ESTRO 36 Abstract Book

S961 ESTRO 36 _______________________________________________________________________________________________

All fields acquired in the study have 20x20 cm size, 5 MU each. To obtain the RC of the EPID at 0º gantry, four fields are used, at 0, 90, 270 and 180 º of collimator and 0º gantry with a radiopaque crosshair attached to the LINAC head. RC is calculated with two methods: using the radiation field limits and with the radiopaque crosshair center. Second series of measures are acquired with a BB (bearing ball) placed in laser isocenter and with a tray with four smaller BB fixed in it, in the periphery of the field. Images are obtained over a 360º arc, with 15 º gantry steps at 0º collimator angle. Cross reference of the 4 smaller BB positions with the RC (determined in the first step) are made at 0º collimator and 0º gantry. This gives to the 4 BB the ability to determine RC position in subsequent gantry angles. EPID sag is calculated for all gantry angles taking into account the laser isocenter BB position in each EPID image and compared with 0 º gantry angle. EPID + Gantry sag is determined taking into account the mean position of the BB fixed in the tray. The Gantry sag is obtained after subtraction of EPID sag from EPID + Gantry sag. Changes in SDD (Source-Detector Distance) are obtained measuring the distance between two tray BB (d) and comparing then to the distance (d 0 ) for SDD for 0º gantry angle (SDD 0 ) in the way as Eq. reflects. -1) A MATLAB in-house software is developed to make the image analysis. The BBs and the center of radiopaque crosshair is determined in each direction (in-plane and cross-plane) with sub-pixel accuracy, 3 profiles near de BB are obtained and fitted to Gaussian curves, the mean maximum of the 3 curves is calculated. Radiation field center is obtained calculating the 50% pixel value of a vertical and horizontal profile displaced from the center in case of BB in the image center. Results The LINAC measurements take no longer than 2 hours. The RC for the EPID at 0º gantry obtained with radiopaque crosshair is 1.11 and -1.02 mm for cross-plane and in-plane directions, respectively. The RC using radiation field limits is less than 0.3 mm away from this. EPID RC is not plotted in Fig. 1 for clarity but is obtained from the EPID + gantry sag measurements after adding the RC for 0º gantry angle. ΔSDD = SDD 0 · (d/d 0

Conclusion The method presented is a useful, necessary and not too time expending tool to characterize the EPID and Gantry sag of a LINAC when EPID will be used in LINAC QA. EP-1746 A new method for exact co-calibration of the ExacTrac X-ray system and linac imaging isocenter H.M.B. Sand 1 , K. Boye 2 , T.O. Kristensen 1 , D.T. Arp 1 , A.R. Jakobsen 1 , M.S. Nielsen 1 , I. Jensen 1 , J. Nielsen 1 , H.J. Hansen 1 , L.M. Olsen 1 1 Aalborg University Hospital, Department of Medical Physics- Oncology, Aalborg, Denmark 2 Zealand University Hospital, Radiotherapy Department, Næstved, Denmark Purpose or Objective To evaluate a new user independent sub-millimetre co- calibration method between the X-ray isocenter of the ExacTrac® (ET) system and the imaging isocenter of the linear accelerator (linac). Material and Methods The new calibration method was evaluated on five linacs from Varian, three Clinacs with the On Board Imager system and two TrueBeams, all equipped with ET and robotics from Brainlab. A BrainLAB isocenter calibration phantom with five infrared markers attached on the top and a centrally embedded 2 mm steel sphere was used in the setup. Orthogonal MV-kV-image pairs of the calibration phantom were acquired at the four quadrantal gantry angles using the linac imaging system (LIS). In- house made software detected the 2D position of the steel sphere in each acquired image and from this determined the 3D couch translation required to move the steel sphere to the LIS isocenter. To accurately perform the translation, we applied the sub-millimetre real-time readout feature of the ET infrared system, which was set to track the infrared markers of the phantom. Subsequently, the origin of the ET system was calibrated to match the optimal phantom position and hence the LIS isocenter. Regular runs of the Varian IsoCal-routine assured correspondence between the radiation isocenter and the LIS isocenter . In the standard calibration method former used, the calibration phantom was positioned based on one set of MV-kV-images, manually interpreted by the user. Results The deviation between the ET X-ray isocenter and the LIS isocenter was determined by evaluating the 3D deviation vector for the new user independent optimal positioning of the calibration phantom relative to the LIS isocenter. In ten successive calibrations performed by different users in a time period of nearly half a year, the 3D deviation vector ranged from 0.03 mm to 0.10 mm with an average of 0.07 mm and a standard deviation (SD) of 0.02 mm. Simultaneously, the 3D deviation vector was determined for the standard calibration method, also in ten successive calibrations and performed by different users. Here the 3D

The major change in SDD is less than 1.4 cm (for 180º) from SDD at 0º gantry angle (see Fig. 2).