ESTRO 36 Abstract Book

S947 ESTRO 36 2017 _______________________________________________________________________________________________

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 Thanks to this analysis, the steps of increased risk have been discovered and new quality management tools have been proposed. However, the multidisciplinary team will perform this analysis periodically to increase the safety and quality of TSEI treatment. EP-1745 EPID and Gantry sag characterization in Elekta LINAC F. Tato de las Cuevas 1 , J. Yuste Lopez 1 1 Hosp. Univ. de Canarias, Medical Physics Dept., Santa Cruz de Tenerife, Spain Purpose or Objective The EPID (Electronic Portal Imaging Device) is a well- known useful tool for LINAC QA (like MLC QA). The position of the EPID radiation center ( RC ) is crucial for this kind of tasks. The purpose of this study is analyse the mechanical performance of EPID and Gantry for LINAC QA with EPID. Material and Methods The LINAC used is an Elekta Synergy equipped with Agility MLC and iViewGT EPID and 6 MV photons energy.

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