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were delineated on the planning CT and the contours and
CT data was duplicated. On the first series the original CT
the HU were kept as is, and on the second CT set the HU
on the gas pockets were overridden with HU zero. Thus,
we simulate a worst-case scenario where the gas pocket is
present on the planning CT but disappear during the
course of treatment. The original treatment plan
optimized using the type C algorithm was recalculated on
both CT data sets. The volume and maximum diameter of
these gas pockets were measured and the difference in
average and maximum dose in these pockets were
calculated.
Results
The average volume of the pockets was 28.6 cm
3
[range:
1.7-77.7 cm
3
] and the average maximum extent of the gas
pocket was 4.2 cm [range: 2.8–5.3 cm]. Volumes up to 20
cm
3
show a decrease about 1 % in the average and
maximum dose to the delineated air pocket for type C and
an increase of less than 1% for type B algorithm. For
volumes larger than 20 cm
3
the average and max dose to
the delineated air pocket increases with more than 5% and
up to 30%, respectively for the type C algorithm. The type
B algorithm shows a decrease up to 2% in average dose and
a small increase in maximum dose.
Conclusion
Gas pockets above a volume of 20 cm
3
in the initial CT scan
can induce dose hotspots during the actual treatment,
when using the type C dose calculation algorithm for
treatment planning and when deriving dose to medium for
VMAT plans. This could increase risk for radiation toxicity
in the worst case scenario. This effect is smaller, opposite
and probably clinically negligible for the type B algorithm.
EP-1463 MONET: an accurate model for the
evaluation of the ion dose in water
A. Embriaco
1,2
, E.V. Bellinzona
1,2,3
, A. Fontana
2
, R.
Alberto
1,2
1
Pavia University, Physics, Pavia, Italy
2
Istituto Nazionale di Fisica Nucleare INFN, Physics
Departiment, Pavia, Italy
3
Ludwig Maxmilians University, Medical Physics, Munich,
Germany
Purpose or Objective
MONET (Model of ioN dosE for Therapy) is a code for the
computation of the 3D dose distribution for protons in
water. MONET accounts for all the physical interactions
and is based on well known theories.
Material and Methods
The first part is the evaluation of the lateral profile.
Our model is based on the Molière theory of multiple
Coulomb scattering. To take into account also the nuclear
interactions, we add to Molière distribution the Cauchy-
Lorentz function, where two free parameters are obtained
by a fit to a FLUKA simulation (Bellinzona et al., PMB
2016).
The next step is the passage from the projected lateral
distribution to a 2D distribution. The projected
distributions are uncorrelated but not independent and we
have to use the Papoulis theorem that allows, in case of
cylindrical symmetry, to rebuild the radial distribution
starting from projected one. We have implemented the
Papoulis algorithm in the code.
The second part is the study of the energy deposition in
the longitudinal profile.
We have implemented a new calculation of the average
energy loss that is in agreement with simulations and other
formulas published in the literature.
The inclusion of the straggling is based on the convolution
of energy loss with a Gaussian function.
In order to complete the longitudinal dose profile, also the
nuclear contributions are included in the calculation using
a linear parametrization with only two free parameters for
energy.
The total dose profile is calculated in a 3D mesh by
evaluating at each depth the 2D lateral distributions and
by scaling them at the value of the energy deposition.
Results
We have compared MONET results with the FLUKA
simulations and we have obtained a good agreement for
different energy of protons in water.
We have reproduced a lateral scan as a sum of many pencil
beams in order to estimate the accuracy of the model
focusing on the tails of the distribution that give rise to
the low-dose envelope. Also in this case, the agreement
between MONET and FLUKA is good.
We have also estimated the calculation time: for each
depth, it is about 2 seconds for the single beam and 4
seconds for the beam scan.
Conclusion
The advantages of MONET are the physical foundation, the
fast calculation time and the accuracy.
A possible development of this study is the creation of a
dose database of clinical interest and an online fast dose
evaluation tool. In the next future, we would like to
extend MONET to the case of Helium beam and other ions.
Preliminary results for helium ion will be shown.
EP-1464 Investigation on beam width tolerances for
proton pencil beam scanning
B. Ackermann
1
, S. Brons
1
, M. Ellerbrock
1
, O. Jäkel
1,2
1
Heidelberg Ion Beam Therapy Center HIT, Medical
Physics, Heidelberg, Germany
2
German Cancer Research Center, Medical Physics in
Radiation Oncology, Heidelberg, Germany
Purpose or Objective
Beside beam spot position and proton range, the beam
spot width is one of the central parameters for the correct
application of a proton therapy plan utilizing pencil beam
scanning techniques. The aim of this work is to investigate
the influence of variations of the nominal beam width on
the dose distribution of cubic dose volumes, which are
often part of a typical QA program.
Material and Methods
For QA purposes, three cubic dose volumes with a spread
out bragg peak (SOBP) of 3x3x3 cm³ are optimized with
the treatment planning system (TPS), syngo PT Planning
(Siemens, Germany). The nominal dose in the SOBP is 0.5
Gy. The depth in water of the centre of the cubes is 50,
125 and 200 mm, respectively.
To perform dose calculations on a water phantom with
variation of initial beam width, the needed algorithms and
base data of our TPS are transfered to MATLAB (The
Mathworks Inc., USA). These are mainly the depth dose
distributions and the double gaussian parametrization of
the beam width. Plans are recalculated with MATLAB with
nominal and varied beam width. Dose distributions are
analysed performing a 3D gamma index analysis with
criterias of 1mm distance to agreement and 5% dose
deviation, normalized to global maximum. The maximum
negative and positive tolerable beam width deviations are
determined where all points still pass the gamma index
acceptance criteria (e.g. gamma index < 1).
Results
The maximum tolerable beam width variations for the
dose cube in a depth of 50 mm amounts to -7% and +10%,
while for the dose cube in 125 mm depth the found values
are -12% and +17%. The most interesting result is found for
the dose cube in 200 mm depth. While the high dose region