Dosimetric Impact of Direct CBCT-Based Dose Calculation Using the Ethos Hypersight Imaging System
Abstract
Purpose
The purpose of this study was to characterize the dosimetric consequences of direct CBCT dose calculation for abdominal stereotactic adaptive radiotherapy by comparing dose distributions calculated on simulation CT (simCT) versus HyperSight CBCT (HsCBCT). Emphasis was placed on quantifying direct dose discrepancies attributable to CBCT image artifacts and, HU accuracy in the largest cohort studied in literature to date.
Methods
Twenty abdominal cancer patients treated on Ethos 2.0 were evaluated, with five adaptive plans per patient calculated on HsCBCT. HUmean and Artifact Index(AI) were assessed in artifact and non-artifact regions. HsCBCT images were target-registered to simCT, adaptive plans were transferred, and doses were recalculated on simCT. Target dose–volume histogram (DVH) metrics were compared using equivalence testing(ET) (±3%, 95% confidence intervals). Dose agreement was evaluated using 3D gamma analysis (1%/1mm, 10% threshold).
Results
Artifact regions exhibited notable HUmean and AI differences with median values of 22.3(IQR:3.1-37.6) and 12.8(IQR:-32.3, 118.1), respectively. Moreover, ET demonstrated that AI is not clinically equivalent. However, PTV D98%, D50%, D2%, and D0.1cc calculated via ET displayed ±1.5% difference which indicates HU variability will not translate into clinically meaningful dose discrepancies. This is further supported by ±2.6% difference for External V33Gy and Dmean of artifact regions confirming the absence of clinically significant dosimetric differences across 200 plans. Gamma analysis yielded an average pass rate >95% , confirming dosimetric robustness despite HU variation.
Conclusion
Direct dose calculation on HyperSight CBCT demonstrated dosimetric equivalence to simCT-based calculations, with no clinically significant impact on dose distribution. These findings in a large cohort dataset indicate that HsCBCT imaging is suitable for accurate dose calculation, even in settings of high bowel gas artifact, in Ethos adaptive radiotherapy workflows.