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Conclusion
To more sophisticated analysis the gamma criteria should
be less than 3%/3mm or/and local dose method should be
used. The resolution of used detector is crucial and should
be high for better interpretation of results. Gamma
method presents some statistic data, for scrutiny analysis
the clinical interpretation should be assessed.
EP-1493 Machine record parameters or Epid based
data for ART QA. A comparison of two scenarios.
P. Haering
1
, C. Lang
1
, M. Splinter
1
1
DKFZ, E040, heidelberg, Germany
Purpose or Objective
Using machine record files and Epid based dosimetry is
popular for machine and patient related QA, as this may
also work for adaptive treatment approaches. The
Siemens Artiste treatment machine used here, allows a
comparison of both methods in one session. Exit images
and all relevant machine parameters are included in the
image header collected during treatment. Here we
present results of a comparison between QA dose
recalculations based on the two sources, exit images and
machine recorded parameters.
Material and Methods
A software tool was developed that allows for the
extraction of the relevant parameters (MLC-positions, MU,
etc.) from the machine records as well as from the Epid
measured exit fluencies. While machine data had to
undergo a reformat to be used for recalculation, the exit
fluencies need more attention. Here both, the delivered
fluence as well as the absorption in the
patient do play a
role. Therefore both have to be separated to receive
reliable MLC positions. The algorithm used first generates
an image containing only absorption information for the
beam using this to remove this influence on the MLC
positions. MUs were used from the parameter file, as the
fluence uncertainties on the EPID images have shown to
be to large to be used for that purpose. The extracted
parameters are then inserted in a newly generated Dicom
RT-Plan file that then can be used in the treatment
planning system (here Raystation, Raysearch) to
recalculate the dose. Dose distributions (Epid based,
parameter file based and originally planned) are then
compared.
Results
Measuring exit doses with the EPID was a simple task and
could be done for all coplanar field sets. The software tool
made it simple to extract all the needed parameter from
the files and images resulting in 2 new Dicom plan files.
Dose recalculation was done by just importing the new
plan files to Raystation. Comparing the original dose
distribution to the machine file based one showed almost
no difference at all (< 0.7%), as MU and leaf position
differences where quite small. This might also be
grounded in the used calculation grid of 2mm size. MLC
positions derived from EPID images show much larger
differences. Here detection uncertainties, EPID
positioning and the resulting image resolution of 0.3mm
do play a major role. This resulted in in noticeable
differences in the dose gradients regions. Absolute dose
differences where below 1.5%.
Conclusion
Recalculating doses based on EPID and machine based
parameters is a possible way for QA in an adaptive
treatment approach. As QA parameters are taken from
information that is given anyway or that can be easily
generated, it does not complicate the procedure of
frequent replanning. Results are as expected quite good
for the machine file approach while higher discrepancies
were found using EPID data. Main problem we face here is
that especially for the machine file based version we do
not have full independent data sources.
EP-1494 The MedAustron proton gantry: nozzle design
recommendations based on Monte Carlo simulations
H. Fuchs
1,2
, L. Grevillot
2
, A. Elia
2
, A. Carlino
2,3
, J.
Osorio
2
, V. Letellier
2
, R. Dreindl
2
, M. Stock
2
, S. Vatnitsky
2
1
Medizinische Universität Wien Medical University of
Vienna, Department of Radiation Oncology & Christian
Doppler Laboratory for Medical Radiation Research for
Radiation Oncology, Vienna, Austria
2
MedAustron lon Therapy Center, Department of Medical
Physics, Wr. Neustadt, Austria
3
University of Palermo, Department of Physics and
Chemistry, Palermo, Italy
Purpose or Objective
MedAustron is equipped with one vertical and three
horizontal fixed beam lines and one proton gantry based
on the PSI gantry 2 design for patient treatments. This
work focuses on simulations and design considerations for
the proton gantry nozzle, allowing an optimization of
beam delivery properties at isocenter.
Material and Methods
Different gantry nozzle designs were evaluated using
Gate/Geant4 Monte Carlo (MC) simulations: air filled
nozzle, helium filled nozzle, full vacuum nozzle, moving
snout, compacting of nozzle elements (vacuum window
and monitors). Design considerations were based on the
use of similar components as in the other beam lines as
well as a static beam monitoring system. Simulations were
performed for 5 representative energies (62, 96.5, 157.4,
205, and 252.3 MeV) also considering the use of range
shifter and ripple filter. Nozzle designs were based on the
MC model of the MedAustron fixed beam line nozzle, which
was verified with measurements at isocenter for 20
representative energies.
Results
The influence of the gantry nozzle filling with vacuum or
helium (as used in some commercial systems) was found
to have only a minor impact on spot size (< 2%).
Compacting all nozzle elements towards the nozzle exit
and reducing the nozzle dimension in beam direction by
25% lead to a reduction of the spot size of up to 20%,
depending on the initial energy as depicted in Fig. 1. Using
higher energies in combination with range shifter also
decreased the delivered spot size for shallow seated
tumors, as already demonstrated in other studies (Parodi,
et al. PMB 57(12), 2012).