S1846
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
system on the Varian Edge linear accelerator represents a significant advancement in image-guided radiotherapy (IGRT). The purpose of this study is to compare the Hounsfield Unit (HU) accuracy between the HyperSight CBCT and conventional CT images and to evaluate the dosimetric impact of using HyperSight CBCT images for patient dose calculation, in comparison with CT-based planning. Material/Methods: An HU-to-electron density calibration curve was generated using the CIRS Electron Density Phantom for both HyperSight CBCT and CT acquisitions. Planning CT images were acquired at 120 kV, while CBCT images were obtained at 125 kV using both Standard and iCBCT modes. Regions of interest (ROIs) corresponding to each tissue-equivalent insert were delineated, and mean HU values were extracted from both datasets. The correlation between CT and CBCT HU values was analysed to evaluate image calibration performance. Additionally, patient-specific dose calculations were performed for different anatomical sites, including pelvis, brain, and thorax cases, using HyperSight CBCT. The CBCT-based dose distributions were compared with those obtained from planning CT through gamma analysis. Dose variations to organs of interest were also assessed to evaluate the clinical feasibility and accuracy of CBCT-based dose calculation. Results: The comparison of HU values between CT and HyperSight CBCT showed a better correlation for the iCBCT mode than for the Standard mode. For iCBCT acquisitions, HU differences were within 50 HU for near-equivalent materials, while for Standard acquisitions reached up to 100 HU for near-equivalent materials and approximately 200 HU for dense bone inserts. A saturation effect in HU values was observed for both HyperSight acquisition modes when evaluating materials with electron densities higher than titanium. Dosimetric comparisons between CT- and CBCT-based plans demonstrated good agreement. The mean gamma passing rate (1%/1 mm, 10% threshold) was approximately 90% for iCBCT and 85% for Standard mode. The dose to organs of interest showed smaller variations between CT and iCBCT mode (< 3%) compared to Standard mode ( ≈ 9%), supporting the improved image accuracy and clinical feasibility of the HyperSight iCBCT mode for dose verification purposes.
(criteria met in 11/20 vs 1/20, p<0.0001). After normalization adjustments, all AP plans except one (minor sigmoid deviation) were considered clinically acceptable. The mean normalization adjustment was 2.9±1.9%, balancing CTV coverage and rectal sparing. Suboptimal bladder or rectum preparation was noted in three cases but did not compromise plan quality. Patient-specific QA confirmed excellent deliverability, with gamma pass rates above 99% (3%/2mm).
Conclusion: The proposed AP pipeline enables fast, consistent, and clinically acceptable VMAT plan generation for ultra- hypofractionated prostate radiotherapy across different TPS and linac models. Integration of MRI- based urethral delineation is expected to further enhance OAR sparing. These results support clinical implementation and further expansion to additional platforms such as Halcyon machine. References: [1] Lapierre, Ariane et al. “Highly hypofractionated schedules for localized prostate cancer: Recommendations of the GETUG radiation oncology group.” Critical reviews in oncology/hematology vol. 173 (2022): 103661. doi:10.1016/j.critrevonc.2022.103661 Keywords: auto-planning, prostate cancer
Digital Poster 1581 Evaluation of the dosimetry accuracy of
HyperSight CBCT on Varian Edge linear accelerator Dalila AR Mateus 1,2 , João Dias 1 , Joao Costa 1 , Nicolle Gomes 1 , Ana Maximiano 1 , Carlo Greco 1 1 Radiotherapy, Champalimaud Foundation, Lisbon, Portugal. 2 Radiotherapy, Mercurius Health, Lisbon, Portugal
Purpose/Objective: The recent implementation of the HyperSight CBCT
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