ESTRO 36 Abstract Book

S954 ESTRO 36 2017 _______________________________________________________________________________________________

device (EPID) and make this process as fast and accurate as possible. Material and Methods The LINAC is an Elekta Synergy with Agility MLC and 6 MV photons. A software is developed in MATLAB with some remarkable points: 1. Elekta iCOMCAT software was employed to generate and send the strip-test with multiple segments as a unique treatment, as is much faster than creating and irradiating a beam for each segment. With the software of Elekta iView is difficult to acquire a complete image of each full segment as this is not fast enough, so fluency corrections of these segments were performed, in order to avoid erroneous pixel values (PV) in the way: a) In a 23x23 open field is acquired a horizontal profile and measure the % PV (in the center position of each future segment), this % is related to the PV of the position of a reference segment. b) Measure the mean PV in the center of each strip-test segment, and obtain the % PV related to the reference segment PV. c) Rescale the image of each segment in order to obtain the % PV (respect the reference segment). Finally make the sum of all images.

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Segments of 2 x 20 cm (cross-plane x in-plane) to form series of strip-test images with gaps overlapping from 1.2 to 3 mm are acquired for taking the MLC reference after calibration. The strip-test need bigger gap spread than other MLC in order to detect the gap position correctly, because of the lower penumbra. To correct the collimator angle is used the filtered back projection method, because is very tricky to use the interleaf leakage, as this MLC have much lower interleaf transmission than other MLC, like Millenium (from Varian). To localize the radiation center (RC) of the EPID is used a LINAC tray with centered radiopaque mark. Four 20x20 fields are obtained with this tray at 4 collimator angles. RC is determined for gantry 0º detecting the mark position in each image and obtaining the mean. A vector displacement is created to obtain RC with one image at 0º collimator. Tray images for various gantry angles at 0º collimator are acquired, so that with just one tray image is enough to detect RC exactly. This method is faster than using field edges, where at least 2 images at different collimator angles must be acquired for each gantry angle.

Results The differences in leaf positions compared with film and light field are beyond 0.1 mm and 1 mm (light field edge detection has a much bigger uncertainty). The acquisition and analysis for one strip-test take less than 4 min. Conclusion The methodology employed analyzes a MLC strip-test in an Elekta LINAC in a fast and accurate way. EP-1758 Towards Clinical Implementation of an Online Beam Monitoring System M. Islam 1 , M. Farrokhkish 2 , Y. Wang 2 , B. Norrlinger 2 , R. Heaton 1 , D. Jaffray 1 1 Princess Margaret Cancer Centre and University of Toronto, Medical Physics, Toronto, Canada 2 Princess Margaret Cancer Centre, Medical Physics, Toronto, Canada Purpose or Objective Continual advancement of Radiation Therapy techniques and consequent complexity in planning and delivery require constant vigilance. To address this, the idea of independent real-time beam monitoring has been proposed. In this presentation, we describe initial steps towards introducing the Integral Quality Monitoring (IQM) system into clinical practice. Material and Methods The IQM system (manufactured by iRT, Germany) consists of a large-area ion chamber mounted at Linear Accelerator’s (Linac) accessory slot, which provides a spatially dependent “dose -area- product” per field segment. The system monitors beam delivery in real-time by comparing the expected and measured signals. Initial evaluation of the system included: reproducibility and stability, agreements between calculated vs. measured signals, sensitivity and specificity for errors. A multiphase approach was considered for clinical implementation. First, IQM data are collected during conventional dosimetric QA tests. Results of the QA tests with and without the IQM chamber in the beam are compared. During this phase a reference dataset of measured IQM signals vs. calculated is generated to help determine the tolerance in the predicated signals. Second, IQM system is introduced as the primary pre-treatment QA tool. Gain in work-flow efficiency and

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Measurements of leaf positions using light projection are made. Also are obtained strip-test with films and analyzed with RIT software.