ESTRO 36 Abstract Book

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S503 ESTRO 36
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(RPN) was calculated from the product of three indexes:
likelihood of occurrence (O), severity of effect (S) and lack
of detectability (D). Forty tests were examined just above
the expected tolerance levels and indexes O, S, and D
were scored from 1 (lowest risk) to 10 (highest risk) using
two methods:
1) A survey was submitted to each of the medical
physicists of our institute involved in the linac QC
2) The QC data over a period of three years were analyzed
and some FMs were simulated with the treatment planning
system.
The average RPN for each test was obtained taking into
account both the methods. For each linac, the tests were
then sorted by their frequency (daily, monthly or annual)
and RPN value.Two different Varian linacs (DBX, Unique)
were considered, the first used only for conformal therapy
and the second one used essentially with volumetric
modulated arc therapy (VMAT) technique.
Results
A high variability was found in the O-D-S scores of the
survey, as shown in the box plots of figure 1 for the
dosimetric tests of the Unique linac. Nevertheless, a lower
variability was obtained for RPNs, highlighting at the same
time the more relevant tests.
Both the FM simulations and the analysis of the QC trend
allowed to reduce the subjectivity of the FMEA score.
Integration of both evaluations provided the RPN-based
ranking of tests: an example is shown in figure 2 for
monthly tests for DBX and Unique linacs.
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. Yang 1, T.L. Chiu1, C.W. Cheung1, H. Geng1, W.W.
Lam 1, K.Y. Cheung1, S.K. Yu 1
1Hong 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