ESTRO 37 Abstract book
S1202
ESTRO 37
guided-RT devices require new quality assurance (QA) methods. One important aspect is the development of new methods for machine QA-procedures, especially the simultaneous measurement of the radiation and imaging isocenter. For this purpose we demonstrate the feasibility of a polymer gel (PG)-based isocenter accuracy test, evaluated with MRI directly after irradiation. In addition the influence of a magnetic field on such measurements were investigated. Material and Methods Experiments were performed with both, EBT3 films (ISP, Wayne, USA) and PAGAT polymer gel (PG). The PG was evaluated by MR imaging, which is especially interesting 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 rotational dimensions (rotation, roll, pitch). The misalignments were within three degrees for each rotational parameter while the translational misalignment was within a few millimeters. For all nine test positions, the 6DOF movement required to align the phantom with the CT scan was determined by obtaining two paired kV images on the ET-system and an OBI CBCT- scan. Subsequently the 6DOF movement was performed by the ET-system and the robotic couch. The precision of this movement is within tenth of a millimeter. In four of the nine cases after repositioning of the phantom a second pair of kV images from the ET-system and another CBCT-scan was obtained to verify the couch-movement. The ET X-ray and the OBI-isocenter was calibrated to correspond within half a millimeter. Results For the nine test positions the translational deviations (3D vector) between the ET- and OBI-match ranged from 0.5mm to 1.4mm with an average of 0.8mm and a standard deviation (SD) of 0.3mm. In the same nine positions the deviation for the couch rotations ranged from 0deg to 0.1deg and the deviation for the robotics (roll, pitch) part of the couch movement ranged from 0deg to 0.8deg with an average of 0.2deg and a SD of 0.2deg. In the four cases of verification of the repositioning of the phantom the average of the translational 3D vector between the ET- and OBI-match was 0.3mm with a SD of 0.3mm. The deviations on the rotational parameters for these measurements were 0-0.1deg. The average of the deviations for the robotics parameters was 0.2deg with a SD of 0.1deg. A summary is listed in table 1.
Conclusion The correlation between a 6DOF repositioning performed by the ET-system with robotics and verified by a CBCT- scan was tested on a thorax-phantom. The correlation was within tenth of a millimeter for the translational parameters and within tenth of a degree for the rotational parameters. EP-2173 Dosimetric verification of synthetic CT using Cone Beam CT in an MR only workflow of cancer prostate E. Palmér 1,2 , E. Persson 2,3 , P. Ambolt 2 , C. Gustafsson 2,3 , L.E. Olsson 3 1 Lund University, Medical Radiation Physics- Department of clinical sciences, Lund, Sweden 2 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden 3 Lund University, Medical Radiation Physics- Department of Translational Medicine, Malmö, Sweden
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