AbstractID: 7046 Title: A Comparison of Monte Carlo simulated electron percentage depth
doses to those measured with four ionization chambers
Monte Carlo (MC) simulated electron percentage depth dose (PDD) curves were
compared to those measured with 4 different ionisation chambers (PTW/Markus,
Attix-449, Wellhofer/IC10 and Capintec/PR-06). The nominal electron beam
energy ranged from 6 MeV to 20 MeV. Measurements were performed in a water
phantom and in the RMI 457 solid water phantom. MC simulations were performed
using the EGSnrc MC code using the full phase space files produced from the
EGS4/BEAM simulation of the head of a medical linear accelerator. PDD
measured by all the chambers except the Attix ionisation chamber agree to within 1
mm at most depths and for all the electron energies. The PDD measured by the
Attix chamber showed an under-response for the 6 MeV beam (< 2.0% compared to
the other ionization chambers) at depths less than dmax and an over response (up to
6% higher than that given by the Capintec/PR-06 ionisation chamber) at depths
greater than dmax. Water PDD generated by MC simulations with a depth bin
resolution of 2 mm matches the Attix ionisation chamber measurements within 1
mm. The agreement between MC simulations and experimental measurements of
PDD depends on the scoring bin thickness (depth resolution) used with the MC
calculations. The MC simulated PDDs can be made to match the measured PDDs
by simply changing the depth resolution. The dependence of the MC calculated
PDD on depth resolution is mainly caused by the effect of averaging dose over the
scoring thickness and the dependence of the stopper-to-crosser ratio on depth in a
water phantom.
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