ESTRO 36 Abstract Book

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S802 ESTRO 36
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Conclusion
No other CFs than the EDCF have to be applied for skin
dose measurements with EBT3 films.
This work has been partially financed by the grant
Singulars Projects 2015 of the Spanish Association Against
Cancer (AECC).
[1]Detector comparison for dose measurements in the
build-up zone. M.A Duch et al. 3rd ESTRO FORUM. 2015.
EP-1496 A portal dosimetry dose prediction method
based on CT images of Electronical Portal Imaging
Device
J. Martinez Ortega1 , N. Gomez Gonzalez1, P. Castro
Tejero 2, M. Pinto Monedero1, N. Tolani3, L. Nuñez
Martin1, R. Sanchez Montero 4
1 Hospital Universitario Puerta de Hierro, Radiofisica y
PR, Majadahonda - Madrid, Spain
2 Hospital Universitario La Princesa, Radioterapia,
Madrid, Spain
3 ME De Bakey VA Medical Center, Radiotherapy
Department, Houston, USA
4 Universidad de Alcala, Signal Theory and
Communications Department, Alcala de Henares-Madrid,
Spain
Purpose or Objective
In this study, we present a new method for portal
dosimetry. CT images of the Electronical Portal Imaging
Device (EPID) were used as phantom images for dose
calculation. The clinical beam model and beam energy, in
the treatment planning system, were used to calculate
dose over the EPID.
Material and Methods
The method was developed for a Varian Clinac 21-EX
(Varian Medical Systems, USA), with a nominal photon
energy of 6 MV, equipped with a Varian aS1000 EPID.
Pinnacle 8.0m (Philips Medical Systems, NL) was used for
treatment planning calculations. Matlab® v2012a
(Mathworks, USA) was employed to develop code for
calculations involving backscatter and output correction
factors.
The EPID was calibrated, following the manufacturer
procedure, and then unmounted from the linear
accelerator and scanned to acquire CT images of the EPID
(Fig. 1) on an Aquilion LB (Toshiba Medical Systems,
Japan). These CT images were imported into the Pinnacle
planning system. The imported images were used as a
quality assurance phantom to calculate dose on the image
plane, which was considered as the predicted portal dose.
Two sliding-window IMRT treatment plans, a prostate and
a head and neck case, were delivered, measured and
analyzed with both with the EPID and with MatriXX (IBA
Dosimetry, Germany), as an independent measurement
method.
Matlab code was used to calculate EPID arm
backscattering and output factor corrections. Gamma
index comparison (3 %, 3 mm) was made for the EPID and
MatriXX dose planes versus the calculated dose planes with
OmniPro ImRT (IBA Dosimetry).
Figure 1. Acquired CT images of the Varian aS1000
EPID. Results
For plans verified with EPID, Gamma index pass rate were
98.6% and 96.5% for prostate (Table 1) and head and neck
case, respectively. Dose differences (EPID vs planned)
were -0.7% and -0.4%.
For MatriXX measurements, the results are very similar:
gamma pass rate of 97.2% for prostate and 97.9% for head
and neck, and dose differences (MatriXX vs planned) of -
1.4% and -0.8%, respectively.
Field
Gamma
(3 %, 3
mm)EPID
Dose
diff EPID
(%)
Gamma (3 %,
3
mm)MatriXX
Dose
diffMatriXX
(%)
1 98.5% -2.0
2 98.6% -1.0 96.5% -1.6
3 98.9% -0.5 98.1% -1.1
4 98.6% -0.6 96.0% -1.5
5 99.0% -0.3 96.5% -1.1
6 98.7% -0.1 99.0% -1.5
7 98.0% -0.6 97.3% -1.2
Average 98.6% -0.7 97.2% -1.4
Table 1. Gamma index and dose difference results for
prostate treatment.
Conclusion
The obtained results show the validity of the method
presented here. This method can be easily implemented
into clinic, as no additional modeling of the clinical beam
is necessary. The main advantage of this method is that
portal dose prediction is calculated with the same
algorithm and energy beam model used for patient
treatment planning dose distribution calculations.
EP-1497 Dosimetric effect of the Elekta Fraxion cranial
immobilization system and dose calculation accuracy
C. Ferrer1, C. Huertas1, R. Plaza1, A. Serrada1
1Hospital universitaria La Paz, Radiofísica y
Radioprotección, Madrid, Spain
Purpose or Objective
Devices external to the patient may cause an increase in
the skin dose, as well as modify the dose distribution and
hence the tumor dose. This study describes the effect on
this parameters caused by the Elekta Fraxion cranial
immobilization system. The effect of the inclusion of
Fraxion in ElektaMonaco treatment planning system (v.
5.00.00) was also checked.
Material and Methods
To study the dose attenuation a cylindrical phantom was
placed over the Elekta Fraxion with a CC13 Scanditronix-
Wellhofer ionization chamber located in the central insert
at the linac isocenter. Dose measurements were
performed for two open fields, 10x10 cm and other smaller
5x5 cm, as Fraxion is used mainly for radiosurgery
treatments. The gantry angles were the ones which cross
Fraxion (135º - 225º, 5º-10º increment, IEC gantry angles).