Validation of a Monte Carlo model of a large-format kV flat-panel system

Online adaptive radiation therapy (ART) offers a paradigm shift in radiotherapy by enabling adjustments to the planned dose based on daily anatomical variation. In the context of cone-beam computed tomography (CBCT) for online ART on a standard linac, thoracic and abdominal treatment sites in particular present unique challenges due to the typically large treatment volumes, mobile anatomy, scatter-induced image quality degradation, and hounsfield unit (HU) limitations. A recent hardware and software upgrade for a standard linac, Varian TrueBeam (TB) v4.1 HyperSight, seeks to overcome these challenges through implementation of a larger kV imager panel (i.e., 43 × 43 cm), increased gantry speed (i.e., from 6 to 9°/s), and improved HU accuracy. However, investigation of the new upgrade is essential to harness the full potential of these advancements.

We report on physical characterization and a digital Monte Carlo (MC) model of the new imaging system hardware.

The open-source GEANT4 Application for Tomographic Emission (GATE) MC toolkit, which allows scintillation systems, including CBCT, to be accurately modeled, was utilized. All physical components of the new TB upgrade were modeled from vendor-provided geometry and material specifications. The model was validated using physical measurements acquired on the upgraded system. Specifically, the modulation transfer function (MTF), noise power spectrum (NPS), profiles across the physically larger detector, scatter-to-primary ratio (SPR), and loss in spatial resolution as a function of the increased gantry speed and an object's distance from isocenter. The latter was quantified using the pixel distance between the 15% and 85% intensities of the over-sampled edge spread function (ESF) for source-to-edge-phantom distances (SEPDs) of 80, 100, and 120 cm. Focal spot motion was also characterized by the MTF at SEPD of 100 cm.

The MTF₅₀ was 0.901 and 0.889 mm^-1 for the measurement and simulation, respectively, for a 125 kVp beam. The normalized root mean square error (nRMSE) was 0.013. While small, the model displayed degraded spatial resolution accuracy for other beam qualities. The general trend of the physically measured normalized noise power spectrum (nNPS) curves was reproduced with the model at all beam energies; however, a small systematic offset was observed. Excellent agreement was observed between central-image x- and y-profiles of measured and MC-generated projections, indicating correct modeling of the larger imaging detector. Simulated SPRs closely agreed with those of measurement. The measured ESF widths for the 6 and 9°/s acquisitions were both 2.5 pixels, indicating no reduction in spatial resolution in the projection space as a result of the increased acquisition speed. The effect of focal spot blurring due to increased gantry speed was accurately modeled, considering ESF width differences no larger than 0.5 pixels were observed for all SEPDs. Decreased spatial resolution in projection images was observed for SEPDs of 80 and 120 cm compared to 100 cm.

The MC model of the novel TB 4.1 HyperSight upgrade accurately reproduced physical measurements acquired on the new system. The model will be used alongside physical testing on the new TB platform, working towards an online CBCT-based ART protocol for thoracic and abdominal treatments.