S2188
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
novel CBCT
CBCT acquisition time. [2] These images have Hounsfield Unit (HU) accuracy, comparable to a standard fan-beam CT image, enabling direct dose calculation on the image. [3] The goal of this research is to evaluate potential dosimetric benefits of oART when using the improved CBCT images for the local population of cervical cancer patients. Material/Methods: This study included anonymized imaging data from five past patients with locally advanced cervical cancer at our institution. The dataset consisted of the planning CT simulation scan, the original plan contours, and six CBCT’s that were evenly spaced throughout the patient’s treatment. (Fraction 1, 5, 10, etc) The CTV-to-PTV margin was reduced to 5mm, based on previous literature. For each CBCT, the adaptive treatment was simulated using VMAT treatment on the system’s treatment emulator, forming an adapted and scheduled plan. Both plans were compared with the dose goals and constraints outlined in our institution’s cervical cancer guidelines. Results: The adapted plans demonstrated improved target coverage compared to the scheduled plans. In average of all patients, CTV V98% increased from 98.65% (scheduled) to 99.5% (adapted) with p<0.001. The maximum dose delivered to the PTV was also significantly reduced, 107.8% for the adapted plans to 109.4% for the scheduled plans (p=0.006). The dose delivered to the OARs were statistically lower for the adapted plans. The maximum dose delivered to the bladder, rectum and bowel bag decreased by 6.5% (p<0.001), 1.05% (0.004) and 2.15% (p<0.001), respectively. Further work will be done to further analyze the use of the upgraded imaging system and potential patient benefit. Conclusion: oART using upgraded imaging technology, provided an acceptable treatment plan with improved coverage of the target volumes while limiting the dose to OARs when compared to non-adaptive radiotherapy. References: [1] N. Tyagi et al., “Daily online cone beam computed tomography to assess interfractional motion in patients with intact cervical cancer,” Int J Radiat Oncol Biol Phys, vol. 80, no. 1, pp. 273–280, May 2011, doi: 10.1016/j.ijrobp.2010.06.003.[2] J. L. Robar et al., “Novel Technology Allowing Cone Beam Computed Tomography in 6 Seconds: A Patient Study of Comparative Image Quality,” Pract Radiat Oncol, vol. 14, no. 3, pp. 277–286, May 2024, doi: 10.1016/j.prro.2023.10.014.[3] C. Fallone et al., “PD- 0662 Evaluation of Ethos HyperSight imaging performance compared to standard CBCT and FBCT,” Radiotherapy and Oncology, vol. 182, pp. S554–S555, May 2023, doi: 10.1016/S0167-8140(23)08956-9. Keywords: adaptive radiotherapy, cervical cancer,
Digital Poster 4535 Automated prostate dose accumulation workflow: comparison of planned vs delivered dose and correlation with daily transit dosimetry Joachim Marichal Faculty of Medicine and Health Sciences, University of Antwerp, Anwterp, Belgium. Department of Radiation Oncology, Iridium Network, Antwerp, Belgium Purpose/Objective: The growing interest in adaptive radiotherapy (ART), both offline and online, has emphasized the need for robust dose accumulation methods to accurately assess delivered dose distributions. Despite the availability of commercial tools, clinical implementation remains limited. This study presents an automated workflow for prostate dose accumulation and evaluates the agreement between planned and accumulated delivered doses. Additionally, correlations between dose deviations and electronic portal imaging device (EPID) transit dosimetry were investigated. Material/Methods: Eight prostate cancer patients treated with 20-fraction radiotherapy were retrospectively analyzed. Daily cone-beam CT (CBCT) images acquired on a Varian TrueBeam system were transferred to RayStation. A customized script in a research version of RayStation (v2025-ReC, RaySearch) automatically executed the following steps:Generation of a synthetic CT using the Corrected CBCT algorithm;Automatic segmentation of
the prostate, seminal vesicles, bladder, and anorectum;Creation of a deformable image registration (DIR) and propagation of target
structures;Dose computation on each CBCT and deformation of the resulting dose to the planning CT.Manual corrections were applied to contours only when large errors were identified. Daily transit dosimetry was acquired for all fractions.Differences between planned and accumulated doses were quantified for the clinical target volume (CTV) and organs at risk (OARs) using D2%, D98%, and Dmean metrics. The Spearman rank correlation between gamma pass rate (2%-2 mm) from transit dosimetry and fraction dose deviations was calculated. Results: The largest discrepancies between planned and accumulated doses occurred in the bladder and anorectum, particularly at CTV boundaries, as shown in Figure 1. Overall, accumulated doses closely matched planned distributions, except in one patient with a hip prosthesis that introduced HU inconsistencies in the synthetic CT. While individual
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