ESTRO 2020 Abstract Book

S708 ESTRO 2020

scanner was used to obtain a digitized image of the film with a resolution of 150DPI, corresponding to a pixel spacing of 0.17 mm. A homemade software (Matlab, The MathWorks Inc.) was used to apply a threshold to the film image and identify fifteen regions of interest (ROI); the beam central axis position at film plane was automatically calculated as the barycenter of the ROIs irradiated with the same couch rotation. The mean distance between beam central axis positions and couch rotation axis, determined through two complementary couch angles (90° and 270°), was assumed as metric to quantify the amount of the induced shift (fig .2). Results were compared to MPC data, performed with the Enhanced Couch option activated. In this configuration, MPC determines the center of rotation through a motion on all available couch rotational axes; the rotation-induced couch shift measured with MPC is the offset of the center of rotation and the radiation isocenter. The sensibility of the film-based method was determined by processing a film having the same ROI pattern, which was obtained irradiating the film with fixed couch angle. The repeatability was evaluated processing three consecutive scans of the same film.

between MPC and films. The mean difference between the two outcomes was -0.01±0.03 mm and -0.02±0.05 mm for TB STX and TB, respectively. Conclusion We demonstrated the validity of MPC tool for evaluating couch rotation-induced shift using an independent method based on films. Since the overall MPC procedure is less cumbersome than film handling, it represents an ideal tool for daily QA, especially when high accuracy in the LINAC geometric setup are mandatory. PO-1339 Initial preclinical EPID dosimetry results for small animal radiotherapy A. Anvari 1 , A. Sawant 2 1 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA ; 2 University of Maryland, Department of Radiation Oncology, Baltimore, USA Purpose or Objective The accuracy of dose delivery in the preclinical small animal image-guided (SA-IGRT) systems depends on a number of operator- and system-related factors that can individually or cumulatively contribute to significant dosimetric errors. Aim of this work is to initiate an independent dose verification technique using electronic portal imaging device (EPID) to verify the delivered dose to small animal during radiation therapy. Material and Methods We calibrated a camera-based EPID on the small animal radiation research platform (SARRP, Xstrahl, Inc.) to measure transit dose at the imager plane. Transit dose was back-projected at object’s exit surface to determine exit dose. First, we validated developed EPID dosimetry technique against film measurements through homogeneous and heterogeneous phantoms. Second, we implemented this technique in administered image-guided radiotherapy fraction for 20 rats with prostate cancer. We compared the planned dose distributions calculated by the treatment planning system (TPS) with delivered dose measured by the EPID. Results Transit central axis dose values measured with the EPID showed close agreement with film measurements, differences were within 4.9%. For inhomogeneous phantom, the EPID and film exit dose measurements agreed within ≤2%. For animal study, average difference between TPS and EPID was 3%, with a maximum of 9.3%. Gamma analyses for exit dose verification between TPS- calculated and EPID-measured dose distributions showed average 90% passing rate under global 2mm/5% gamma criteria. Conclusion We implemented a kilovoltage EPID dosimetry technique to verify accuracy of SA-IGRT to improve fidelity of preclinical radiation research. Beyond the current scope, we expect that this framework may be applied to any SA- IGRT system that incorporates an EPID. PO-1340 Optical surface tracking for non-coplanar SRS treatments verified by film in a RandoAlderson phantom M. Oellers 1 , A. Swinnen 1 , F. Verhaegen 1 1 MAASTRO Clinic, Medical Physics, Maastricht, The Netherlands Purpose or Objective To perform a dosimetric verification with EBT-XD film of a non-coplanar plan for multiple brain metastases monitored by a commercial optical surface tracking system (OST). Material and Methods A 3-camera OST system was used (Catalyst HD TM , C-RAD, Sweden) on a Varian Truebeam STx linac with a 6DoF couch. The set-up accuracy and agreement between the OST system and cone beam CT (CBCT) and kV-MV imaging at respectively couch angles 0 o and 270 o were examined.

Results A sensibility of 0.02 mm was achieved by measuring the rotation-induced couch shift for the film exposed with fixed couch angle. The repeatability resulted within 0.01 mm. The table reports the offsets measured with film and MPC, performed in the same day, for three different days of measurement. Results showed a good agreement