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A Monte Carlo investigation of low-Z target image quality generated in a
linear accelerator using Varian's VirtuaLinaca)
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The focus of this work was the demonstration and validation of VirtuaLinac with
clinical photonbeams and to
investigate the implementation of low-Z targets in a TrueBeam linear accelerator(Linac) using
VirtuaLinac, a cloud based web application utilizing Geant4 Monte Carlo code, was used to
treatment head components. Particles were propagated through the lower portion of the
treatment head using BEAMnrc. Dose distributions and spectral distributions were
calculated using DOSXYZnrc and BEAMdp, respectively. For validation, 6 MV flattened and
flattening filter free (FFF) photonbeams were
generated and compared to measurement for square fields, 10 and 40 cm wide and at
dmax for diagonal profiles. Two low-Z targets were investigated: a 2.35 MeV
and the proposed 2.50 MeV commercial imaging target for the TrueBeam platform. A 2.35 MeV
was also simulated in a 2100EX Clinac using BEAMnrc. Contrast simulations were
made by scoring the dose in the phosphor layer of an IDU20 aSi detector after propagating through a 4
or 20 cm thick phantom composed of water and ICRP bone.
Measured and modeled depth dose curves for 6 MV flattened and FFF beams agree within 1% for
98.3% of points at depths greater than 0.85 cm. Ninety three percent or greater of
points analyzed for the diagonal profiles had a gamma value less than one for the
criteria of 1.5 mm and 1.5%. The two low-Z target photon spectra produced in
TrueBeam are harder than that from the carbon target in the Clinac. Percent dose at depth 10 cm
is greater by 3.6% and 8.9%; the fraction of photons in the diagnostic energy range (25–150 keV) is
lower by 10% and 28%; and contrasts are lower by factors of 1.1 and 1.4 (4 cm thick phantom)
and 1.03 and 1.4 (20 cm thick phantom), for the TrueBeam 2.35 MV/carbon and commercial
VirtuaLinac is a promising new tool for Monte Carlomodeling of
novel target designs. A significant spectral difference is observed between the low-Z
target beam on
the Clinac platform and the proposed imagingbeam line on
TrueBeam, with the former providing greater diagnostic energy content.
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