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on the kV CBCTs were compared to corresponding dose
distributions derived from the ArcCHECK measurements.
Secondly, the PerFRACTION3D reconstructed dose
distributions were compared to corresponding RayStation
dose calculations (RaySearch) using the kV CBCTs and the
original treatment plan data. The comparison was in
either case carried out by the use of 3D gamma analysis as
well as by comparing traditional dose volume histogram
statistics. Following the testing, a clinical deployment into
the clinical routine was carried out in order to perform
daily dose monitoring of delivered patient dose.
Results
A 3D gamma analysis (2%/2mm) showed gamma passing
rates
>98%
when
comparing
ArcCheck
measurements/RayStation plans to the doses
reconstructed in PerFRACTION3D for all plans studied.
Preliminary results from comparing daily PerFRACTION3D
dose reconstructions to planned dose distributions in
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. Talamonti
1
, S. Russo
2
, M.D. Falco
3
, A. Bartoli
4
, E.
Cagni
5
, L. Strigari
6
, M. Esposito
2
, F.R. Giglioli
7
, C.
Fiandra
7
, C. Marino
8
, S. Clemente
9
, M. Stasi
10
, E.
Villaggi
11
, P. Mancosu
12
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. Granero
1
, L. Brualla
1
, A. Vicedo
1
, M. Trinitat
1
, J.
Rosello
1
1
Eresa-Hospital General Universitario, Department of
Radiation Physics, Valencia, Spain