ESTRO 2025 - Abstract Book

S3132

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

progression and 79.1% patients needing replanning. In this cohort the average treatment break was 3 days, which was statistically lower than the pre-TLP cohort (p= 0.007). Factors that significantly contributed to reduction in replanning post-TLP were nutritional intervention and <5% weight loss. Conclusion: Implementing the traffic light protocol shows promise for HNC patients treated with VMAT. It facilitated identifying patients needing replanning earlier and an average reduction in treatment breaks by at least 4 days than prior TLP implementation, ensuring treatment continuity. Larger cohorts are warranted to confirm these findings. References: [1] DiBartolo D, Carpenter TJ, Santoro JP, Ebling DW, Haas JA, Lischalk JW, et al. Novel VMAT Planning Technique Improves Dosimetry for Head and Neck Cancer Patients Undergoing Definitive Chemoradiotherapy. Int J Radiat Oncol Biol Phys 2022 [2] Sousa FR, Jourani Y, Somoano M, Dragan T, Beauvois S, Gestel DV. Evaluation and Validation of an Image-Guided Traffic Light Protocol for Head and Neck Cancer Patients Undergoing Radiotherapy. J Med Imaging Radiat Sci 2019;50:S2–3 [3] van Beek S, Jonker M, Hamming-Vrieze O, Al-Mamgani A, Navran A, Remeijer P, et al. Protocolised way to cope with anatomical changes in head & neck cancer during the course of radiotherapy. Tech Innov Patient Support Radiat Oncol 2019;12:34–40. Digital Poster The utility of three-dimensional ultrasound for plan of the day selection in cervical cancer external beam radiation therapy Lei Wang 1 , Sarah Mason 2 , Sophie Alexander 1 , Helen McNair 1 , Emma Harris 3 , Susan Lalondrelle 1 1 Radiotherapy and Imaging, Royal Marsden Hospital, Sutton, United Kingdom. 2 Physics, Royal Marsden Hospital, Sutton, United Kingdom. 3 Radiotherapy and Imaging, Institute of Cancer Research, Sutton, United Kingdom Purpose/Objective: Cervical cancer plan-of-the-day (PotD) improves low-risk clinical target volume (CTV LR ) coverage and reduces normal tissue irradiation 1 . Use is limited due to poor CBCT image quality and substantial training required to select PotD accurately. We investigated the potential of ultrasound to make PotD selection easier, faster and more accurate. Material/Methods: This was a cohort study of consecutive cervical cancer patients treated with primary chemoradiotherapy, PotD technique and brachytherapy. Four plans were created per library, each covering a sub-range of anticipated CTV LR motion. Patients who consented received Clarity (Elekta, Sweden) transabdominal ultrasound scanning 2 (Fig 1) in the treatment position, prior to CBCT. Clarity scan acquisition depended solely on departmental workflow and operator availability for that fraction. Ultrasound images were analysed offline and did not affect on-set PotD selection. For each fraction, three experts determined the gold standard PotD, which optimised CTV LR coverage while sparing bladder and bowel. CBCT-only and co-registered US-CBCT images were presented for each fraction, in random order, to five observers. PotD PTV contours were overlaid, and observers selected the PotD independently. The accuracy score of each image was the number of observers selecting the gold standard plan, out of five. Observers also rated their plan selection confidence (0-3) and uterocervix visibility (0-3). The primary outcome was the mean difference in accuracy on US-CBCT vs CBCT. Secondary outcomes were mean differences in confidence and visibility. Keywords: Traffic Light Protocol, Setup Uncertainty 2314