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S178
ESTRO 36
_______________________________________________________________________________________________
1
Université Catholique de Louvain- Institute of
Experimental & Clinical Research, Molecular Imaging-
Radiotherapy & Oncology, Brussels, Belgium
2
Centre Antoine Lacassagne, Medical Physics, Nice,
France
3
EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
4
National Physical Laboratory, Acoustics and Ionising
Radiation Division, Teddington, United Kingdom
5
Cliniques Universitaire St-Luc, Radiotherapy and
Oncology Dep., Brussels, Belgium
6
IBA Dosimetry GmbH, Schwarzenbruck, Germany
Purpose or Objective
The main application of calorimeters in standards
laboratories is as primary standard of absorbed dose to
water against which ionisation chambers (ICs) are
calibrated. At present, no calorimeter is established as a
primary standard instrument in proton beams.
Based on the absorbed dose-formalism of IAEA TRS-398,
this work describes a direct comparison between a water
calorimeter (WCal) and plane-parallel ICs in clinical pulsed
pencil beam scanning (PBS) proton beams, delivered by a
synchrocyclotron. The temporal beam characteristics and
the absence of a dosimetry protocol for such beams create
significant challenges in absorbed dose determination.
The aim of this work is to demonstrate the feasibility a
water calorimetry in pulsed PBS beams.
Material and Methods
The method consisted in comparing the response of WCal
and ICs (PPC40 and PPC05) in the same reference
conditions. Measurements have been performed at a
depth of 3.1 cm using two mono-layers maps of proton
beams (10 x 10 cm²), with incident beam energies of 96.17
MeV (range in water = 6.8 g/cm²) and 226.08 MeV (range
in water = 31.7 g/cm²), respectively. The response of the
WCal is corrected for heat transfer (calculated using
numerical simulations based on finite element method)
and non-water material inside the WCal (using
experimentally derived factors). Using hydrogen-
saturated high-purity water in the WCal, the chemical
heat defect is assumed to be zero. Classical correction
factors are applied to the response of ICs: temperature
and pressure, polarity and recombination (k
s
). k
s
was
studied in detail due to the very high beam dose rate used
with the delivery method.
Results
Table 1 shows preliminary relative differences of D
w
measured with WCal and IC, during two independent
experimental campaigns, for both energies. A small
positioning uncertainty could explain that the ratios
obtained during campaign B are higher for the low energy
beam. For campaign A, however, ratios are higher for the
high energy beam, which cannot be explained by a
positioning uncertainty. A new campaign is planned to
repeat the measurement of correction factors to improve
the statistics of the results.
Conclusion
The preliminary results are very encouraging and
demonstrate that water calorimetry is feasible in a clinical
pulsed PBS proton beam. The absolute relative differences
between D
w
derived from WCal and IC are inferior to 2%,
which is within the tolerance of the IAEA TRS-398 protocol.
Due to the depth-dose distribution, a depth inferior to 3.1
cm (e.g. 2 cm where the gradient is lower) would be more
suitable to minimise the uncertainty in positioning.
Further numerical and experimental investigations are
planned to confirm and consolidate correction factors and
determine the overall uncertainty on absorbed dose-to-
water obtained using each system. The next experimental
step is to perform the same experimental comparison for
a real clinical situation: a dose cube of 10 x 10 x 10 cm³,
created by a superposition of mono-energetic layers.
OC-0340 Validation of HU to mass density conversion
curve: Proton range measurements in animal tissues
J. Góra
1
, G. Kragl
1
, S. Vatnitsky
1
, T. Böhlen
1
, M.
Teichmeister
1
, M. Stock
1
1
EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
Purpose or Objective
Proton dose calculation in the treatment planning system
(TPS) is based on HU information taken from the CT scans
and its relation to the relative stopping powers (RSP).
However, tissue equivalent substitutes commonly used in
the process of conversion curve definition may not reflect
precisely the properties of real, human tissues. Therefore,
various animal tissues were used for validation of the CT
number to mass density (MD) conversion curves
implemented in the TPS (RayStation v5.0.2).
Material and Methods
10 animal tissue samples (pig) were used in this study
(muscle, brain, bone, blood, liver, spleen, lung, fat,
kidney and heart). Each sample was prepared and
wrapped separately. 3-4 tissues were placed in dedicated
phantoms (head and pelvis) at a time and CT scans were
taken in the clinically accepted planning protocols.
Specially designed PMMA phantoms where composed of
two parts: a) an internal box, which could fit the animal
tissues inside, b) the outer PMMA cover, designed to
simulate pelvis (see fig.1c) and head during CT scan. The
design of the phantoms not only helped to reduce imaging
artefacts but also allowed to apply a slight pressure on the
tissues in order to remove unwanted air. Subsequently,
the tissue phantom was attached to the front of the water
phantom, where with the use of 2 Bragg peak chambers,
range measurements were performed. All measurements
were performed within 24h after the animal was
slaughtered with the use of one, central, 160.3 MeV pencil
beam. For each sample, multiple irradiation positions
were chosen in a very precise matter, as it was extremely
important to choose the most homogeneous path through
which the proton beam would pass. Acquired CT data was
used to read out the HU, correlate them with the
measured RSP and validate against implemented CT
number to MD conversion curves.
Results
Figure 1, shows the comparison between measured RSP
and HU for real tissue samples and implemented
conversion curve in the TPS a), CT scan of the adult,
abdomen protocol b), and measurement set-up c). The
measured data for all soft tissues were found to be within
1% agreement with the calculated data. Only for lung
tissue the deviations were up to 3.5%. For bone, both the
difficulty in assessing the actual thickness of the part
where the beam was passing through, as well as the
inhomogeneous nature of this tissue, prevented us from
the accurate RSP assessment. However, for 2
measurements out of 3, the measured RSP where within
3.5% uncertainty.
Conclusion
The experimental validation of the conversion curve
resulted in good agreement between measured and
calculated data, therefore we can use it in the clinical set-
up with confidence. There is a number of uncertainty
sources related to these measurements, starting from HU
to RSP model, real tissue heterogeneities or uncertainties
related to acquisition of the CT data due to beam
hardening. The last one, we tried to minimize by using
especially dedicated phantom.