S2224
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
dose spinal nerve ablation. Preclinical and clinical evaluations demonstrated good agreement with our existing clinical standards. This approach has the potential to improve treatment accuracy by providing real-time internal anatomical information during treatment delivery. References: (1) Hazelaar C, Verbakel W, Mostafavi H, van der Weide L, Slotman BJ, Dahele M. First Experience With Markerless Online 3D Spine Position Monitoring During SBRT Delivery Using a Conventional LINAC. Int J
Radiat Oncol Biol Phys. 2018;101(5):1253-8. Keywords: real-time tracking, markerless, radiosurgery
Digital Poster 1928 Towards postoperative online adaptive radiotherapy for endometrial cancer: CTV-PTV margin and intrafractional target motion Liselotte A ten Asbroek- Zwolsman, Lisanne GM Zwart, Stan Scholten, Hans Ligtenberg, Judith J Dasselaar Radiotherapy, Medisch Spectrum Twente, Enschede, Netherlands Purpose/Objective: Online adaptive radiotherapy (OART) for postoperative treatment of endometrial cancer is feasible in theory (1). This previous retrospective study demonstrated a dosimetric benefit of OART when applying standard, image-guided radiotherapy (IGRT) based clinical target volume (CTV) to planning target volume (PTV) margins. Given OART’s ability to account for daily anatomical variations, margins may potentially be reduced. This study aimed to determine a CTV–PTV margin to correct for intrafractional target motion during postoperative OART in endometrial cancer patients. Material/Methods: In consecutive patients with endometrial cancer treated with IGRT in 2025, intrafractional motion of the postoperative target area was evaluated. The target area consists of the postoperative target area (CTV-t), including parametrial tissues, surgical area, the top of the vagina and elective lymph nodes (CTV-n), based on FIGO staging for endometrial cancer and extend of surgery. Previous unpublished research on pelvic OART showed that a 5 mm margin for CTV-n can be used safely. Therefore, this study solely investigated CTV-t. CTV-t was re-delineated on both pre-treatment cone-beam CT (CBCT1) and post-treatment CBCT (CBCT2). For each fraction, the minimum CTV–PTV margin required to cover the motion of CTV-t from CBCT1 to CBCT2 was calculated. To assess whether target motion increased over time, the measured motion was plotted against time between CBCT1 and CBCT2 and a Pearson correlation test was used.
Conclusion: Offline analysis of RapidTrack tracking results indicated successful real-time monitoring during high-
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