Author: Casey E. Bojechko, Tricia Chinnery, Grace Gwe-Ya Kim, Xenia Ray 👨🔬
Affiliation: University of California San Diego 🌍
Purpose: To evaluate the use of a calibration curve generated from the CT simulator for direct dose calculation on cone beam computed tomography (CBCT) images taken with the on-board imaging system. This novel approach allows for dose calculation on CBCT scans, as opposed to synthetic CTs, during online adaptive treatments. These CBCTs can be used for offline replanning or provide an alternative for when the CT simulator is unavailable.
Methods: High quality CBCTs, enabled by the HyperSight platform, were performed on a Sun Nuclear electron density phantom, using thorax, pelvis, and head protocols (100-125 kV). The CT HU-to-mass density calibration curve was generated from a GE CT scanner. The measured CBCT Hounsfield Units (HU) were compared to the CT HUs. Plans calculated on the CTs of homogeneous and anthropomorphic phantoms, as well as patient scans, were compared to the same plans re-calculated on the respective HyperSight scans using 3D global gamma analyses with 1%/1mm, 2%/2mm and 3%/3mm criteria. A pass rate criterion of 90% was employed.
Results: Overall, differences in HU between the CT and CBCT were minimal, with noticeable differences only in the least (lung) and most dense (bone) material inserts for all scanning protocols. Of the three protocols, the pelvis protocol had the smallest residuals between the CT and CBCT HUs. For the dose comparisons using the homogeneous and anthropomorphic phantoms, the gamma pass rates were above 98.7% (1%/1mm). For the patient plans, gamma pass rates were slightly lower due to anatomical changes between the time of CT simulation and the first fraction. All gamma pass rates were above 94.6% (1%/1mm).
Conclusion: The CT-generated calibration curve is acceptable for dose calculation on HyperSight images. This forgoes the need to implement a CBCT-specific HU-to-mass density curve and relieves the task of selecting the correct calibration curve for dose calculation.