ESTRO 36 Abstract Book

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S410 ESTRO 36 2017
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MATLAB, allowing determination of the correction for
scanner inhomogeneity and calibration of film optical
density (OD) response in terms of absorbed dose to water.
Regions of interest (ROIs) of various sizes were used to
sample image data, quantifying the uncertainty associated
with variations within each film, from film to film within
the same lot, and from lot to lot. 35 sheets of film were
used, taken from 7 boxes across 3 lots. Three channels of
optical density (OD) data were analysed statistically, both
directly as OD and also in the ratios red/blue and
green/blue. Net values were obtained by subtracting pre-
irradiation values, and a normalisation correction factor,
based on large dose saturation values, was applied.
Results
The figure shows the net ratio of OD, green/blue, before
and after applying the normalisation correction, as a
function of dose, for ROIs which are 10 x 10 mm2. The
table lists the relative standard deviation of absorbed dose
measurements made using EBT-3 in the present work.
Conclusion
By combining the subtraction of pre-irradiation values,
with a normalisation correction based on large dose
saturation values, it should be possible to reduce the
contribution to measurement uncertainty arising from
intrinsic variations in the characteristics of EBT-3 film to
0.7 % ( k=1) for doses in the range of up to 5 Gy.
PO-0782 New liquid ionization chamber detector of
high resolution for treatment verification in
Radiotherapy
L. Brualla-Gonzalez1, A. Vázquez-Luque2, M. Zapata 3,4,
D.M. González-Castaño3,5 , V. Luna-Vega4, J. Guiu-Souto4,
D. Granero1, A. Vicedo 1, M.T. García-Hernández1, J.
Roselló1,6 , M. Pombar3,4, F. Gómez3,7, J. Pardo-Montero3,4
1 Hospital General Universitario de Valencia, Servicio de
Radiofísica ERESA, Valencia, Spain
2 Detection And Radiation Technologies SL, Development,
Santiago de Compostela, Spain
3 Instituto de Investigación Sanitaria IDIS, Grupo de Imaxe
Molecular, Santiago de Compostela, Spain
4 Complexo Hospitalario Universitario de Santiago de
Compostela, Servizo de Radiofísica e Protección
Radiolóxica, Santiago de Compostela, Spain
5 Universidade de Santiago de Compostela, Laboratorio
de Física de Partículas RIAIDT, Santiago de Compostela,
Spain
6 Universidad de Valencia, Departamento de Fisiología de
la Facultad de Medicina, Valencia, Spain
7 Universidade de Santiago de Compostela, Departamento
de Física de Partículas, Santiago de Compostela, Spain
Purpose or Objective
In this work we present a new liquid ionization chamber
array prototype for patient treatment verification. The
objective of its design is to offer a high spatial resolution
with 100% fill factor.
Material and Methods
The prototype has 2041 liquid ionization chambers of
2.5x2.5 mm2 effective area and 0.5 mm thickness. The
detection elements are arranged in a central square grid
of 43x43, covering an area of 107.5x107.5 mm2. The
central inline and crossline are extended to 227 mm and
the diagonals to 321 mm. The active medium is liquid
isooctane.
We have studied short- and medium-term stability, dose
rate dependence, depth and field size dependence,
anisotropy and leaf positioning detectability.
We have measured output factors, tongue-and-groove,
garden fence, small field profiles and irregular fields.
Finally we have used it for the verification of patient
treatments.
Results
The detector presents dependency on energy that is
reflected in the response variation with depth and field
size (2.2% under-response for 6 MV, 20x20 cm2 at 20 cm
depth).
The anisotropy study shows important deviations: 28% for
lateral incidences and 7% for posterior incidence.
The detector sensitivity for leaf positioning measurement
is 1.8 % per tenth of millimeter in the penumbra.
The output factor corresponding to 6 MV and 1x1 cm2
shows +2% deviation compared with the measurements
obtained using a SFD diode and a CC13 gas ionization
chamber. The results are normalized to a 5x5 cm 2. For a
10x10 cm2 this deviation is -1%. If the energy increases the
deviations decrease (+1% for 1x1 cm2 and -0.5% for 10x10
cm2 in 10 MV and 15 MV).
In the measurement of small field profiles the gamma
comparison between measurements with the liquid
ionization array and radiographic film shows 100% passing
rates with tolerances 1% - 1mm.
Several patient treatments have been verified. In table 1
the comparison between the treatment planning system
and the array measurement for a particular case is shown.
We show differences in gamma passing rates when
anisotropy corrections are applied or not. Figure 1 shows
one of such comparisons.
Conclusion
A new detector array is presented for the verification of
patient treatments of high complexity.
The detector presents a small dependence on
energy, which causes a small over-response for the output
factors of small fields and an under-response for output
factors of large fields. The anisotropy of the device is
significant (28% and 7% for lateral and posterior
incidences), but can be compensated during treatment
verification by using angle-dependent correction factors.
The usefulness for the patient treatment verification has
been demonstrated by measuring different patient
treatments. The results obtained confirm the validity of
this array for dose distribution measurements of complex
treatments with small fields and high gradients.
PO-0783 Planverification in Robotic Stereotactic
Radiotherapy with the Delta4-Dosimetry-System
W. Baus1, G. Altenstein1
1Universität zu Köln, Department of Medical Physics,
Köln, Germany
Purpose or Objective
Stereotactic robotic radiotherapy with the CyberKnife
(Accuray, Sunnyvale) might not be fluency modulated
radiotherapy (IMRT) in the strict sense. However, the
technique is comparable in complexity because of a large
number of small (5 to 60 mm), highly non-coplanar fields.
Therefore, the manufacturer recommends individual plan
verification (DQA, Delivery Quality Assurance), though
only on a point dose measurement basis. The report of the