Truebeam Hypersight Hounsfield Unit Accuracy for Dose Calculation
Abstract
Purpose
To evaluate the dosimetric accuracy of plans calculated on the Varian TrueBeam HyperSight cone beam CT (CTHS) imager upgrade compared to current fan beam simulation CT (CTSim) solutions towards online adaptive radiotherapy.
Methods
The HyperSight imager was calibrated for HU to electron density conversion using a Gammex 167 plug phantom with known electron densities. A Rando thoracic phantom was scanned at 120 kVp CTSim, 125 and 140 kVp CTHS. Retrospective patient images acquired on CTSim and CTHS were gathered for evaluation. In thoracic cases without a discernible tumor in the lung, a virtual 2.5cm lung tumor was added prior to planning. Treatment plans were generated on sim volumes according to clinical lung planning aims (Chestwall Dmax< 120%Rx, Heart Dmax<30Gy, Great Vessels Dmax<54 Gy). The CTHS was deformed to match the anatomy of the CTSim, resulting in a deformed CBCT (dCT). Beam parameters from the CTSim were copied to the dCT, and patient contours were rigidly copied to the dCT from CTSim. Dose maps were calculated on both the dCT and CTSim image volumes for the purpose of evaluating CTHS HU performance compared to CTSim. The dose volumes were then compared with a global 10% threshold 1%/1mm gamma criterion.
Results
Phantom generated plans showed a gamma pass rate of 99.74% and 98.60% for 140 and 125 kVp HyperSight potentials respectively with 1%/1mm, 10% global threshold, compared against the CTSim dose volume as ground truth. Patients imaged at 125 kVp CTHS pass rate ranged from 97.99 to 98.21%, with an average of 98.09%, with the same gamma criteria.
Conclusion
The pass rate being greater than 98% in all cases indicates that the HU accuracy of plans generated on the HyperSight imager is comparable CTSim for accurate dose calculation, which could avail online adaptive radiotherapy workflows involving direct dosimetry on daily images.