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cylinder applicator (Figure 1b) with 5 catheters. Inter-
dwell distances of 2 and 5 mm were employed and the
experiments performed for source activities between 5
and 10 Ci. The EPID response is proportional to the source
activity so it is possible to obtain the activity by sending
the
source
to
pre-defined
dwell
position.
Results
3D Cartesian coordinates can be obtained with 0.2 mm
accuracy using a single EPID panel. The panel can clearly
identify dwell positions 2 mm apart even with the catheter
at 24 cm distance (Figure 1c) from the panel. Absolute
coordinates can be obtained by adding reference points
(representing the corners of the water phantom) in the
treatment plan that can be related with the position of
the water phantom over the panel during the experiments.
An
in-house
developed software compares all dwell
positions/times against the treatment plan. The software
can also monitor the sequence of the treatment
identifying the afterloader channel connected to each
catheter. Therefore, it is possible to detect catheter
misplacements, swapped transfer tube connections,
wrong dwell times and/or positions and also verify the
source activity.
Conclusion
This work describes an experimental system that can be
implemented in the clinic
providing experimental pre-
treatment verification that is not currently available. This
method provides several advantages when compared
against other dosimeters such as films or MOSFETs as it
combines a 2D dosimeter, which has an online response.
Our system can detect several problems that would be
unnoticed during the treatment if only traditional QA is
performed.
PO-0946 Entropic model for real-time dose
calculation: I-125 prostate brachytherapy application.
G. Birindelli
1
, J.L. Feugeas
1
, B. Dubroca
1
, J. Caron
1,2
, J.
Page
1
, T. Pichard
1
, V. Tikhonchuk
1
, P. Nicolaï
1
1
Centre Lasers Intenses et Applications, Interaction-
Fusion par Confinement Inertiel- Astrophysique,
Talence, France
2
Institut Bergonié Comprehensive Cancer Center,
Department of radiotherapy, Bordeaux, France
Purpose or Objective
This work proposes a completely new Grid Based
Boltzmann Solver (GBBS) conceived for the description of
the transport and energy deposition by energetic particles
for brachytherapy purposes. Its entropic closure and
mathematical formulation allow our code (M
1
) to calculate
the delivered dose with an accuracy comparable to the
Monte Carlo (MC) codes with a computational time that is
reduced to the order of few seconds without any special
processing power requirement.
Material and Methods
The kinetic M
1
model, is based on the spherical harmonic
expansion of the distribution function, solution of the
linear Boltzmann equation. The first two angular moments
equations, combined with the Continuous Slowing Down
Approximation, are closed using the Boltzmann's principle
of entropy maximization. The algorithm computes at the
same time all primary and secondary particles created by
the interactions of the beam with the medium. Thanks to
the implementation of the interaction cross sections for
electrons and photons in the energy range from 1keV up
to 100 MeV, the algorithm can simulate different
treatment techniques such as the external radiotherapy,
brachytherapy or intra-operative radiation therapy.
As a first validation step, a large number of heterogeneity
shapes has been defined for various complex numerical
phantoms both for electron and photon monoenergetic
sources. Dose profiles at different positions have been
measured in water phantoms including inhomogeneity of
bone ( ρ = 1.85 g/cm
3
), lung ( ρ = 0.3 g/cm
3
) and air ( ρ =
10
-3
g/cm
3
). Secondly, taking as reference the Carleton
Laboratory for Radiotherapy Physics Database, different
radioactive seeds have been implemented in the code.
Moreover, several simulations based on CT scan of
prostate cancer have been performed. The M
1
model is
validated with a comparison with a standard, accurate but
time consuming, statistical simulation tools as PENELOPE.
Results
The M1 code is capable of calculating 3D dose distribution
with 1mm
3
voxels without statistical uncertainties in few
seconds instead of several minutes as PENELOPE. Thanks
to its capability to take into account the presence of
inhomogeneities and strong density gradients, the dose
distributions significantly differ from those calculated
with the TG-43 approximations. More in detail: inter-seed
attenuation is treated, the real chemical composition of
the different tissues can be taken into account and the
effects of patient dimensions are considered.
Conclusion
In the comparison with the MC results the excellent
accuracy of the M
1
model is demonstrated. In general, M
1
,
as the MC codes, overcomes the approximations that are
formalised in TG-43 in order to decrease the complexity
of the calculations. Thanks to its reduced computational
time and its accuracy M
1
is a promising candidate to
become a real-time decision support tool for
brachytherapists.
PO-0947 Image-guided brachytherapy with 106Ru eye
plaques for uveal melanomas using post implantation
MRI
G. Heilemann
1
, N. Nesvacil
2
, M. Blaickner
3
, L. Fetty
1
, R.
Dunavoelgyi
4
, D. Georg
2
1
Medical University of Vienna/ AKH Vienna, Department
for Radiotherapy, Vienna, Austria
2
Medical University of Vienna/ AKH Vienna, Department
for Radiotherapy/ Christian Doppler Laboratory for
Medical Radiation Research for Radiation Oncology,
Vienna, Austria
3
Austrian Institute of Technology GmbH, Health and
Environment Department Biomedical Systems, Vienna,
Austria
4
Medical University of Vienna/ AKH Vienna, Department
for Ophthalmology and Optometry, Vienna, Austria
Purpose or Objective
In radiation oncology magnetic-resonance imaging (MRI) is
an important modality for tissue characterization, target
delineation and allows image-guidance due to its high soft
tissue contrast as a tool for better cancer treatment. In
106
Ru-brachytherapy of uveal melanomas MRI is mainly
used for pre-treatment planning scans to assess tumor size
and location. However, post-implantation MR scans yield
additional information on the plaque position in relation
to the target volume and critical structures. Together with