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Conclusion
Brass bolus may be used for surface dose enhancement in
external beam radiotherapy with megavoltage photons.
The surface dose increased from 20 % to 57 % of dose at d-
max for a 10 cm x 10 cm 6 MV field. The non-uniform
surface dose distribution should have minimal clinical
impact for multi-fraction radiotherapy regimes where
multiple layers and the random orientation of brass links
relative to skin surface will vary with daily setup.
EP-1503 The effect of tandem-ovoid applicator on the
dose distribution in GYN brachytherapy using Ir-192
M.H. Sadeghi
1
, A. Mehdizadeh
1
, M. Tafi
1
, R. Faghihi
1
, S.
Sina
2
, A.S. Meigooni
3
, A. Shabestani Monfared
4
1
Shiraz University, nuclear engineerning department,
Shiraz, Iran Islamic Republic of
2
Shiraz University, Radiation Research Center, Shiraz,
Iran Islamic Republic of
3
Comprehensive cancer center of Nevada, Las Vegas-
Nevada, USA
4
Babol University of Medical Sciences, Babol, Iran Islamic
Republic of
Purpose or Objective
The dosimetry procedures by simple superposition
accounts only for the source shield, and does not take in
to account the attenuation of photons by the applicators.
The purpose of this investigation is estimation of the
effects the tandem ovoid applicator on the dose
distribution inside the phantom by MCNP5 Monte Carlo
simulations.
Material and Methods
In this study, the superposition method is used for
obtaining the dose distribution in the phantom for a
typical GYN brachytherapy. Then the sources are
simulated inside the tandem ovoid applicator, and the
dose at points A, B, bladder and rectum was compared
with the results of supper position. The exact dwell
positions, and times of the source, and positions of the
dosimetry points were determined from images of a
patient. The MCNP5 Monte Carlo code was used for
simulation of the phantoms, applicators, and the sources.
Results
The results of this study showed no significant differences
between the results of superposition method, and the MC
simulations for different dosimetry points. The difference
in all important dosimetry points were found to be less
than 4%. The maximum dose differences were found at the
tip of the detectors.
Conclusion
According to the results, the superposition method, adding
the dose of each source obtained by the TG-43 algorithm,
can estimate the dose to point A, B, bladder,and rectum
points with good accuracy.
EP-1504 Monte Carlo modeling of non-isocentric proton
pencil beam scanning treatments
A. Elia
1,2
, L. Grevillot
1
, A. Carlino
1,3
, T. Böhlen
1
, H.
Fuchs
1,4,5
, M. Stock
1
, D. Sarrut
2
1
EBG MedAustron GmbH, Medical Department, A-2700
Wiener Neustadt, Austria
2
CREATIS- Université de Lyon- CNRS UMR5220- Inserm
U1044- INSA-Lyon- Université Lyon 1, Centre Léon
Bérard, 69007 Lyon, France
3
University of Palermo, Department of Physics and
Chemistry, 90128 Palermo, Italy
4
Medical University of Vienna / AKH, Department of
Radiation Oncology, Vienna, Austria
5
Medical University of Vienna, Christian Doppler
Laboratory for Medical Radiation Research for Radiation
Oncology, Vienna, Austria
Purpose or Objective
Monte Carlo (MC) calculation is the gold standard to
support dose calculation analytically performed by
Treatment Planning Systems (TPS). This work is built upon
a preliminary beam model of a fixed beam line based
mainly on measurements performed at isocenter. For non-
isocentric treatments, accurate description of beam spot
size for reduced air-gaps is of paramount importance for
accurate treatment planning. This work extends the
previous beam model based on final medical
commissioning data, with special emphasis on beam optics
modeling in non-isocentric conditions.
Material and Methods
GATE 7.2 based on GEANT4 10.02, using physics-builder
QBBC_EMZ and both
range cut
and
step limiter
of 0.1 mm
were used. Mean energy and energy spread were
optimized in order to match the clinical range (R80) and
the Bragg peak width measured in water. An initial set of
beam optics parameters (beam size, divergence and
emittance) was predicted at nozzle entrance (1.3 m
upstream the isocenter) for five key energies. At this step
of the study, a symmetrical proton pencil beam was
considered. A sensitivity study in order to understand the
influence of beam optics parameters at nozzle entrance
on the spot size in air for different air gaps was performed.
The beam optics parameters were then adjusted
empirically, in order to reach 1 mm in absolute deviation
or 10% in relative deviation within a treatment area
(defined from 58 cm upstream the isocenter to the
isocenter). Eventually, optical parameters were
extrapolated for 20 clinical energies.
Results
Differences obtained between simulated spot sizes and
the measured spot sizes seem to be due to systematic
differences in the modeling of beam scattering through
the nozzle and air gap. These differences are most
probably due to combined intrinsic uncertainties from
Multiple Coulomb Scattering (MCS) algorithm and nozzle
geometry implemented in the simulation. The achieved
agreement between measured and simulated spot FWHM
is within clinical tolerances of 1 mm in absolute deviation