Abstract
Introduction:
Monte Carlo simulations are the gold standard for radiation dosimetry. However, developing accurate models for kilovoltage cone-beam computed tomography (kV-CBCT) systems can be challenging because accurate spectral and geometry specifications from vendors are not always available. This study presents equivalent spectral and filtration models that may be used for Monte Carlo modeling of the kV-CBCT beamline on-board the Halcyon 2.0 linear accelerator.
Material and methods:
Equivalent energy spectra for the 100, 125, and 140 kV beams were determined by matching beam half-value layers and air kerma outputs calculated using SPEKTR 3.0 software with measurements. The bowtie filter profile was reconstructed from transmission measurements, enabling the generation of a 3D filter model. The model was incorporated into Geant4/GATE Monte Carlo simulations for the purpose of computing depth and off-axis dose characteristics. Calculated percentage depth doses (PDDs) and off-axis profiles were benchmarked against those measured in a water phantom.
Results:
The maximum differences between measured and computed PDDs were 3.47% (100 kV), 3.65% (125 kV), and 3.27% (140 kV). For off-axis profiles, maximum differences were 3.73% (100 kV), 6.84% (125 kV), and 4.44% (140 kV) within the central beam. Larger discrepancies up to 22% occurred in high-gradient penumbral regions due to mismatches in spatial resolution between detectors and Monte Carlo scoring geometry.
Conclusions:
This study presents the first Monte Carlo model of the Halcyon 2.0 kV-CBCT system using measurement-derived equivalent models. The agreement between computed and experimental dose characteristics validates the accuracy of the model for imaging dose calculations until vendor supplied data is made accessible for geometry and spectral specifications.