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S807
ESTRO 36
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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
and 10% in relative deviations for five key energies within
the treatment area. As an example, FWHM in function of
the air gap for three key energies are reported in Figure
1. Deviations observed are presented in Figure 2.
Agreement achieved in terms of ranges in water is within
0.1 mm in absolute deviation for all the energies
considered.
Conclusion
We extended a preliminary beam model based on a first
predictions at nozzle entrance. The final beam model
describes spot sizes within clinical tolerances of 1
mm/10%, for the treatment area considered. Detailed
validation of this MC beam model is on-going and is based
on beam scattering of the core pencil beam, transverse
dose profiles in the low dose region (nuclear halo),
absolute dose in reference conditions, evaluation of the
delivery of 3D cubes (depth-dose and transverse profiles).
Special emphasis will be given to non-isocentric set-up,
including the use of range shifters
.
EP-1505 Use of Portal dosimetry to monitor treatment
consistency throughout the course of treatment
S. Deshpande
1
, A. Sutar
1
, S. Naidu
1
, M. Vikram
1
, V.
Anand
1
, R. Bajpai
1
, V. Kannan
1
1
P.D. Hinduja National Hospital, oncology, Mumbai, India
Purpose or Objective
Use of portal dosimetry software to check treatment
delivery consistency and to monitor changes in patient
anatomy during course of treatment.
Material and Methods
Varian portal dosimetry software and Electronic Portal
Imaging Device (EPID) aS1200 were used to study
consistency of treatment. Patients undergoing VMAT
treatment were enrolled in this study. Patient plan was
delivered after correcting set up error and transmitted
images were acquired by the EPID aS 1200 during the
treatment. The transmitted dose images were acquired by
EPID after the beam passes through patient. Images were
acquired in continuous mode at source to imager distance
SID = 150cm on the 1,2,3,5,10,15,20,25 fraction number.
Before measuring transmitted dose images cone beam CT
was performed to eliminate any set up error. Day one
transmitted dose images were defined as base line images.
On an average 8 images were acquired during treatment
for each patient. These images were compared with base
line image. Gamma index evaluation was performed with
1mm and 1% parameter using Varian portal dosimetry
software.
Results
For the first five images i.e. up to tenth fraction we got
average gamma index passing 98.3% which is within action
level threshold of 97%. Depending upon the site of
treatment we observed gamma passing percentage varies
during fag end of treatment
Conclusion
Dosimetric measurement during treatment is good tool to
investigate error during the treatment. Portal vision is
mostly used for patient set up and pre treatment QA of
patient. We found that portal dosimetry is useful tool for
checking consistency of treatment delivery and monitoring
changes in patient contours.
EP-1506 Temperature dependent dose readout of
Gafchromic EBT3 and EBT-XD film and clinical relevance
in SRT
K. Buchauer
1
, L. Plasswilm
1
, J. Schiefer
1
1
Kantonsspital St. Gallen, Departement of Radiation
Oncology, St Gallen, Switzerland
Purpose or Objective
Modern radiation therapy modalities regularly produce
SRT/SRS/SBRT plans with highly irregular and steep dose