ESTRO 38 Abstract book

S932 ESTRO 38

remarkable variations tended to remain without strong consensus across different energies and machine classes.

introduced such as adapting the imaging protocol, recalculating treatment plans on CBCT or a new CT, weekly weight control of patients at risk, adjusting rectal and bladder filling protocol. In 17% of cases no actions were taken. Due to the high resolution of transit images, most of the conventional portal images in non VMAT could be omitted. A learning curve was observed during feedback moments with staff, which resulted into a wide acceptance by repeated training using practical examples. Conclusion A standardized transit dosimetry program using a decision tree was introduced in a busy department. Some treatment and imaging protocols were adjusted based on this experience and pre-treatment imaging could be omitted for some treatments. After 6 months half of the observed deviations could be reduced, contributing to a wide acceptance level in our department. A detailed report of further experience using an adapted workflow on the first years’ experience will be presented as well as the preparatory work needed to establish a standardized QC program. EP-1729 Evaluation of beam modeling parameter variations among radiotherapy institutions using common TPS M. Glenn 1 , D. Followill 1 , R. Howell 1 , J. Pollard-Larkin 1 , S. Zhou 2 , S. Kry 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 2 The University of Texas MD Anderson Cancer Center, Biostatistics, Houston, USA Purpose or Objective Previous studies indicate that a major contributor to erroneous radiotherapy treatments is the inaccuracy of the dose calculation itself. This study evaluates the variation of several treatment planning system (TPS) beam modeling parameters to determine the degree to which the radiotherapy community shows consensus in how clinical beam models are established. Simultaneously, this study provides reference values for TPS parameters that can assist clinical physicists in the TPS commissioning process. Material and Methods Beginning January 2018, TPS beam modeling parameter surveys were distributed to users of IROC services through online facility questionnaires. These surveys, designed for Eclipse, Pinnacle, and RayStation users, instructed physicists to report parameter values used to model the radiation source and multileaf collimator (MLC) for each treatment machine and beam energy used clinically for IMRT. Parameters collected included the effective source/spot size, MLC transmission, dosimetric leaf gap, tongue and groove effect, as well as other nondosimetric parameters specific to each TPS. To facilitate survey participation, instructions were provided on how to identify requested beam modeling parameters within each TPS environment. Survey results were then isolated according to calculation algorithm, beam energy, machine class, and MLC type, and then examined for trends. Results To date, parameters for 2171 beam models from 533 radiotherapy institutions have been aggregated via online questionnaires. 76% of reported beam models were commissioned using Eclipse, 18% of responses were for Pinnacle, and 6% of responses reported values for RayStation beam models. Some parameters, such as the effective target spot size (Eclipse AAA and AcurosXB), exhibited very good uniformity (>75% reported the same value for a given machine class). Other variables presented broad distributions of values, especially for factors that are physically measured for the characterization of the MLC (e.g. MLC transmission and dosimetric leaf gap). These parameters that showed

Conclusion This study demonstrates that high variation exists in several TPS beam modeling parameters, thus highlighting the need for further exploration to determine what effects these discrepancies may have on dose calculations. These results can be employed by the radiotherapy community to compare parameter values obtained during commissioning to better inform what are considered reasonable values for model creation. EP-1730 Systematic Monte Carlo dose verification of VMAT treatment plans for TrueBeam linac using PRIMO A. Sottiaux 1 , V. Baltieri 1 , A. Monseux 1 , C. Leclercq 1 , D. Vanache 1 , M. Tomsej 1 1 CHU Charleroi, Radiotherapy, Montigny-leTilleul, Belgium Purpose or Objective Complex techniques like VMAT require appropriate QA to check if the predicted dose is delivered to patient, including TPS dose verification. Regarding dose verification, Monte Carlo methods are often considered as gold standard. PRIMO is a Monte Carlo software self-containing (geometry included), publically available and able to handle VMAT plan for TrueBeam linacs. The purpose of this study is to check if PRIMO is suitable for systematic VMAT plans dose verification. Material and Methods PRIMO v0.3.1 combines linacs geometry, Graphical User Interface and 2 Monte Carlo engines: PENELOPE/PenEasy and DPM (Dose Planning Method, optimized for radiotherapy). DPM is faster but less accurate in low density materials. Download, Installation and PRIMO setup is very easy. We used a PRIMO beta-version v0.3.1.1625 that provides 2 additional features: Macro mode and start from command line. Macro mode allows required task to simulate VMAT plan (load DICOM, set parameters, start simulation and perform Gamma Index (GI) analysis) to be executed with simple commands, without user action nor risk of error. We developed scripts to automate the process. After DICOM export from TPS, appropriate Macro file is generated, and simulation is launched (including GI) without any user action. As TrueBeam head’s geometry (beam production) is not implemented in PRIMO, Phase Space Files provided by Varian are used. They are designed to match TrueBeam standard tuning. We compared PRIMO simulation against PDD measurements for 3x3 cm 2 and 10x10 cm 2 beams, on 2 different TrueBeam. 50 VMAT plans (calculated with Eclipse/Acuros 13.7, dose to medium) have been compared for various cases and PTV size. All plans were calculated with DPM, and 8 recalculated with PENELOPE because they failed our criteria with DPM. We performed GI (3%/3mm and 2%/2mm, 95% passing