Abstract Book

S933

ESTRO 37

and localizing dwell positions will reduce the time required for routine applicator quality assurance relative to film-based measurement. Material and Methods The Raven detector system (LAP Laser, Germany) is an optical and radiosensitive tool designed for EBRT quality assurance consisting of a fluorescent screen imaged using a CCD camera with variable signal integration time. For HDR brachytherapy measurements, the CCD camera was set to integrate signal for 100 ms per image (10 fps). An algorithm was implemented in Matlab (Mathworks, USA) to automatically detect and localize source dwell positions within the resultant images by analyzing changes in signal intensity between consecutive frames. To demonstrate feasibility, a plastic interstitial applicator was attached to the surface of the fluorescent screen and connected to an Ir-192 Flexitron afterloader (Elekta, Sweden) with 24.0 mGy m 2 h -1 source strength. For comparison, a piece of EBT3 radiochromic film (Gafchromic, USA) was placed between the applicator and fluorescent screen. A treatment plan was delivered consisting of four dwell positions with 1.0 cm spacing and uniform 6.8 s dwell times. Following delivery, the dwell positions were automatically identified using the Raven images and custom software. The film was digitized using a flatbed scanner and registered to the Raven coordinate system using fiducial marks using 3D Slicer. The automatically-identified Raven-based dwell positions were compared to the manually-identified film-based dwell positions in terms of 2D Euclidean distance. Results Figure 1 displays the co-registered film and four Raven images with horizontal lines for visual reference, and Table 1 summarizes the manually and automatically identified dwell positions relative to film-based dwell position 1. The mean±SD 2D distance between the Raven and film-based dwell position centers was 0.4±0.1 mm.

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 brachytherapy 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

Conclusion We have demonstrated the feasibility of automatically identifying brachytherapy source positions using a fluorescent screen-based optical detector. Systematic differences in dwell positions between film and Raven may be due to residual film registration error or bias in the automatic localization algorithm. We will present further characterization of device performance for brachytherapy applicator quality assurance. EP-1743 Comparison of IBA Stealth with CC13 for use as reference detector in beam data scanning G. Beyer 1 , N. Hindocha 2 , R. Paiva 1 , N. Abushena 2 , R. Patel 2 , D. Mateus 1 1 Medical Physics Services Intl Ltd, Medical Physics, Cork, Ireland 2 University College London Hospitals, Radiotherapy, London, United Kingdom Purpose or Objective The purpose of this work was to evaluate the feasibility of using the IBA Stealth chamber as a reference detector in beam data scanning for different field sizes.