Abstract
Introduction For Varian Medical Systems (VMS) accelerators with ”Eclipse” TPS, whatever the intensity modulated technique (IMRT or RapidArc) or type of MLC (M120 or M120HD), the latter is modeled for a given energy by two parameters: the transmission (T) and the Dosimetric Leaf Gap (DLG). This study presents the results obtained for a Novalis Truebeam STx (NTB) and Truebeam (TB) installed in 2015 at the Integrated Center for Oncology. Material and methods Two NTB and TB accelerators (6 MV) equipped with MLC M120HD and M120 respectively. (T, DLG) couples were determined in three steps: (i) Varian methodology [1] ; (ii) 1D ionometric measurements (CC13, IBA Dosimetry) in IMRT SW technique: sliding window, dynamic ”chair” and ”squares” with different level of doses, 10 IMRT SW and RapidArc plans; (iii) 10 IMRT SW and RapidArc plans analysis via radiochromic films (EBT3, Ashland) and Portal Dosimetry (PDIPv11.031, VMS). Results For NTB, T and DLG were set to 1.3% and 0.38 mm respectively. This result was determined from VMS methodology as good results were obtained in steps (ii) and (iii) (respectively 95% (3%/3 mm GA)). For TB, transmission was also determined from VMS methodology. DLG determination was not that simple because for a fixed DLG while acceptable results were obtained with IMRT SW technic, unacceptable results were obtained with RapidArc plans and vice versa. A compromise was found to match the tolerances regardless the type of plan: T and DLG were set to 1.5% and 1.45 mm respectively. These values are close to those found in publications [2] , [3] , [4] , [5] . Conclusion Transmission and DLG were determined per energy for each accelerator. T and DLG choice is a compromise between VMS methodology and measurements when both IMRT SW and rapidArc technics are implemented on the same accelerator. An extreme solution would be to set T and DLG in Eclipse per energy and technique.
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