Abstract Book

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ESTRO 37

possibility of using of Catalyst™ system to replace CBCT- based IGRT. Material and Methods Set-up displacements from Catalyst TM (C-RAD, Uppsala, Sweden) and CBCT were compared for a total of 928 set- ups in 52 patients. Amongst them, 15 patients treated in the head and neck (A), 18 patients in the thorax (B) and 10 patients in abdomen (C), all patients in these three groups were fixed with the thermoplastic film. In addition, there are 9 patients treated in abdomen-thorax were fixed with vacuum pad, 5 of them with free breath (D), the other 4 patients used active breathing control (ABC) (E). Patients were using CBCT at first positioned and then scanned with the Catalyst TM , surface data captured by Catalyst TM at the first treatment fraction (CSref) was used as a reference for the Catalyst TM , while bony structures from the planning CT were used as a reference for the CBCT method. As comparison, the patient outline extracted from the planning CT was used as Catalyst TM system reference (CTref). The displacements detected by the CBCT was also used for comparison. Results The mean differences and standard deviation of 52 patients between the Catalyst TM and CBCT displacements in 6 dimensions direction (lateral/longitudinal/vertical/rotation/roll/pitch) were - 0.24±3.64mm, 1.63±5.88mm, 0.58±3.41mm, -0.11±1.55°, 0.05±1.40° and 0.10±1.14°. Which in group A were - 0.16±2.67mm, -0.81±2.31mm , 0.56±2.53mm, 0.40±1.36°, -0.16±1.33° and 0.36±1.17°; in group B were 0.00±4.08mm, 0.22±5.57mm, 0.46±3.04mm, -0.36±1.43°, 0.23±1.26° and -0.10±1.02°; in group C were 0.03±4.01mm, 4.90±8.07mm , 0.50±3.46mm, - 0.10±1.10°, 0.29±1.66° and 0.19±1.14°; in group D were - 2.36±5.14mm, 4.57±7.81mm , 1.91±4.45mm, - 1.45±1.96°, 0.50±1.26° and -0.44±1.09°; in group E were -0.23±2.34mm, 0.74±3.56mm, -0.01±2.76mm, 0.46±2.52°, 0.92±1.30° and -0.17±1.21°, respectively. Compared with CBCT, the Catalyst TM for radiotherapy guided placement in the head and neck region and abdmen–thorax (application with ABC) are more accurate (0.70

for MR-Linac devices, since it allows for online 3- dimensional (3D) dose evaluation. The PG consists of monomers embedded within a gelatine matrix. After irradiation, the gel polymerizes locally and alters its local relaxation rates R 2 depending on the absorbed dose. Two types of irradiation experiments were performed: (1) a star shot measurement performed with a clinical linear accelerator (Artiste, Siemens Healthineers, Erlangen, Germany) with 9 equidistant gantry angles on EBT3 film and PG. The PG was scanned at different times (1 h, 24 h, 48 h and 216 h) after irradiation and compared to film. Both dosimeters were evaluated with the commercial software Mephisto (PTW, Freiburg, Germany). (2) A star shot performed within an electro magnet (AGEM 5520, Schwarzbeck Mess-Elektronik, Schönau, Germany) with and without applied magnetic field of 1 T. Due to the positioning of the electro magnet, the gantry rotation was limited to angles between 260° and 80°. To obtain a comparable beam arrangement as for (1) the irradiation was performed in two steps: After irradiating the upper hemisphere (gantry angles of 288°, 0° and 72°), the gel containers and films were rotated manually by 180° and the remaining beams were irradiated also from the upper side to mimic an irradiation from gantry angles of 216° and 144°. Results (1) No significant changes over time for the radius of the iso-circle (smallest circle that intersects with all beam axes) of the PG (0.27 ± 0.02 mm (1 h), 0.27 ± 0.03 mm (24 h), 0.26 ± 0.04 mm (48 h) and 0.26 ± 0.04 mm (216 h)) were found. Gel and film measurements show comparable results (0.26 ± 0.02 mm). (2) For the measurements inside the electromagnet the iso-circle radii showed a comparable increase from 0.39 ± 0.01 mm to 1.37 ± 0.01 mm for the film and from 0.44 ± 0.02 mm to 0.97 ± 0.02 mm for the PG-measurements, when a magnetic field of 1T was applied. Conclusion Polymer gels may be employed to measure the isocenter accuracy of clinical irradiation devices. As an important result, geometric evaluation of the polymer gel directly after irradiation is feasible. Although the used polymer gel requires a higher effort than films, they allow for a simultaneous measurement of the radiation and imaging isocenter accuracy of MR-Linac devices in 3D. EP-2172 Positioning using six degrees of freedoms. Does the ExacTrac X-ray system and a CBCT scan agree? H.M.B. Sand 1 , T.O. Kristensen 1 , M. Nielsen 1 1 Aalborg University Hospital, Department of Medical Physics, Aalborg, Denmark Purpose or Objective The need of a precise, quick and low-dose bony six degrees of freedoms (6DOF) repositioning followed by a tumor-match may be of clinical relevance i.e. in advanced lung-radiotherapy, when you treat more than one target-location and where tumor deformation in general is correlated to the precision of the bony positioning. The aim is to evaluate the agreement of the 6DOF positioning performed by the ExacTrac® (ET) X-ray system and a subsequent Cone Beam Computed Tomography (CBCT) scan recorded by the On Board Imager (OBI) system on a linear accelerator (linac). Material and Methods The 6DOF positioning was obtained, performed and evaluated on a Clinac from Varian, equipped with Varian OBI system, ET and a robotic 6DOF couch from Brainlab. A thorax-phantom with an inner skeleton was CT-scanned and transferred to the OBI- and ET-systems, respectively. On the couch, the phantom was misaligned compared to the CT-scan in nine successive different test positions. The misalignments were performed in the three translational dimensions (vert, lng, lat) and in the three