- Email: [email protected]
PO-0909: QA test of MLC speed using a fluorescent screen-CCD based dosimetry system
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.
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.
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. Lam1, 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
S504 ESTRO 36 _______________________________________________________________________________________________
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 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.
Agility® collimator system included 160 MLC, minimum leaf width was 5 mm. MLC effective speed was 6.5 cm/sec and leaf travel was 15cm over the central axis. VMAT plans were generated on Monaco 5.1® (Crawley, Elekta) treatment planning system with Monte Carlo algorithm. All calculation parameters were grid spacing 0.3 cm, minimum segment width 1.0 cm, Max. 180 of control Points Per Arc, Fluence smoothing medium, Statistical Uncertainty 1% per plan, increment of gantry 30° and dose to medium. The VMAT-TBI tecnique consisted of three isocentres and three dual overlapping arcs from top of head to the bottom of pelvis region. The prescribed dose was 90% of target volume receiving dose of 12Gy. Mean dose to lung and kidney were restricted less than 10Gy and maximum dose to lens were restricted less than 6Gy. The plans were verified using 2D array IBA Matrixx® and CC13 ion chamber. The comparison between calculation and measurement were made by γ-index (3%-3mm) analysis and absolute dose measurement at the isocentre. Results An average total delivery time was determined 923±34 seconds and an average monitor unit (MU)s was determined 2614±231MUs for dual arc VMAT technique. When we evaluated organ at risk(OAR)s, mean dose to lungs was 9.7±0.2Gy, mean dose to kidneys was 8.8±0.3Gy, maximum dose to lens was 5.5±0.3Gy and maximum point dose was 14.6±0.3Gy, HI of PTV was 1.13±0.2, mean dose to PTV was 12.6±0.15Gy and mean γindex (%3-3mm) pass rate was %97.1±1.9. Absolute doses were measured by CC13 ion chamber and we determined %2.0±0.6 dose difference between measurement and treatment planning system's (TPS) calculation at the isocentre. Conclusion The results show that dose coverage of target and OAR’s doses are feasible for TBI using VMAT tecnique on the coach. A benefit could be demonstrated with regard to dose distribution and homogeneity and dose-reduction to organs at risk. Additionally,we determined highly precise dose delivery by patient QA and point dose measurement at the isocentre. Based on the dose distributions we have decided to plan TBI in our clinic with dual arc VMAT technique on the treatment coach. PO-0911 Can the therapeutic benefits of microbeam radiation therapy be achieved using a clinical linac? N. Suchowerska1, V. Peng1, L. Rogers1, E. ClaridgeMackonis1, D.R. McKenzie2 1 Chris O'Brien Lifehouse, Radiation Oncology, Camperdown- Sydney, Australia 2 University of Sydney, School of Physics, Sydney, Australia Purpose or Objective The increasing availability of high definition multileaf collimators (HDMLCs) with 2.5mm leaves provides an opportunity for ‘grid’ therapy to more closely approach the clinical outcomes of Microbeam Radiation Therapy (MRT). However, periodic spatial modulation of the dose in the target volume runs counter to current clinical practice. To optimize the modulation, a better understanding of cell dose responses to such treatments is needed. The aim of this study is to determine if some of the therapeutic benefits of MRT can be achieved using a clinical linac with HDMLCs and if so, to develop a predictive model to optimize the benefits of such treatments. Material and Methods Varian Novalis TxTM HD120-MLCs were used to generate grid patterns of 2.5mm and 5.0mm spacing, which were dosimetrically characterized using GafchromicTM EBT3 film [Figure 1]. Clonogenic survival of normal (HUVEC) and cancer (lung NCI-H460, breast HCC-1954, melanoma MM576) cell lines were compared in vitro for the same
(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.
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.
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. Lam1, 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
S504 ESTRO 36 _______________________________________________________________________________________________
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 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.
Agility® collimator system included 160 MLC, minimum leaf width was 5 mm. MLC effective speed was 6.5 cm/sec and leaf travel was 15cm over the central axis. VMAT plans were generated on Monaco 5.1® (Crawley, Elekta) treatment planning system with Monte Carlo algorithm. All calculation parameters were grid spacing 0.3 cm, minimum segment width 1.0 cm, Max. 180 of control Points Per Arc, Fluence smoothing medium, Statistical Uncertainty 1% per plan, increment of gantry 30° and dose to medium. The VMAT-TBI tecnique consisted of three isocentres and three dual overlapping arcs from top of head to the bottom of pelvis region. The prescribed dose was 90% of target volume receiving dose of 12Gy. Mean dose to lung and kidney were restricted less than 10Gy and maximum dose to lens were restricted less than 6Gy. The plans were verified using 2D array IBA Matrixx® and CC13 ion chamber. The comparison between calculation and measurement were made by γ-index (3%-3mm) analysis and absolute dose measurement at the isocentre. Results An average total delivery time was determined 923±34 seconds and an average monitor unit (MU)s was determined 2614±231MUs for dual arc VMAT technique. When we evaluated organ at risk(OAR)s, mean dose to lungs was 9.7±0.2Gy, mean dose to kidneys was 8.8±0.3Gy, maximum dose to lens was 5.5±0.3Gy and maximum point dose was 14.6±0.3Gy, HI of PTV was 1.13±0.2, mean dose to PTV was 12.6±0.15Gy and mean γindex (%3-3mm) pass rate was %97.1±1.9. Absolute doses were measured by CC13 ion chamber and we determined %2.0±0.6 dose difference between measurement and treatment planning system's (TPS) calculation at the isocentre. Conclusion The results show that dose coverage of target and OAR’s doses are feasible for TBI using VMAT tecnique on the coach. A benefit could be demonstrated with regard to dose distribution and homogeneity and dose-reduction to organs at risk. Additionally,we determined highly precise dose delivery by patient QA and point dose measurement at the isocentre. Based on the dose distributions we have decided to plan TBI in our clinic with dual arc VMAT technique on the treatment coach. PO-0911 Can the therapeutic benefits of microbeam radiation therapy be achieved using a clinical linac? N. Suchowerska1, V. Peng1, L. Rogers1, E. ClaridgeMackonis1, D.R. McKenzie2 1 Chris O'Brien Lifehouse, Radiation Oncology, Camperdown- Sydney, Australia 2 University of Sydney, School of Physics, Sydney, Australia Purpose or Objective The increasing availability of high definition multileaf collimators (HDMLCs) with 2.5mm leaves provides an opportunity for ‘grid’ therapy to more closely approach the clinical outcomes of Microbeam Radiation Therapy (MRT). However, periodic spatial modulation of the dose in the target volume runs counter to current clinical practice. To optimize the modulation, a better understanding of cell dose responses to such treatments is needed. The aim of this study is to determine if some of the therapeutic benefits of MRT can be achieved using a clinical linac with HDMLCs and if so, to develop a predictive model to optimize the benefits of such treatments. Material and Methods Varian Novalis TxTM HD120-MLCs were used to generate grid patterns of 2.5mm and 5.0mm spacing, which were dosimetrically characterized using GafchromicTM EBT3 film [Figure 1]. Clonogenic survival of normal (HUVEC) and cancer (lung NCI-H460, breast HCC-1954, melanoma MM576) cell lines were compared in vitro for the same