ESTRO 36 Abstract Book

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S499 ESTRO 36 2017
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To note that, except for output constancy, the differences
in ranking order in the first positions are due to the
treatment techniques implemented on the two linacs:
VMAT for Unique, requiring accurate tests on dose
modulation and multi leaf collimator speed; treatment
with multiple isocenters and/or junctions between
adjacent fields for DBX, requiring accurate tests on couch
and jaw position indicators.
Conclusion
FMEA is a useful tool to optimize and prioritize the linac
QCs. It allowed to identify the more relevant tests for
patient safety by taking into account the specific
equipment, treatment modalities and clinical practice.
The variability and subjectivity of the FMEA scoring,
mostly caused by individual differences in risk perception
and professional experience of the involved physicists, can
be limited by a semi-quantitative analysis of each failure
mode and of the QC trend.
PO-0909 QA test of MLC speed using a fluorescent
screen-CCD based dosimetry system
B. Yang1, T.L. Chiu1, C.W. Cheung1, H. Geng1, W.W.
Lam 1, K.Y. Cheung1 , S.K. Yu1
1 Hong Kong Sanatorium & Hospital, Medical Physics and
Research Department, Happy Valley, Hong Kong SAR
China
Purpose or Objective
The purpose of this study is to demonstrate quality
assurance (QA) test on the speed accuracy of multileaf
collimator (MLC) which is crucial for intensity modulated
radiotherapy treatment (IMRT) modality, using a
fluorescent screen-CCD based dosimetry system.
Material and Methods
Our fluorescent screen-CCD based dosimetry system
consisted of a fluorescent screen sandwiched by two
transparent PMMA blocks and a low dark noise CCD
camera. The fluorescent screen was aligned
perpendicularly to the radiation beam line and the
fluorescent light was directed to the CCD camera by a 45º
mirror underneath. All components were assembled in an
L-shape light-tight box. The median filter was applied to
remove the radiation induced spike noise. Test delivery
plans with fixed 1cm MLC gap and constant movement
speed for both carriage A and B sliding from one side to
another were created for QA of MLC speed. During the
delivery of these plans, the CCD camera captured the
images continuously with a fixed exposure time 0.1s at its
maximum frame per second (fps) under different settings
of pixel binning. The maximum fps of our current system
is limited to 0.98, 1.61 and 3.11 under 1×1, 2×2 and 4×4
pixel binning setting which corresponds to a spatial
resolution of 0.259, 0.518 and 1.036 mm/pixel
respectively. By tracking the movement of the edge of
leaves, the speed could be calculated. Further the
machine trajectory log files were also analyzed for
comparison and t-test was performed to evaluate the
statistical significance between our measured speeds and
those calculated from log file.
Results
The calculated speed of leaf #30 for both carriage A and B
is listed in Table 1. By analyzing the machine log file, the
speed of the same leaf was calculated to be 25.00±0.10,
15.05±0.12 and 4.99±0.12mm/s for carriage B;
25.00±0.12, 15.05±0.11 and 4.99±0.13mm/s for carriage A
under nominal speed 25, 15 and 5mm/s respectively. Our
measured MLC speed for 1×1 pixel binning setting and that
extracted from log data are also plotted in figure 1. T-test
results show that the p values are all larger than 0.3,
which suggest the measured results are not statistically
distinguishable from log data and our measurement is
accurate compared with log data. Similar results were also
obtained for other leaves.
Conclusion
The fluorescent screen-CCD based dosimetry system can
serve as an independent and reliable tool for QA of MLC
speed, whose temporal resolution as a motion monitor can
be further improved by using the camera with higher fps.
PO-0910 Is Linac-Based Total Body Irradiation (TBI) on
the coach by VMAT Feasable?
B. Tas1, I.F. Durmus1, A. Okumus1, O.E. Uzel1
1Yeni Yuzyil University Gaziosmanpasa Hospital,
Radiation Oncology, Istanbul, Turkey
Purpose or Objective
In our study, we investigate the use of Linac-Based TBI by
VMAT tecnique at nominal SAD on the coach. Eight TBI
patient’s treatment planning were performed using
Monaco5.1® treatment planning system with dual arc
VMAT tecniques for each patient.
Material and Methods
For treating patients, Versa HD® (Crawley, Elekta) linear
accelerator with 6 MV, equipped with Agility® collimator
system, XVI 5.0 cone beam CT was used as a Image Guided
Radiation Therapy (IGRT) method for VMAT delivery.