Ardu, V.

Borzì, G.

Clemente, S.

D'Alesio, V.

Falco, M.D.

Giglioli, F.R.

Linsalata, S.

Loi, G.

Mameli, A.

Mancosu, P.

Marino, C.

Menghi, E.

Miceli, R.

Moretti, E.

Nigro, R.

Palleri, F.

Pasquino, M.

Pastore, G.

Raza, G.H.

Russo, S.

Stasi, M.

Talamonti, C.

Vaiano, A.

Villaggi, E.

Vittorini, F.

Elsevier - Publisher Connector 

S386                                                                                                                                                    ESTRO 35 2016 
______________________________________________________________________________________________________ 
Daily measurements were examined for consistency and again 
the EPID and Starcheck performed similarly, with comparable 
standard deviations, as shown in Table 1.  
 
 
 
Conclusion: Our results show that for FFF QA measurements 
such as field size and symmetry, using the EPID is a viable 
alternative to other QA devices. The EPID performs 
particularly well on geometric measurements, as it is able to 
detect small changes in positon (~1mm) with good 
consistency. This is to be expected due to its high resolution 
when compared to the other QA devices used (EPID 0.34mm, 
Starcheck 3mm, QA3 5mm). Therefore the EPID could 
potentially be used for a wider range of QC measurements 
with a focus on geometric accuracy, such as MLC positional 
QA.  
References [1] Fogliata, A., Garcia, R., et Al (2012). 
Definition of parameters for quality assurance of flattening 
filter free (FFF) photon beams in radiation therapy. Med. 
Phys., 39(10), p.6455. 
 
PO-0817  
Characteristics and performance of the first commercial 
MLC for a robotic delivery system 
P. Prins
1Erasmus Medical Center Cancer Institute, Department of 
Radiation Oncology, Rotterdam, The Netherlands 
1, C. Fürweger2, H. Coskan1, J.P.A. Marijnissen1, 
B.J.M. Heijmen1 
2European Cyberknife Center, Munich, Germany 
 
Purpose or Objective: To assess characteristics and 
performance of the “InciseTM” MLC (41 leaf pairs, 2.5mm 
width, FFF linac) mounted on the robotic SRS/SBRT platform 
“Cyberknife M6TM“ in a pre-clinical 5 months test period and 
to ensure quality of clinical treatments. 
 
Material and Methods: Beam properties were measured with 
unshielded diodes and EBT3 film. Bayouth tests for leaf / 
bank position accuracy were performed in standard (A/P) and 
clinically relevant non-standard positions, before and after 
exercising the MLC for 10+ minutes. Total system accuracy 
was assessed in End-to-End tests. Delivered dose was verified 
with EBT3 film for exemplary and clinical plans. Stability over 
time was evaluated in Picket-Fence- and adapted Winston-
Lutz-tests (AQA) for different collimator angles. 
 
Results: Penumbrae (80-20%, with 100%=2*dose at inflection 
point; SAD 80cm; 15mm depth) parallel/perpendicular to leaf 
motion ranged from 2.7/2.2mm for the smallest 
(0.76x0.75cm2) to 3.7/3.6mm for larger (8.26x8.25cm2) 
square fields. MLC penumbrae are slightly wider than 
penumbrae fixed cones (2.1 to 2.8mm for 5 to 60 mm cones). 
Interleaf leakage was <0.5%. Average leaf position offsets 
were ≤0.2mm in 14 standard A/P Bayouth tests and ≤0.6mm 
in 8 non-standard direction tests. Pre-test MLC exercise 
slightly increased jaggedness (range +/-0.3mm vs. +/-0.5mm) 
and allowed to identify one malfunctioning leaf motor. Total 
system accuracy with MLC was 0.39+/-0.06mm in 6 End-to-
End tests. Delivered dose showed good agreement with 
calculated dose (typically Gamma(3%,3mm)<1 for >95% of 
pixels with D > 0.1 Dmax). Picket-Fence and AQA showed no 
adverse trends (> 1 yr). 
 
Conclusion: The InciseTM MLC for CyberKnife M6TM displays 
high mechanical stability and accurate dose delivery. The 
specific CK geometry and performance after exercise demand 
dedicated QA measures. 
PO-0818  
Multicentre small field measurements using a new plastic 
scintillator detector 
M. Pasquino
1A. O. Ordine Mauriziano di Torino - Ospedale Mauriziano 
Umberto I, Medical Physics, Torino, Italy 
1, S. Russo2, P. Mancosu3, E. Villaggi4, G. Loi5, R. 
Miceli6, G.H. Raza7, A. Vaiano8, M.D. Falco9, E. Moretti10, F.R. 
Giglioli11, R. Nigro12, C. Talamonti13, G. Pastore14, E. Menghi15, 
F. Palleri16, S. Clemente17, C. Marino18, G. Borzì19, V. Ardu20, 
S. Linsalata21, A. Mameli22, V. D'Alesio23, F. Vittorini24, M. 
Stasi1 
2Azienda Sanitaria di Firenze, Medical Physics, Firenze, Italy 
3Humanitas, Radiotherapy, Milano, Italy 
4AUSL Piacenza, Medical Physics, Piacenza, Italy 
5AOU Maggiore delle Carità, Medical Physics, Novara, Italy 
6AOU Tor Vergata, Medical Physics, Roma, Italy 
7Ospedale San Pietro Fatebenefratelli, Medical Physics, 
Roma, Italy 
8USL 3, Medical Physics, Pistoia, Italy 
9Policlinico SS. Annunziata, Radiotherapy, Chieti, Italy 
10AOU "Santa Maria della Misericordia", Medical Physics, 
Udine, Italy 
11AOU Città della Salute e della Scienza, Medical Physics, 
Torino, Italy 
12O.G.P. S.Camillo de Lellis, Radiotherapy, Rieti, Italy 
13AUO Careggi, Medical Physics, Firenze, Italy 
14Ecomedica, Radiotherapy, Empoli, Italy 
15I.R.S.T., Medical Physics, Meldola, Italy 
16AO Parma, Medical Physics, Parma, Italy 
17IRCCS CROB, Medical Physics, Rionero in Vulture, Italy 
18Humanitas, Medical Physics, Catania, Italy 
19Centro REM Radioterapia, Radiotherapy, Catania, Italy 
20Policlinico San Donato, Radiotherapy, San Donato M.se, 
Italy 
21USL Lucca, Medical Physics, Lucca, Italy 
22Campus Biomedico, Radiotherapy, Roma, Italy 
23Malzoni Radiosurgery Center, Radiotherapy, Agropoli, Italy 
24ASL1 Abruzzo, Medical Physics, L'Aquila, Italy 
 
Purpose or Objective: Small field dosimetry standardization 
is fundamental to ensure that different institutions deliver 
comparable and consistent radiation doses to their patients. 
The current study presents a multicenter small field 
evaluation including: Tissue Phantom Ratio (TPR), dose 
profiles FWHM and penumbra, and output factors (OF), for 
the two major linear accelerator manufacturers and different 
X-ray energies. 
 
Material and Methods: The project enrolled 31 Italian 
centers, 15 equipped with Elekta Linacs and 16 with Varian 
Linacs. Each center performed TPR measurement, in-plane 
and cross-plane dose profile of 0.8x0.8cm2 field and OFs 
measurements for field sizes ranging from 0.6x0.6 cm2 to 
10x10 cm2 defined by both secondary jaws and MLC. Set-up 
conditions were: 10cm depth in water phantom at SSD 90cm. 
Measurements were performed using the new Exradin W1 
plastic scintillator detector (Standard Imaging). The two 
canals SuperMAX electrometer (Standard Imaging) to 
automatically correct for Cherenkov radiation was used. Two 
identical W1 were used to speed up the data collection. 
 
Results: The analysis included 13 Varian and 13 Elekta 
centers; 7 centers were excluded due to a condenser problem 
in an electrometer. As reported in Table 1 for the two most 
representative linac models, TPR measurements showed 
standard deviations (SD)=0.6%; penumbra values of dose 
profiles showed SD=0.5mm, while FWHM measurements 
showed a greater variability. As illustrated in Figure 1, OF 
measurements showed standard deviations within 1.5% for 
field size greater than 2x2 cm2; for field size less than 2x2 
cm2 measurements’ variability increases with decreasing 
field size. OF values show no dependence from the effective 
field size. 
Table 1. TPR, FWHM and penumbra values measured with W1 
PSD for the two most representative linacs of the multicenter 
study 
 
ESTRO 35  2016                                                                                                                                                  S387 
________________________________________________________________________________ 
 
 
 
Fig. 1. OF values measured with W1 PSD for the two most 
representative linacs of the multicenter study 
 
Conclusion: High TPR and penumbra values consistency were 
obrained over the centers. FWHM and OF showed greater 
variability, also considering Linac with the same model of the 
head. Measurements confirm W1 PSD as a good candidate for 
small field clinical radiation dosimetry in advanced radiation 
therapy techniques. 
 
PO-0819  
Analysis of liquid embolic agents on flattening filter free 
dose deposition with Monte Carlo method 
D. Akcay
1Dokuz Eylul University Medical Faculty, Radiation Oncology, 
IZMIR, Turkey 
1 
 
Purpose or Objective: Brain Arteriovenous mal-formation 
(AVM), in some cases, is treated with Onyx34 liquid embolic 
system (LES) containing tantalum. Moreover, stereotactic 
radiosurgery (SRS) may be required when total obliteration is 
not achieved after embolization. Presence of tantalum in 
radiation field not only generates artefacts in computed 
tomography (CT) images but also it could arise dose 
distribution perturbations. Goal in this study was to analyze 
the perturbation effect of Onyx34 in flattening filter free 
(FFF) photon beams using GAMOS Monte Carlo (MC). Artefact 
cause analysis was also included in the study. 
 
Material and Methods: GAMOS simulations for 6 FFF and 10 
FFF photons were done in three different conditions: a) 
depth dose simulations in a water phantom containing 2x2x2 
cm3 Onyx34 (inc. %35wt/vol Ta) medium at 5 cm depth, b) 
depth dose simulations in the same condition with compact 
bone instead of Onyx34, c) simulations in homogenous water 
phantom. Dose and photon flux scorers were used at central 
beam axis with 5x5x2 mm3 grid sizes. For comparison 
purposes, photon fluxes were also scored with broad photon 
beam with single photon energy of 80 keV. All of MC 
calculations were done with 1.5% and 0.5% statistical noise 
respectively. 
 
Results: In 80 keV photon simulations, photon flux decreased 
around 50% at post Onyx34 region relative to homogeneous 
water simulations. However, for compact bone falloff was 
around 10% at the same geometry. Also, there was a 
remarkable flux reduction in pre-Onyx34 region which is not 
as much as post Onyx34 region. For both 6 and 10 FFF photon 
beams, around 5% of decrease was seen in photon flux and 
depth dose after Onyx34 and compact bone inhomogeneties . 
Pre-Onyx34 region doses were increased by 15% for 6 FFF and 
10% for 10 FFF.  
 
 
Conclusion: Photon flux calculations for 80 keV beam, 
showed a considerable photon attenuation and lateral 
scattering due to presence of Onyx34. As a result, photon 
starvation causes black-white streak artefacts in CT images. 
In conclusion, in AVM SRS planning the presence of LES can be 
taken into account by defining high density artefact region as 
a compact bone. However in vicinity of critical structures, 
the possible dose peaks must be considered at pre-Onyx 
regions which might not be calculated in treatment planning 
systems.  
 
PO-0820  
Volumetric quality assurance of RapidArc plans for multiple 
intracranial targets using gel dosimetry 
N. Khater
1Hotel Dieu de France - University of Saint Joseph, Radiation 
Oncology, Achrafieh, Lebanon 
1, C. El Khoury1, M. Sarraf2, J. Barouky1, D. Nehme 
Nasr1, F. Azoury1, T. Felefly1, R. Sayah1, N. Farah1, S. 
Achkar1, E. Nasr1 
2Clinatec-Cea-Grenoble, University of Joseph Fourier - 
University of Saint Joseph, Grenoble, France 
 
Purpose or Objective: Given the unlimited spatial 
arrangements of multiple intracranial tumors, an evaluation 
of a planar sampling for end-to-end test is insufficient as it 
could provide no information about one or more tumors. 
Hence, a volumetric approach is needed. Here, we evaluate 
polymer gel dosimetry for three-dimensional (3D) patient-
specific quality assurance (QA) in multiple brain lesions 
stereotactic radiosurgery (SRS) plans using volumetric 
modulated arc therapy (VMAT) technique. 
 
Material and Methods: End-to-end test using polymer gel 
dosimeters was performed for an intracranial SRS case 
involving two lesions treated with VMAT – single isocenter 
approach. The following was performed: (1) BANG-3 polymer 
gel was prepared for two opaque spherical glass phantoms, 
one for patient plan QA and one for calibration; (2) the 
patient plan was delivered to the patient phantom and a 
simple non-modulated plan with predetermined doses was 
given to the calibration phantom; (3) 1.5T MRI was performed 
on both phantoms; (4) an in-house program was used to 
determine the relaxation rate maps (R2) from proton density 
and T2-weighted images; (5) CT scans were acquired with 
markers triangulating the isocenter of irradiation setups; (6) 
CTs were imported into Eclipse treatment planning system 
for dose computation in corresponding gel phantoms; (7) CTs 
and MRs were registered in Eclipse and registration 
transformations used to resample the R2 maps in 
corresponding CT positions using MATLAB. Then, data analysis 
was performed using an in-house visual-C++ code which took 
as input all 3D images, 3D dose, patients’ structures exported 
from Eclipse and performed the following: (8) A calibration 
was extracted from the calibration gel and used in the 
patient gel QA; (9) the patients’ structures were registered 
with the patients’ gel using CT isocenter marks on the gel 
phantom and through the isocenter on the patients’ plan; and 
(10) compared measured versus planned 3D isodose, dose 
volume histogram (DVH) analyses, and multi-slice 2D gamma 
evaluation. 
 
Results: The measured isodose lines and surfaces were well 
visualized and qualitatively reproduced the calculated dose 
distribution (Figure 1). Gamma analysis between the dose 
matrices were carried out using gamma criteria 3% 3mm and 
5% 5mm, % dose difference – distance to agreement 
combination within the volume enclosed by the 50% and the 
80% isodose surface, respectively. Representative transverse 
slices yielded gamma pass rates of greater than 90%. 
Measured and planned DVH analyses showed agreement for 
planning target volume and organs at risk. 
 


PO-0818: Multicentre small field measurements using a new plastic scintillator detector 

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PO-0818: Multicentre small field measurements using a new plastic scintillator detector

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