843 / 1082
S827
ESTRO 36 2017
_______________________________________________________________________________________________
experienced planners who took less time to reach an
“optimal” solution.
Conclusion
There is a relatively
consistent approach to H&N VMAT planning within our
institution. This study has highlighted where differences
between optimisation objectives and priorities and the use
of dummy structures can lead to subtle variations in dose
distributions which may not be detected in quantitative
analysis but may affect acceptability. More experienced
planners
familiar
with
clinician
expectations
demonstrated improved judgement when determining an
optimal plan. Shared learning has enabled a more
consistent approach to H&N plan optimisation and an
improved understanding of what is achievable and
clinically acceptable. This has benefitted both planners
and clinicians. In addition, the creation of plan
optimisation templates, based on the findings of this
study, are aimed at a consistent optimisation resulting in
improvements in the patient pathway.
EP-1556 Dosimetric commissioning of a TPS for a
synchrotron-based proton PBS delivery system
G. Kragl
1
, T. Böhlen
1
, A. Carlino
1,2
, L. Grevillot
1
, H.
Palmans
3
, A. Elia
1
, B. Knäusl
1
, J. Osorio
1
, R. Dreindl
1
, J.
Hopfgartner
1
, S. Vatnitsky
1
, M. Stock
1
1
EBG MedAustron GmbH, Medical Department, Wiener
Neustadt, Austria
2
University of Palermo, Department of Physics and
Chemistry, Palermo, Italy
3
National Physics Laboratory, Radiation Dosimetry,
Teddington, United Kingdom
Purpose or Objective
To provide an overview regarding dosimetric
commissioning of the TPS RayStation for proton PBS
delivery installed at a synchrotron-based dual particle
facility. 1D/2D commissioning consisted of benchmarking
the dose calculation algorithm against measured IDDs, on-
axis lateral spot profiles in air and field size factors as well
as comparisons of absolute dosimetry. 3D commissioning
consisted of absolute dose comparisons in the SOBP of
cubic targets in water as well as the characterization of
3D dose distributions with increasing complexity. A robotic
patient positioning system was used rather than
extendable snouts to reduce the air gap between patient
and nozzle. Therefore, special attention was paid to non-
isocentric setups.
Material and Methods
Commissioning was performed for the PB algorithm
(version 3.5) integrated in RayStation (version 5.0.2). IDDs
were acquired with a Bragg peak chamber (PTW) and
corrected for insufficient detector size by means of MC
simulations (GATE/GEANT4). Spot profiles in air were
acquired with a scintillating screen (Lynx, IBA) at 7 air
gaps. Absolute dosimetry was performed with a Roos
chamber (PTW) in 12 x 12 cm
2
fields (2 mm lateral spot
spacing) for 20 energies. Field size factors were acquired
with a semiflex ionization chamber (PTW) at 3 depths in
water for field sizes ranging from 2 x 2 to 20 x 20 cm
2
. 3D
dose distributions were characterized using 24-PinPoint
chamber arrays (PTW).
Results
Calculated ranges agreed within 0.2 mm with measured
ranges. The integrals of measured and calculated IDDs
agreed within 0.5% for clinically relevant ranges. At
isocenter, calculated and measured spot sizes (FWHM)
differed on average less than 0.4 mm in x- and y-
directions. For non-isocentric setups differences were
within 0.5 mm. Field size factors always agreed within 4%;
deviations were generally low (<1%) and increased only at
small field sizes and the highest energies (range >30 cm).
For isocentric arrangements, absolute dose agreed within
2.2% in the center of SOBPs of cubic targets with different
sizes and at different depths. As expected, the deviations
increased for plans with range shifter for non-isocentric
arrangements. Variations of up to 3.5% were obtained for
modulation widths of 6 cm. Results for more complex
geometries are currently under investigation.
Conclusion
Clinically acceptable results were obtained for open
beams. For plans with range shifter, a scaling of dose
distributions might be considered until the upcoming MC
dose calculation algorithm is available. Minimizing the air
gap to reduce modelling inaccuracies with respect to
scattered protons in air is beneficial for these cases and
realized by non-isocentric treatments.
EP-1557 Minimum prescription concept for dose
painting increases robustness towards geometrical
uncertainty
S. Korreman
1
1
Aarhus University Hospital, Department of Oncology,
Aarhus C, Denmark
Purpose or Objective
Dose painting radiotherapy with heterogeneous dose
escalation is vulnerable to geometrical errors, which
potentially deteriorate the benefits of dose escalation
substantially. This study investigates use of a minimum
prescription concept to increase plan robustness towards
geometrical uncertainties.
Material and Methods
Dose escalation was prescribed based on PET Cu-ATSM
tracer uptake for a head and neck cancer patient, with a
high degree of heterogeneity in the uptake. The minimum
dose was 60Gy, and dose escalation was prescribed based
on a linear correspondence model to the tracer uptake,
with a maximum escalation up to ~90Gy. Dose painting
plans were optimized using the Eclipse treatment planning
system, using modulated arc therapy technique in a
contour-based dose escalation scheme (5 levels). Two
planning strategies were tested: (1) Minimum and
maximum dose constraints imposed on all subvolumes
(exact-map), and (2) minimum constraints on all
subvolumes with only one overall maximum constraint
(minimum-map). Geometrical error was simulated by
displacing the isocenter with up to 2 mm. Quality index
metrics were compared for the two planning strategies.
Results
For both strategies, optimizations could be performed
with good adherence to dose constraints. For the exact-
map technique, the fraction of voxels with quality index
within plus/minus 5% of prescription dose was ~79%, and
the fraction of voxels above 95% of prescription dose was
~93%. For the minimum-map technique, the fraction of
voxels above 95% of prescription dose was ~97%. With
displacement of 2 mm, the >95% fraction changed to ~85%
for exact-map, and ~95% for minimum-map technique.
Conclusion
Using a minimum dose concept for dose painting with only
an overall maximum constraint gives more robust plans
than a voxel-by-voxel exact dose prescription, while
maintaining maximum dose constraints. Highest
adherence to dose painting degree of heterogeneity could
be obtained with the minimum-map approach.
EP-1558 Dosimetric evaluation of incidental radiation
of internal mammary chain in breast cancer with 3D RT
A.C. Ciafre
1
, J. Castilla
1
, C. Domingo
1
, E. Jorda
1
, D.
Dualde
1
, E. Ferrer
1
1
Hospital Clinico Universitario, Oncologia Radioterapica,
Valencia, Spain
Purpose or Objective
The internal mammary chain represents a less common yet
important rout of lymphatic drainage of breast cancer,
constituting a matter of debate in the current clinical
practice. Poortmans P. 2015 suggests an improvement of
disease-free survival, distant disease-free survival and a
reduction of breast cancer mortality when they included
the internal mammary and medial supraclavicular lymph-