ESTRO 37 Abstract book

S933

ESTRO 37

Finally, the total dose is obtained by summing the 3D dose associated for each gantry angulation. The estimated dose from the EPID was compared to the one calculated by the TPS (Treatment Planning System) with a local and global gamma index of 3% and 3mm also developed with Matlab. Our algorithm has been tested for 20 IMRT and VMAT prostate and head and neck treatments in a homogeneous phantom (cylinder of 25 cm diameter). Results The global gamma index obtained is greater than 99% and more than 90% for local gamma index (3%,3mm). Conclusion We have developed and validated a reconstruction algorithm that can be used to verify the 3D dose distribution for the IMRT and VMAT fields from in vivo EPID images. EP-1741 X-ray/Light field Congruence with varying gantry angle using the EPID F. Tato de las Cuevas 1 , J. Yuste López 1 , F. Fernández Belmonte 1 , I. Ribót Hernández 1 1 Hosp. Univ. de Canarias, Medical Physics Dept., Santa Cruz de Tenerife, Spain Purpose or Objective To develop a method to perform X-ray/Light field Congruence ( XLC ) QA using the EPID (Electronic Portal Imaging Device). The method must be fast, accurate and capable of make the analysis for different gantry angles. Material and Methods The LINACs: 1) CLINAC 2100 with Millenium MLC with 6 MV and an EPID with 512x384 pixels, 2) Elekta Synergy with Agility MLC, 6 MV and EPID with 1024x1024 pixels. Field size used is 20x20 cm. A jig is made with a Bearing ball (BB) in its centre and 4 moving radiopaque discs. The jig is aligned with the crosshair light projection over the EPID, the discs are placed inside and tangent the light field. The moving discs allow to work with nominal field size and permit the images to be acquired in clinical mode and exported from the network easily. The images are analysed with an In- House Software ( IHS ): - Rotation correction of the image is performed (when angle > 0.1º). - Radiation field limits are obtained with the 50 % pixel value (PV). - 3 profiles parallel to every field edge are acquired. Discs edge points are obtained with subpixel precision from the 50 % PV. These six disc edge points are used to obtain geometrically every disc centre. Light field edge is obtained from the nominal disc radio and centre. - The BB position is obtained from Gaussian curve fit of cross-profiles over the max PV in central ROI, this gives the cross-hair light field projection over the EPID. Validation of IHS 1. Image J software and IHS are used to perform XLC measurements of 10 EPID Elekta images. XLC analysis is made with RIT113 software and Vidar VXR-16 scanner. Every operator marks each light field edge twice, punching the envelope with a sharp needle. Radiation field edge is determined with the 50% of the central dose. XLC is measured with RIT software tools. LINACs QA EPID images are acquired following the steps described above. 15 tests are made in each LINAC (one test/day) for 3 gantry angles (0, 90, 270º), i.e.: 45 images per LINAC. Results IHS validation 1. The image analysis with the algorithm (IHS) and with the external software (Image J) gives discrepancies in XLC of less than one pixel. 2. Fig. 1 shows the congruence spread is slightly bigger 2.

for the film than for the IHS .

LINACs QA The acquisition and analysis process for one image take less than 10 min. Relative shifts in X-ray/Light field congruence due to change of gantry angle are negligible compared with the error spread (Fig. 2). The XLC results for all gantry angles are under tolerance for both LINACs (< 2mm). Conclusion The jig and algorithm developed to perform the X- ray/Light field congruence test with EPID have proved to be at least as accurate as film. The method is also useful and accurate to make the test at different gantry angles, being also reasonably fast. EP-1742 Automatic brachytherapy source localization using a fluorescent screen-based optical detector W. Hrinivich 1 , A. Robinson 1 , M. Morcos 1 , B. Yi 2 , J. Wong 1 1 Johns Hopkins Kimmel Comprehensive Cancer Center, Radiation Oncology and Molecular Radiation Sciences, Baltimore, USA 2 University of Maryland, Radiation Oncology, Baltimore, USA Purpose or Objective To demonstrate the feasibility of automatic high-dose- rate (HDR) brachytherapy source dwell position identification and localization using a fluorescent screen- based optical detector for applicator quality assurance. AAPM task-group report 56 recommends routine verification of source dwell positions within brachy- therapy applicators, which is typically performed using radiosensitive film. Exposing and digitizing film can be time consuming and manual source localization can be user-dependent. A system for automatically identifying