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Abstract
The effects of the physical parameters of an electron beam from a Siemens PRIMUS clinical
linear accelerator (linac) on the dose distribution in water were investigated by
Monte Carlo simulation. The EGS4 user code, OMEGA/BEAM, was used in this study. Various
incident electron beams, for example, with different energies, spot sizes and distances
from the point source, were simulated using the detailed linac head structure in the
6 MV photon mode. Approximately 10 million particles were collected in the scored
plane, which was set under the reticle to form the so-called phase space file. The
phase space file served as a source for simulating the dose distribution in water
using DOSXYZ. Dose profiles at Dmax (1.5 cm) and PDD curves were calculated following
simulating about 1 billion histories for dose profiles and 500 million histories for
percent depth dose (PDD) curves in a 30 x 30 x 30 cm3 water phantom. The simulation
results were compared with the data measured by a CEA film and an ion chamber. The
results show that the dose profiles are influenced by the energy and the spot size,
while PDD curves are primarily influenced by the energy of the incident beam. The
effect of the distance from the point source on the dose profile is not significant
and is recommended to be set at infinity. We also recommend adjusting the beam energy
by using PDD curves and, then, adjusting the spot size by using the dose profile to
maintain the consistency of the Monte Carlo results and measured data.