ESTRO 36 Abstract Book

S504 ESTRO 36 _______________________________________________________________________________________________

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 γ- index (%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. Suchowerska 1 , V. Peng 1 , L. Rogers 1 , E. Claridge- Mackonis 1 , D.R. McKenzie 2 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 Tx TM HD120-MLCs were used to generate grid patterns of 2.5mm and 5.0mm spacing, which were dosimetrically characterized using Gafchromic TM 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

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. 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.