Monte Carlo modelling of the kV and MV imaging systems on the Varian TrueBeam STx Linac.

In this work we developed and validated a Monte Carlo simulation framework of the imaging systems of the Varian TrueBeam STx Linac, and studied the scatter contribution in MV and kV cone-beam CT images. The simulations were validated by comparing measured PDD (Percentage Depth Dose) curves, off-axis profiles and transmission curves with those calculated with the DOSXYZnrc code and a dosimetric model of the detector response using the theoretical energy spectra. The transmission curves were obtained by imaging Al and Cu step-wedge phantoms with the MV system. A parallelized code was used to estimate cone-beam CT projections separating the scatter and primary contributions, optimizing the code using variance reduction techniques (VRTs). The models developed in this work show very good agreement with experimental measurements and with previous results reported in the literature. The use of VRTs allowed quick estimation of the scatter contributions with a speed up factor of 20. This MC framework was used to develop an iterative scatter correction method based on fast simulations of low-resolution voxel models and sparse angular sampling. The iterative scatter correction method showed very good results in both kV and MV cone-beam CT images, improving image quality by reducing the cupping artifact and improving the estimation of the CT numbers. [ABSTRACT FROM AUTHOR]