The BEAMnrc
code was used for the simulation of the Theratron Equinox-80 telecobalt machine.
The phase space of radiation beam was generated at treatment distance of 80 cm for
various field sizes. The phase spaces in air were analyzed by BEAMdp data processing
program. The electron energy fluence with respect to photon energy was 0.09% and
0.34% for field size of 05 × 05 and 35 × 35 cm2 respectively
and it was maximum at the central axis which gradually decreases beyond this. The
profiles for photon fluence were in symmetry for all the fields. The full width
at half maximum of profiles in photon energy fluence shows good agreement with the
field size. The photon energy fluence was flat till the field size of 27 cm2,
after which it decreases gradually till the edge in larger field sizes. The air-kerma
output factor from the simulation was in good agreement with measured value. We
analyzed the dose data scored in the voxels in a large water phantom by
simulation using dosxyznrc code. The percentage depth dose for all field sizes was
in good agreement with the BJR supplement 25 and the data supplied by the manufacturer
of machine. Significant deviation of about 20% in isodose line near the edge of
the profile was observed for 35 × 35 cm2 field size. The
penumbra widths of all field sizes were comparable except for 35 × 35 cm2,
which has a penumbra width of 4.1 cm at 10 cm depth. The significant under dose
near the edge as compared to central axis for larger field sizes may be the indication
for its careful use in treatment.
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