Author: Jochen Cammin, Shifeng Chen, Arun Gopal, Kai Huang, Jason K Molitoris, Amit Sawant, Kai Wang 👨🔬
Affiliation: University of Maryland, Department of Radiation Oncology, University of Maryland School of Medicine, Maryland University Baltimore, Department of Radiation Oncology, University of Maryland Medical Center 🌍
Purpose: The HyperSight CBCT optional feature on Varian TrueBeam linacs offers a larger field-of-view, improved Hounsfield units (HU) accuracy, and overall improved image quality, including metal artifact reduction, compared to conventional on-board CBCT, potentially enabling direct dose calculations on CBCT. In this study, we evaluate HyperSight image quality and dose calculation accuracy to assess HyperSight’s feasibility for offline adaptive replanning for pelvic RT.
Methods: The image quality and HU accuracy in phantom images obtained with pelvis HyperSight iCBCT modes were compared to the simulation-CT and conventional CBCT images from a TrueBeam without HyperSight, using standard FDK reconstruction. Additionally, clinical treatment plans, simulation-CTs and HyperSight CBCTs from five patients who received radiotherapy to the pelvis were evaluated for both HU accuracy and dosimetric fidelity. The HyperSight CBCTs were deformably registered to the simulation-CT to mitigate anatomical differences for dose calculation comparisons. Dose distributions, DVH parameters, and gamma passing rates were evaluated.
Results: The TrueBeam HyperSight CBCT has higher low-contrast resolution and lower noise level compared to conventional CBCT. The HU values of the pelvis HyperSight CBCT and the simulation CT agreed within ±20 HU for all phantom inserts except bone inserts. For patient scans, HU values agreed within ±40 HU for various tissues except cortical bone. All pelvic targets fit into a single HyperSight CBCT scan. The PTV Dmean, D95%, and D105% agreed within 1.3%, 0.7%, and 1.5%, respectively, between the clinical plans and the dose recalculated on the HyperSight CBCT. The average 1%/1mm gamma passing rate was (98.2±0.8)%.
Conclusion: The TrueBeam HyperSight pelvis CBCT provides improved image quality compared to conventional CBCT and enables accurate dose calculation without the need for density corrections. The system can improve the effectiveness and efficiency of CBCT-based dosimetry and has the potential to enable CBCT-based offline-adaptive replanning.