ESTRO 36 Abstract Book

S503 ESTRO 36 _______________________________________________________________________________________________

(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.

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. Chiu 1 , C.W. Cheung 1 , H. Geng 1 , W.W. Lam 1 , K.Y. Cheung 1 , S.K. Yu 1 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

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.