Creative Imaging Solutions for HDR Brachytherapy Planning Using Linac-Integrated Hypersight CBCT
Abstract
Purpose
Mobile CT and CBCT systems improve flexibility and reduce patient transport burden in HDR gynecologic brachytherapy. However, competition for imaging resources remains a practical limitation. This work evaluates the Varian Ethos linear accelerator equipped with HyperSight CBCT for applicator implantation verification and treatment planning without reliance on a dedicated CT simulator.
Methods
Two applicator configurations were studied in a gynecologic phantom with a silicone uterus and vaginal canal embedded in synthetic medical gel with added layers of folded bolus: tandem and ovoids, and tandem and ovoids with two interstitial titanium needles with coded marker strands. The phantom was imaged using a dedicated brachytherapy CT scanner (Brainlab Airo CT) with 1 mm slice thickness and the Ethos system (6-second CBCT with 2 mm slice thickness). 30-cm x 30-cm field of view was used for both systems. The applicators were digitized on both image-sets using Varian BrachyVision. Image quality was compared using Catphan scans.
Results
HyperSight CBCT provided clear visualization of intracavitary applicators and interstitial needles for accurate reconstruction. While the applicators were more sharply defined in the axial view, inserted coded marker strands were harder to distinguish in the sagittal and coronal views for HyperSight due to metal image artifacts. Catphan analysis showed that Hypersight scans had a high contrast-to-noise ratio (8.17) and a superior contrast detail curve compared to the Airo CT, suggesting an advantage in detecting low contrast targets. Hypersight spatial resolution (4.26 lp/cm), HU constancy, geometric distortion, noise and uniformity were found to be comparable to Airo CT.
Conclusion
This work demonstrates, for the first time, the feasibility of using Ethos HyperSight CBCT for HDR gynecologic brachytherapy planning with hybrid applicators. HyperSight CBCT offers an efficient imaging option that may significantly improve workflow flexibility at centers with limited CT availability, while maintaining clinically acceptable image quality.