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S432
ESTRO 36 2017
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Mevion (Mevion Medical systems, Littleton, MA) whose use
is rapidly increasing in the United States and worldwide.
Material and Methods
Measurements were performed using a 250-MeV passively
scattered proton beam with a range of 20 cm, modulation
of 10 cm with the small aperture in place. Measurements
were done with a solid brass plates fully filling the
aperture opening to achieve a 'closed jaw configuration”.
This configuration was selected because it is the most
amount of high-Z material that can be in the beamline,
thus representing the maximum external neutrons
produced for the small field designation.
We performed measurements at isocenter and off axis at
40 and 100 cm from the isocenter with the gantry rotated
to 90
o
or 0
o
and couch rotated 0
o
or 270
o
, Figure 1. All
measurements were performed using an extended range
Bonner Sphere Spectrometer (ERBS). The ERBS had 18
spheres including the 6 standard Bonner spheres and 12
extended spheres with various combinations of copper,
tungsten, or lead. Each set of measurements was
performed with all 18 sphere combinations in air with the
6
LiI(Eu) scintillator. Data were unfolded using the MAXED
MXD_FC33 algorithm and normalized per unit proton Gy to
isocenter.
Figure 1:
Schematic diagram of measurement locations.
Results
The measured neutron spectral fluence at each of the six
measurement positions are shown in Figure 1. The average
energies, total fluence, and ambient dose equivalents per
proton Gy are listed in the table imbedded within figure
1. The average energy, total fluence, and ambient dose
equivalent were all highest at isocenter and decreased as
a function of distance from isocenter. While the energy
distributions for each of the fluence spectra (Figure 1)
were similar, with a high-energy direct neutron peak, an
evaporation peak, a thermal peak, and an intermediate
continuum between the evaporation and thermal peaks,
there were a higher fraction of direct neutrons at
isocenter compared to 40 and 100 cm from isocenter.
Figure 2:
Measured neutron fluence spectra at each of
measurement position. For each fluence spectrum, the
average energy, total fluence, and ambient dose
equivalent [H*(10)] are listed in the imbedded table.
Conclusion
In this study, we measured spectra for external neutrons
and characterized neutron dose equivalents for a single
gantry proton system, whose use in the United States and
worldwide is increasing.
Poster: Physics track: Treatment plan optimisation:
algorithms
PO-0816 LRPM for fast automated high quality
treatment planning – towards a novel workflow for
clinicians
R. Van Haveren
1
, B.J.M. Heijmen
1
, W. Ogryczak
2
, S.
Breedveld
1
1
Erasmus Medical Center Rotterdam Daniel den Hoed
Cancer Center, Radiation Oncology, Rotterdam, The
Netherlands
2
Warsaw University of Technology, Control and
Computation Engineering, Warsaw, Poland
Purpose or Objective
The aim is to create a novel efficient workflow for
clinicians, where high quality treatment plans are ready
to be inspected minutes after the delineation is finished.
In the current clinical workflow, plans are automatically
generated using the in-house developed Erasmus-iCycle
optimiser, but planning times can be in the order of hours.
Therefore, we propose an extension of Erasmus-iCycle to
substantially reduce computation times, but maintain plan
quality.
Material and Methods
We developed the Lexicographic Reference Point Method
(LRPM), a fast algorithm to automatically generate multi-
criterial treatment plans in a single optimisation run. In
contrast, the currently implemented sequential method in
Erasmus-iCycle requires multiple optimisations to
generate a plan. We validate the LRPM by comparing
automatically generated VMAT plans (mimicked by 23
static beams) with the LRPM and the sequential method
for 30 prostate cancer patients and 15 head-and-neck
cancer patients. For these treatment sites (and others),
Erasmus-iCycle is in clinical use.
Results
For the 30 prostate cancer patients, plan differences
between the LRPM and the sequential method were found
neither clinically nor statistically significant. The LRPM
reduced the average planning time from 12.4 to 1.2
minutes, a speed-up factor of 10. For head-and-neck, the
LRPM reduced the planning times from 99.7 to 4.6