Stability of CT Numbers for Advanced Cone-Beam Computed Tomography Imaging – across Scanners and Time
Abstract
Purpose
To evaluate the CT number stability of advanced cone-beam CT (CBCT) detectors installed on C-arm linacs, across detectors and time, and assess the CBCT-based dose calculation accuracy.
Methods
A Gammex Advanced Electron Density phantom (Sun Nuclear) was scanned at installation of HyperSight CBCT detectors at seven TrueBeam C-arm linacs (Varian), as well as after three and six months. Three CBCT protocols were evaluated: The head protocol with 100 kVp and iterative reconstruction (iCBCT), the thorax protocol with 125 kVp and Feldkamp-Davis-Kress filtered back-projection (FDK), and the pelvis protocol with 125 kVp and iCBCT. The phantom inserts were scanned individually placed in the center of the phantom. The mean CT number was extracted, and the CT number stability was assessed across detectors and timepoints. CT conversion curves were generated, following a consensus guide. The spread in curves due to CT number variation over detectors was assessed. Treatment plans created on planning CT scans (pCT) of ten head-and-neck, twenty-two lung, and fifteen pelvis cancer patients were recalculated on CBCT scans, selected to match the anatomy seen on the pCT. Dose-volume parameters for targets and organs-at-risk were compared between pCT and CBCT.
Results
The CT numbers were consistent over detectors and time, and a single conversion curve could be used per protocol. For iCBCT, median dose differences between pCT and CBCT were within 0.5% for head-and-neck and pelvis and 1.5% for lung, while FDK lung was within 2.5%. The largest deviations with iCBCT were -1.3% for head-and-neck, -1.1% for pelvis, and -3.8% for thorax, but -18.0% for thorax FDK.
Conclusion
The CT number stability was sufficient to allow for a single conversion curve per CBCT protocol to be applied across all the CBCT scanners. A high dose calculation accuracy was found for iCBCT, while large deviations were seen for thorax FDK.