  Modulated electron radiotherapy (MERT) and combined, modulated
photon/electron radiotherapy (MPERT) have received increased research
attention, having shown capacity for reduced low dose exposure to healthy
tissue and comparable, if not improved, target coverage for a number of
treatment sites. Accurate dose calculation tools are necessary for clinical
treatment planning, and Monte Carlo (MC) is the gold standard for electron
field simulation. With many clinics replacing older accelerators, MC source
models of the new machines are needed for continued development, however,
Varian has kept internal schematics of the TrueBeam confidential and electron
phase-space sources have not been made available. TrueBeam electron fields are
not substantially different from those generated by the Clinac 21EX, so we have
modified the internal schematics of the Clinac 21EX to simulate TrueBeam
electrons. BEAMnrc/DOSXYZnrc were used to simulate 5x5 and 20x20 cm$^2$
electron fields with MLC-shaped apertures. Secondary collimating jaws were set
0.5 cm beyond the MLC periphery and to 40x40 cm$^2$. Complete accelerator
models agreed with diode measurement within 2%/2 mm at 12 and 20 MeV, and
within 3%/3 mm at 6 MeV. Comparisons of measured depth and profile data showed
dose-dependencies on jaw position; simulated energy fluences scored just above
the phantom showed that, for small apertures, dose dependencies on jaw position
are dominated by changes in the in-field energy fluence (not scattered by jaws
or MLC) at 6 MeV, while at 20 MeV dose dependencies are dominated by the
scattered component. Our models reproduce these jaw position dependencies,
which is an asset as there is no consensus on the optimal position for jaws in
modulated electron field delivery. Good agreement between simulation and
measurement and flexibility in jaw position make our models appropriate for use
in MERT and MPERT planning.
