ESTRO 36 Abstract Book

S499 ESTRO 36 2017 _______________________________________________________________________________________________

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.

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

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. Tas 1 , I.F. Durmus 1 , A. Okumus 1 , O.E. Uzel 1 1 Yeni 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.