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lines. Negative MLC errors were not performed at
Institution 2, due to differing equipment.
The automated VMAT plans from institution 3 were similar
in pass rate to the manually planned VMAT for collimator
errors, despite the difference (higher magnitude for
manual VMAT plans) in error magnitude. This could be
caused by the higher MLC modulation in the automated
plans.
Conclusion
Not all deliberately introduced errors were discovered for
VMAT plans using a typical 3%/3mm global gamma pass
rate (for 10% threshold with correction off). Consistency
between institutions was low for plans assessed utilising
differing devices and software. A 2%/2mm global analysis
was most sensitive to errors.
PO-0809 A 3D polymer gel dosimeter coupled to a
patient-specific anthropomorphic phantom for proton
therapy
M. Hillbrand
1
, G. Landry
2
, G. Dedes
2
, E.P. Pappas
3
, G.
Kalaitzakis
4
, C. Kurz
2
, F. Dörringer
2
, K. Kaiser
2
, M. Würl
2
,
F. Englbrecht
2
, O. Dietrich
5
, D. Makris
3
, E. Pappas
6
, K.
Parodi
2
1
Rinecker Proton Therapy Center, Medical Physics,
Munich, Germany
2
Ludwig-Maximilians-Universität München, Department
of Medical Physics, Munich, Germany
3
National and Kapodistrian University of Athens, Medical
Physics Laboratory- Medical School, Athens, Greece
4
University of Crete, Department of Medical Physics,
Heraklion, Greece
5
Ludwig-Maximilians-Universität München, Department
of Radiology, Munich, Germany
6
Technological Educational Institute, Radiology &
Radiotherapy Department, Athens, Greece
Purpose or Objective
The high conformity of proton therapy (PT) dose
distributions, attributed to protons stopping in the target,
is also the main source of uncertainty of the modality. PT
is sensitive to errors in relative stopping power to water
(RSP) uncertainties and to density changes caused by
organ motion. The ability to verify PT dose distributions in
3D with a high resolution is therefore a key component of
a safe and effective PT program. Existing 2D dosimetric
methods suffer from shortcomings attributed to LET
dependence, positioning uncertainties, limited spatial
resolution and their intrinsic 2D nature. Recent advances
in polymer gel dosimetry coupled to 3D printing
technology have enabled the production of high
resolution, patient specific dosimetry phantoms. So far
this approach has not been tested for PT.
Material and Methods
A 3D-printed hollow head phantom derived from real CT
data was filled with VIPAR6 polymer gel and CT scanned
for pencil beam scanning (PBS) treatment planning,
following RSP characterization of the gel and the 3D
printer bone mimicking material (see Figure 1). All
irradiations of phantoms were carried out at the Rinecker
Proton Therapy Center in Munich, which is dedicated for
PBS. An anterior oblique SFUD plan was used to cover a
centrally located cerebral PTV, following the standard
operating procedures of the PT facility. The field was
crossing the paranasal sinuses (see Figure 2A) to test the
TPS modelling of heterogeneities. 3D maps of the T2
relaxation time were obtained from subsequent MR
scanning of the phantom and were converted to relative
dose. The dose response linearity and proton range were
verified using separate mono-energetic irradiations of
cubic phantoms filled with gel from the same batch.
Relative dose distributions were compared to the TPS
predictions using gamma analysis.
Figure 1.
3D printed patient-specific head phantom filled
with dosimetric gel during the treatment planning
process.
Results
Results from mono-energetic irradiation of the cubic
phantoms showed proton range agreement to the TPS
within 1 mm for 90 MeV and 115 MeV, supporting the SPR
gel characterization accuracy. Dose-response linearity was
confirmed for the delivered dose range, except at the
Bragg peak position where a LET dependence was
revealed. Gamma index and relative dose distribution
profiles showed good agreement between TPS and gel, as
shown in in Figure 2.
Figure 2.
(A) Slice of the 3D SFUD dose distribution
converted from a T2 relaxation map obtained from MR
scanning
the irradiated 3D printed head phantom filled with
polymer gel. The PTV is indicated in white. (B) Gel
(RTsafe) and
TPS
dose profiles along the path marked in red in (A). (C)
3%/2mm gamma index along the profile.
Conclusion
In this work we have shown that patient-specific 3D
polymer gel dosimetry is applicable to PT using PBS.
Further characterization and correction of the LET
dependence and comparison to MC dose calculations will
be carried out and presented.
Acknowledgements:
DFG-MAP
PO-0810 Absolute dose pre-treatment Portal Dosimetry
using the Varian MAASTRO implementation
A. Taborda
1
, J. Stroom
1
, C. Baltes
2
, A. Seabra
1
, K.
Dikaiou
2
, C. Greco
1
1
Champalimaud Centre for the Unknown, Clinical
Department, Lisboa, Portugal
2
Varian Medical Systems, Varian Medical Systems Imaging
Laboratory, Baden-Dättwil, Switzerland