Author: Sarah Aubert, Harald Keller, Jeff D. Winter 👨🔬
Affiliation: Princess Margaret Cancer Centre 🌍
Purpose: Support for treatment planning directly on CBCT imaging on C-arm linacs will enable adaptive radiation therapy (ART) capacity with increased treatment technique flexibility. The HyperSight imaging feature on Varian’s TrueBeam system, with advanced reconstruction algorithms, may support CBCT dose calculation. Here we performed a within-patient comparison of CBCT dose calculation accuracy and reconstruction time for the different reconstruction algorithms available on the TrueBeam HyperSight platform.
Methods: We compared four CBCT reconstruction algorithms: Standard, iCBCT, iCBCT Fast, and iCBCT Monte-Carlo-based metal artifact reduction (MAR) for head and neck (n = 7) and pelvis (n = 7) cases treated on TrueBeam v4.1 with HyperSight. We performed offline CBCT reconstruction with all four algorithms for a single CBCT and captured the reconstruction time. For the reference dose distribution, we deformed the planning CT to the daily CBCT using deformable image registration. Next, we computed the dose distributions on the deformed planning CT and offline reconstructed CBCTs using our standard CT-to-density table we validated for HyperSight CBCT. We compared dose distributions between deformed CT and CBCT images using 99th percentile absolute dose difference and the global 1% and 2% gamma pass rate.
Results: Reconstruction times averaged over all patients ranged from 19.7±3.6s for iCBCT fast to 46.1±3.1s for iCBCT MAR for offline re-reconstruction. Median 99th percentile absolute dose difference was less than 2% for all reconstruction algorithms. Median 2% gamma pass rate ranged between 99-100% for all algorithms and patients. Median 1% pass rate was lowest for the iCBCT Fast algorithm, with 79% for pelvis and 96% for head and neck.
Conclusion: We found excellent agreement between reference planning CT dose and all CBCT dose distributions using the HyperSight imaging feature on TrueBeam, with minimal variation for both pelvis and head and neck patient cohorts, except for lower agreement with iCBCT Fast Pelvis.