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ESTRO 36
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EP-1469 Flattening filter free beam profile analysis
using two different normalization methods
G. Nicolini
1
, A. Fogliata
2
, E. Vanetti
1
, G. Reggiori
2
, A.
Stravato
2
, P. Mancosu
2
, M. Scorsetti
2
, L. Cozzi
2
1
Radiqa Developments, Medical Physics Team,
Bellinzona, Switzerland
2
Humanitas Research Hospital, Radiotherapy and
Radiosurgery Dept, Milan, Italy
Purpose or Objective
Flattening filter free (FFF) beams present a profile peaked
on the beam central axis (cax), unsuitable for flatness and
symmetry description that usually characterize standard
beam profiles. Definitions of unflatness and slope have
been recently proposed, requiring a preliminar suitable
FFF profile normalization. Two main normalization
processes as far published are: the inflection point IP
(Pönish 2006), and the renormalization factor RF (Fogliata
2012). In both formalisms the FFF dose fall-off at the field
edge is superimposed with the corresponding FF profile.
The present study aims to compare FFF specific profile
parameters using the two normalization procedures.
Material and Methods
Dosimetric data from a Varian TrueBeam with 6 and 10 MV,
FF and FFF modes, have been collected at SSD 100cm and
5 depths. The cax normalization value N was evaluated for
the IP method as N=D
cax
·(D
u
/D
f
), where D
cax
and D
f
are the
doses on cax and at the IP of the penumbra region for the
corresponding FF beam, D
u
is the dose at the IP of the FFF
beam.
The N value for the RF method was evaluated by using the
fit dependent on the field size FS and depth:
N=(a+b·FS+c·depth)/(1+d·FS+e·depth), where the fitting
parameters are taken from published data. The main
profile parameters of FFF photon beams were computed:
field size, penumbra, unflatness, slope, and peak-position
parameters. To systematically investigate the impact of
the N value, they were recomputed with a RF value
modified of + 1,2,3,5,7,10% (perturbed RF).
Results
In terms of cax normalization value, in average, the two
methods show an agreement within the 2%, with a
tendency of a greater N with IP respect RF method for
10MV. In any case, some outliers are present, with a
discrepancy that reaches the 10%; this is expected, since
the IP method suffers of the uncertainty of IP position
determination in the practice. Beam parameters values
derived with the approaches (IP/RF) were computed
showing, e.g., for both energies 1.00+0.00 for unflatness
and, respectively for 6 and 10MV, 0.99+0.05 and 1.02+0.04
for slope. Analysis with perturbed RF values, shows that
with a variation up to 10% of N, the peak position remains
within 0.05mm, the unflattens within 0.5% and 1% for 6MV
and 10MV beams, while the slope has a variation almost of
the same amount of N itself. Field size difference is within
1mm if N variation is within 5%.
Conclusion
The two normalization methods are both suitable for
subsequent FFF profile description. Unflatness parameter
resulted similar when computed using the two different
normalization formalisms with no significant differences.
Slope values are more sensitive to normalization value,
and therefore some outliers were observed due to
uncertainty of IP position in the practice. The RF
procedure, with the published fitting parameters is easier
to use and more robust respect to measurements sampling
and
detector size.
EP-1470 Determination of paramagnetic gel sensitivity
in low energy X-ray beam
Y. Ben Ahmed
1
, J. Coulaud
2
, S. Ken
1
, L. Parent
1
1
Institut Universitaire du Cancer Toulouse, Haute
Garonne, Toulouse, France
2
SIMAD, Haute Garonne, Toulouse, France
Purpose or Objective
The INTRABEAM® system is a miniature accelerator
producing low energy photons (50 keV maximum). The
published dosimetric characterization of the INTRABEAM
system for flat and surface applicators was based on
detectors (radiochromic films or ionization chambers) not
allowing measuring the absorbed dose in the first
millimeters of the irradiated medium, where the dose is
actually prescribed. This study aims at determining the
sensitivity of a paramagnetic gel in order to measure the
dose deposited with INTRABEAM surface applicators in the
first millimeters of irradiated medium.
Material and Methods
The determination of paramagnetic gel sensitivity was
performed with irradiations at different dose levels with
the INTRABEAM® Carl Zeiss Surgical system (Oberkochen,
Germany). The ferrous gel used in this study is a new «
sensitis» material which is described by C. Stien et al and
V. Dedieu et al. Gel irradiation in tin and capsule
containers was carried out for twelve dose levels between
2 Gy and 50 Gy at the gel surface with a 4 cm surface
applicators. The applicator was in contact of the gel
during irradiation. For the calibration curve, one batch gel
was measured without being irradiated. T
2
weighted multi
echo MRI acquisitions were performed on a 1,5 T
Magnetom Aera MR scanner of Siemens with surface flex
head coil technology.
Results
The T
2
signal versus echo times can be fitted with a mono-
exponential function with 95% of confidence. The first
echo time was not considered for the fit. The calibration
curve determined from experiments with tins is a linear
function (R
2
=0.967) with a sensitivity of 1.04*10
-4
s
-1
.Gy
-1
.
Gels Sensitivity with capsules are of 3.67*10
-4
s
-1
.Gy
-1
(R
2
=0.979) and 2.54*10
-4
s
-1
.Gy
-1
(R
2
=0.944). The
calibration curve was applied to the irradiation of a
surface applicator to obtain the 3D dose distribution in the
gel.
Conclusion
The dose distribution obtained after irradiation at low
energies with an INTRABEAM® miniature accelerator can
be measured for the first millimeters thanks to ferrous
gels. The determination of gel sensitivity was possible with
MRI measurements. Results are relevant but must be
confirmed with more irradiations with different dose
levels at the surface and different surface and flat
applicator
diameters.
EP-1471 Comparison of the integral dose of IMRT,
RapidArc and helical tomotherapy prostate treatments
J. Martinez Ortega
1
, P. Castro Tejero
2
, M. Pinto
Monedero
1
, M. Roch Gonzalez
2
, L. Perez Gonzalez
2
1
Hospital Universitario Puerta de Hierro, Servicio de
Radiofísica y PR, Majadahonda - Madrid, Spain
2
Hospital Universitario de la Princesa, Servicio de
Radioterapia, Madrid, Spain
Purpose or Objective
Comparison of integral dose (ID) and normal tissue integral
dose (NTID) for Helical Tomotherapy (HT), RapidArc and
static fields IMRT.
Material and Methods
A cohort of ten prostate patients were selected whose
prescription was 78 Gy mean dose to the Planning Target
Volume (PTV). Seven different plans for every patient
were computed. One sliding-window IMRT with XiO
planning system and Varian Clinac 21EX, equipped with
MLC Millennium 80. Four Intensity-Modulated Radiation
Therapy (IMRT) plans were calculated with Varian Eclipse
planning system, two step-and-shoot and sliding-window