  Purpose: To experimentally validate the Fastcat cone beam CT (CBCT) simulator
against kV and MV CBCT images acquired with a Varian Truebeam linac.
  Methods: kV and MV CBCT images of a Catphan 504 phantom were acquired using a
100 kVp beam with the on-board imager (OBI) and a 6 MV treatment beam with the
electronic portal imaging device (EPID), respectively. The kV Fastcat
simulation was performed using detailed models of the x-ray source, bowtie
filter, a high resolution voxelized virtual Catphan phantom, anti-scatter grid,
and the CsI scintillating detector. Likewise, an MV Fastcat CBCT was simulated
with detailed models for the beam energy spectrum, flattening filter, a high
resolution voxelized virtual Catphan phantom, and the GOS scintillating
detector. Experimental and simulated CBCT images of the phantom were compared
with respect to HU values, contrast to noise ratio (CNR),and dose linearity.
Detector modulation transfer function (MTF) for the two detectors were also
experimentally validated. Fastcat's dose calculations were compared to MC dose
calculations performed with Topas.
  Results: For the kV and MV simulations, respectively: Contrast agreed within
14 and 9 HUs and detector MTF agreed within 4.2% and 2.5%. Likewise, CNR had a
root mean squared error (RMSE) of 2.6% and 1.4%. Dose agreed within 2.4% and
1.6% of MC values. The kV and MV CBCT images took 71 and 72 seconds to simulate
in Fastcat with 887 and 493 projections, respectively.
  Conclusions: We present a multi energy experimental validation of a fast and
accurate CBCT simulator against a commercial linac. The simulator is open
source and all models found in this work can be downloaded from
https://github.com/jerichooconnell/fastcat.git
