ESTRO 36 Abstract Book

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S770 ESTRO 36 2017
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patients treated for cancer in the head and neck region
showed similar agreement.
Conclusion
The dosimetric accuracy of the presented method for
reconstructing the delivered 3D dose distribution daily
CBCT appears very promising. Further studies are however
required to study the sensitivity to dosimetric changes
from changes in patient anatomy and set-up errors not
accounted for by the CBCT based set-up verification
procedure.
EP-1459 Relative Signal Ratios using an unshielded
silicon detector: data from 30 centers
C. Talamonti1, S. Russo2, M.D. Falco3, A. Bartoli4, E.
Cagni5, L. Strigari6 , M. Esposito 2, F.R. Giglioli7, C.
Fiandra7, C. Marino8, S. Clemente 9, M. Stasi10, E.
Villaggi11, P. Mancosu12
1 University of Florence, Dip Scienze Biomediche
Sperimantali e Cliniche, Firenze, Italy
2 Azienda Sanitaria Firenze, Fisica Sanitaria, Florence,
Italy
3 Policlinico Chieti, Fisica Sanitaria, Chieti, Italy
4 Istituto Nazionale Fisica Nucleare, Sezione di Firenze,
Florence, Italy
5 ArciOspedale Santa Maria Nuova, Fisica Sanitaria,
Reggio Emilia, Italy
6 IFO, Fisica Sanitaria, Roma, Italy
7 Ospedale Molinette, Fiisica Sanitaria, Torino, Italy
8 Humanitas, Fisica Sanitaria, Catania, Italy
9 IRCCS CROB, Fisica Sanitaria, Potenza, Italy
10 Ospedale ordine Mauriziano, Fisica Sanitaria, Torino,
Italy
11 AUSL Piacenza, Fisica Sanitaria, Piacenza, Italy
12 Humanitas, Fisica sanitaria, Milano, Italy
Purpose or Objective
The aim of this study was to provide Relative Signal Ratio
(RSR) values of three Linac models (Elekta, Elekta Synergy
BM and Varian) using measurements performed in a
multicenter Italian study. An eventual mathematical
description of the RSR curve was proposed in order to
calculate RSR for arbitrary field sizes with high accuracy.
Material and Methods
Thirty centres with different LINACs joined this project.
All measurements were performed using the new IBA
unshielded silicon diode RAZOR and the Stealth flat
ionization chamber fixed on the gantry as reference. The
RAZOR was positioned at 10cm depth in water and
SSD=90cm. RSRs were calculated for square field size
ranging 0.6-5.0cm. Data were normalized to the 3x3cm 2
field size. In-plane and cross-plane profile were measured
to correct RSR for the effective field size (EFS).The error
assigned to the experimental points was estimated taking
into account the statistical dispersion of the repeated
measurements and the error introduced on RSR by the
detector positioning uncertainty. Collected data were
clustered by linac model and each group was fitted using
the function proposed by Sauer (Med Phys2007). Moreover
the obtained curves were compared with the one
published by Sauer and Wilbert (SWF) calculated from the
fit to the mean values of four solid state detector data
series of an ELEKTA linac.
Results
The experimental data (blue points) with the fit function
(black line) and the SWF (pink dotted line) are shown in
figure 1 for each linac model. In all cases, It is evident how
the proposed analytical functions fit perfectly to the data
for all field sizes and for all the three linac models
investigated. Deviations from the SWF are shown in the
figure 2. The largest difference of the RSR value respect
to the SWF ones are evident below 2cm of effective field
size. This is mainly due to the different geometry of the
head of the linacs and therefore small changes in the
energy spectrum
Conclusion
In general, the adopted methodology removes much of the
ambiguity in reporting and interpreting small field
dosimetric quantities and facilitates a clear dosimetric
comparison across a population of linacs. Moreover it
minimizes the influence of measurement uncertainties
and it allows accurate determination of values for non-
measured field sizes.
EP-1460 Detection of forced errors in VMAT plans using
EPID and Epiqa dosimetric system
D. Granero1, L. Brualla 1, A. Vicedo1, M. Trinitat1, J.
Rosello1
1Eresa-Hospital General Universitario, Department of
Radiation Physics, Valencia, Spain
Purpose or Objective
VMAT treatments in radiotherapy are verified usually by
an experimental method, such as, measurement in
discrete points with an ionization chamber, radiochromic
film, EPID or other dosimetric systems. In this study we did
an analysis of the error detection capability in treatment
plans of aS1200 EPID of TrueBeam 2.0 linac together with
the Epiqa v4.0.12 dosimetric software from EPIdos. To do
this we have introduced forced errors in the treatment
plans of 5 different VMAT patients in order to test if the
dosimetric system can detect different forced errors
introduced in the treatment plans.
Material and Methods
5 different patients have been selected, a
hypofractionated prostate+vesicles treatment, a
hypofractionated prostate+vesicles+lymph nodes a head
and neck patient, a spine SBRT and finally a treatment of
the paraaortic lymph nodes. Treatment plans were
designed using 1 or 2 complete arcs. We have introduced
forced errors in the Pinnacle treatment plans using excel
home made macros. A total of 160 different plans have
been irradiated with the TrueBeam 2.0 linac and dose has
been measured with the EPID. The errors introduced were: