AbstractID: 8241 Title: Are neutrons responsible for the dose discrepancy between Monte Carlo
calculations and measurements in a high-energy photon beam at the buildup region?
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This study investigates a new hypothesis that neutrons in a high-energy photon beams may
be responsible for reported significant dose discrepancies between the Monte Carlo
calculation and measurement at the buildup region in a large field. Borated polyethylene
slabs are inserted between the accelerator head and the phantom in order to remove neutrons
generated in the accelerator head. The thickness of the slab ranges from 2.5 cm to 10 cm.
The superheated drop neutron dosimeter is used to verify the effectiveness of the slab to
remove these neutrons. Dose measurements are performed in water using a WELLHOFER
beam scanner with an IC-10 ionization chamber. The Monte Carlo code BEAM is used to
simulate an 18 MV photon beam from a Varian Clinac-2100EX accelerator. Both
EGS4/DOSXYZ and EGSnrc/DOSRZnrc are used in the dose calculations. The results show
a good agreement (<1%) at 1 cm depth between Monte Carlo calculated and measured dose
distributions for a 40×40 cm2 field when a 10 cm thick borated polyethylene slab is inserted.
However, there are still up to 10% dose discrepancy near the water surface. It is worth
mentioning that without the slab there were over 6% of the dose discrepancy at 1 cm depth
for the same beam. The result indicates that the neutrons may be partially responsible. It
should be noted that the neutron dose measured in water for a 40×40 cm2 open field using the
superheated drop neutron dosimeter is too small to explain such a large discrepancy.