Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

48 / 146

Introduction: Radiotherapy treatments are getting more and more complex,
thus it is of increasing importance to monitor delivered beams to identify
errors. This study analyzes the use of a linac-head integral quality monitor
(IQM, iRT Systems GmbH) for real-time beam delivery control. We evalu-
ate IQM beam attenuation and its ability in detecting VMAT delivery errors.
Materials and Methods: Beam attenuation was calculated at 4 beam size
(from 5 × 5 to 20 × 20 cm2) by the IC Profiler (Sun Nuclear Corporation) at
6 and 10 MV beam energies.
The IQM capability in recognizing errors was performed modifying 4 clin-
ical H&N VMAT plans: +3, 5, 10% errors on total MUs and 3, 5, 10 mm MLCs
shift.
At the same time, we obtained dose distribution maps through the PTW
2D array inserted in a rotating QA phantom (RT-smartIMRT, dose.point
GmbH). Cumulative IQM checksum values and local gamma pass rates (2%/
2 mm) were compared to those of non-modified plan.
Results: Beam attenuations normalized to the profiler central chamber are
6.56 ± 0.03% and 5.27 ± 0.12% for 6 and 10 MV beams, respectively. Flat-
ness deviation is <0.4% for 6 MV and <0.1 % for 10 MV excluding beam
penumbra regions.
Results for modified VMAT plans show that both methods detect specifi-
cally MLC shift errors. IQM shows a linear response with dose (R2 = 0.9995),
while gamma analysis seems to have difficulty in identifying 3% and 5% MUs
variations. In our opinion the reason is that the RT-smartIMRT recollects
a 2D dose map as if the entire plan were delivered at a fixed gantry angle.
Further comparisons should be evaluated with a different kind of phantom.
Conclusion: IQM beam attenuation can be considered to be homogenous
and the machine-specific beam attenuation percentage could be used to
rescale treatment plan dose for clinically IQM use. IQM shows apprecia-
ble features in detecting real-time errors, although the characterization of
IQM response to single segment errors still have to be analyzed.
A.145
CYBERKNIFE BEAM OUTPUT FACTOR MEASUREMENTS: A MULTI-SITE,
MULTI-DETECTOR STUDY
L. Masi S. Russ E. Demartin R. Dor C. Frassanit M.L. Fumagalli
M. Marinell A. Martinott M. Pimpinella G.L. Verona-Rinati E. Rond
S. Vigorito C. Vite P. Mancos a IFCA, Firenze, Italy; b OSMA ASL10, Firenze,
Italy; c Istituto Besta, Milano, Italy; d Mater Dei, Bari, Italy; e Università Tor
Vergata, Roma, Italy; f CDI, Milano, Italy; g ENEA, Roma, Italy; h IEO, Milano,
Italy; i Humanitas Rozzano, Milano, Italy
Introduction: This study was developed within the SBRT AIFM working
group and focused on a multicenter output factors (OF) evaluation for
CyberKnife systems, using 2 detectors of new generation and a silicon diode
considered as a reference after Monte Carlo (MC) correction.
Material and Methods: OF were measured by five centers on CyberKnife
units of various generations (600, 800, 1000 MU/min) for fixed collima-
tors circular fields from 5 to 60 mm in setup conditions 80 cm source to
detector distance and 1.5 cm depth in water. A PTW 60019 synthetic
diamond, a plastic scintillator detector (PSD) (W1 Exradin) and the rou-
tinely used detectors PTW 60017 unshielded silicon diodes were used by
each center. PSD data were corrected for Cerenkov Light Ratio (CLR) ac-
cording to the method by Morin (Med Phys 2013). MC Cyberknife specific
correction factors (Francescon PMB 2012) were applied to silicon diode data.
Results: For collimators above 10 mm, diamond and PSD OF were within
1.8% of MC corrected diode values over all centers. A good agreement
between PTW 60019 and MC corrected silicon diode was found for the three
smallest diameters (5, 7.5, 10 mm): relative differences were below 2.2%
performing a center by center analysis and below 0.6% for average values.
A larger variability among different centers was observed for PSD data: OF
measured by W1 PSD were within 3.2% of MC corrected PTW-60017 data
for 10 mm and 7.5 mm. For the smallest cone a 4.6% disagreement was de-
tected in one center, but differences were within 1.6% for the remaining
units.
Conclusions: CyberKnife OF measured by PTW-60019 diamond showed
a high consistency among centers and a good agreement with MC cor-
rected PTW-60017. The comparison between Exradin W1 PSD and MC
corrected diode results is also promising but the application of CLR remains
a critical point and may explain the observed larger variability. The results
emphasized the usefulness of a multi-center validation over a single center
approach.
A.146
VERIFICATION OF THE GEOMETRIC AND DOSIMETRIC ACCURACY OF THE
CYBERKNIFE SYNCHRONY SYSTEM
E. Mastella S. Vigorito E. Rondi G. Piperno A. Ferrar E. Strat
D. Rozza B.A. Jereczek-Fossa F. Cattan a Fondazione CNAO – Centro
Nazionale di Adroterapia Oncologica, Pavia, Italy; b IEO – Istituto Europeo di
Oncologia, Milano, Italy; c Università degli Studi di Milano, Milano, Italy
Purpose: To evaluate the geometric and dosimetric accuracy of the
CyberKnife Synchrony Respiratory Tracking System (RTS).
Material and Methods: An EasyCube phantom (Sun Nuclear) was mounted
on the ExacTrac (ET) Gating (Brainlab) moving phantom. Eight fiducial
markers were implanted in the EasyCube for the tracking. A radiochromic
EBT3 film (Ashland) was inserted inside the EasyCube (depth = 5 cm) and
irradiated with single fields perpendicular to the phantom. We compared
dynamic measurements (different motions of ET phantom) with static cases.
Our evaluation was performed in two steps: (1 )The field size (FS) and the
penumbra (P) of 6 secondary collimators (15–30–60 mm fixed and 20–
30–40 mm IRIS collimators) were measured along the two main orthogonal
directions. Dynamic cases were also delivered with purposefully simu-
lated errors (RTS switched off or low coverage of the respiratory correlation
model). (2) The delivered and planned dose distributions (from step 1) were
compared with the gamma analysis method. The local gamma passing rates
(GP) were evaluated using 3 acceptance criteria: 3%/3 mm dose threshold
(TH) = 10%, 2%/2 mm TH = 30% and 3%/1 mm TH = 50%.
Results: (1) The field parameters did not vary between the static and
dynamic cases along the left-right direction, while we observed a slight
enlargement of the FS and of the P along the SI direction (on
average < 1 mm). (2) The mean GP were 96.9 ± 1.8% (3%/3 mm), 94.2 ± 3.8%
(2%/2 mm) and 94.9 ± 3.1% (3%/1 mm) for static cases and 97.9 ± 1.4%,
95.4 ± 2.6% and 95.9 ± 2.3% for dynamic cases properly delivered. Only the
stricter distance-to-agreement (DTA) criterion drastically failed the test with
simulated errors.
Conclusion: The presented method provides additional measurements to
verify the geometric and dosimetric accuracy of the Synchrony RTS. Our
method confirms the ability of the RTS, if used properly, to treat a moving
target with great precision. However, we recommend using a tight DTA cri-
terion to evaluate delivery quality assurance plans.
A.147
DELIVERY QUALITY ASSURANCE FOR CYBERKNIFE TUMOR-TRACKING
TREATMENTS
E. Mastella S. Vigorito E. Rondi G. Piperno A. Ferrar E. Strat
D. Rozza B.A. Jereczek-Fossa F. Cattan a Fondazione CNAO – Centro
Nazionale di Adroterapia Oncologica, Pavia, Italy; b IEO – Istituto Europeo di
Oncologia, Milano, Italy; c Università degli Studi di Milano, Milano, Italy
Introduction: The aim of this work was to investigate the dosimetric ac-
curacy of the CyberKnife Synchrony Respiratory Tracking System (RTS) and
to validate a method for pre-treatment delivery quality assurance (DQA).
Materials and Methods: The CT scan of the EasyCube Phantom (Sun
Nuclear) was acquired according to our clinical protocol for CyberKnife pa-
tients. The EasyCube was mounted on the ExacTrac Gating Phantom
(Brainlab) which can move along the cranio-caudal direction to simulate
the tumor motion. Eight fiducial markers were implanted in the EasyCube
for the treatment set-up and for the tracking. A Gafchromic EBT3 film
(Ashland) was positioned between two slabs of the EasyCube, while a
PinPoint ionization chamber (PTW) was placed in an appropriate insert.
Six routine plans were recalculated on the EasyCube and centered in the
sensitive volume of the PinPoint. The DQA plans were delivered in differ-
ent dynamic conditions, for a total of 19 cases. The films were scanned using
an Epson 10000XL scanner (transmission mode, 48-bit, 96 dpi resolu-
tion) and analyzed with the multichannel film dosimetry performed by the
FilmQA Pro software (Ashland). The delivered and planned dose distribu-
e43 Abstracts/Physica Medica 32 (2016) e1–e70