2. Monte Carlo model
In order to include the influence of secondary particle
transport, the Monte Carlo code
EGS
nrc
14
was used to model
the Intrabeam source. A user-code employing a spherical
simulation geometry was utilized which is defined by a series
of radial shells and angular cones.
共This code was originally
created for use with
EGS4
and was kindly supplied by Alex
Bielajew. It was subsequently adapted for use with
EGS
nrc.
兲
In the current investigation, only one angular cone was con-
sidered
共i.e., simulations only considered entire spherical
shells
兲.
Spectrum-scoring routines were added to the user-code to
determine x-ray spectra at each radial shell. Rayleigh scatter-
ing was included in all simulations. Photons were modeled
emanating from a point source at the center of a vacuum of
radius 1.6 mm to simulate the tip of the Intrabeam probe
共as
spectra were measured after filtration through the probe ma-
terials
兲. Modeled phantoms were extended at least 100 mm
past the last spherical shell of interest to fully incorporate
backscatter.
D. Modeled parameters
The following parameters and characteristics were mod-
eled using the above two models with the aims of validating
dosimetric measurements, and of evaluating the efficacy of
the basic primary beam model for obtaining relative dose
distributions for the Intrabeam device.
1. Spectral variation with distance in water
Using the spectra measured for the probe in air, the mod-
els were used to predict spectral variations with depth, and
subsequently changes in water/air mass–energy absorption
coefficient,
关(
¯
en
/
)
w/air
兴
z
, with distance in water (z) ap-
propriate to absolute dose calibration protocols.
15,16
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