration, 13 minutes; range, 5.9–35.6), largely due to the majority of patients receiving a single 24Gy fraction as per an experimental protocol. Mean intrafraction prostate motion was 0.02 mm, 0.50 mm, -1.18 mm for Clarity and - 0.11 mm, 0.26 mm, -0.83 mm for RayPilot, in the lateral, longitudinal, and vertical directions, respectively. Both systems showed predominant posterior motion, with consistent lateral and vertical motion patterns over time. For the longitudinal direction, Clarity recorded a slight systematic displacement in the superior direction after 10 minutes, which was not observed with RayPilot, along with a gradual reduction in motion probability over time. For motion exceeding 2 mm in the longitudinal axis, the probabilities were 2% vs. 5% at 3 minutes, 4% vs. 9% at 5 minutes, 9% vs. 27% at 8 minutes, and 10% vs. 48% at 10 minutes. In any direction, the probabilities of prostate