Impact of Motion on Dose Calculation Differences between Halcyon and Truebeam Hypersight Imaging Systems
Abstract
Purpose
To compare HyperSight imaging systems between O-ring (Halcyon) and C-arm (TrueBeam) machine, with specific emphasis on the impact of motion in resulting dose calculation.
Methods
Scans of the CIRS Lung Phantom were taken at two different locations with Halcyon and TrueBeam machines. Variations in motion settings were implemented, specifically time cycles (4s and 8s) and lengths of oscillations (5mm and 20mm), and an additional still condition. Before planning, the Halcyon and TrueBeam datasets were registered using the still images. The contours were drawn based on the known size of the target and the known amount of motion. Then, the treatment plan was created and optimized on the Halcyon dataset in Eclipse. The same plan was later copied onto the TrueBeam dataset and re-calculated. This process allowed the comparison between the dose differences caused only by imaging datasets. Finally, dose distributions were evaluated using 3D gamma analysis and DVHs calculated for the internal target volume (ITV) in Eclipse.
Results
3D gamma analysis and DVH graphs presented differences, even in the still condition. Once motion was introduced, these differences amplified. The 3D gamma pass rates decreased systematically with increasing amplitude (5mm to 20mm) and with longer cycle times (4s to 8s), and the DVH deviations followed the same trend. These differences in dose seen in the still and motion conditions indicate that the CT number differences between reconstruction protocols are the primary source of variation.
Conclusion
This study demonstrates that dose calculation differences persist between Halcyon and TrueBeam HyperSight datasets even in the absence of motion, primarily due to differences in CT number reconstruction. These differences were non-negligible and became more pronounced under stricter evaluation criteria and increased motion conditions, indicating that care must be taken when using advanced CBCT imaging for dose calculation purposes.