ESTRO 38 Abstract book

S1080 ESTRO 38

1 Medical Radiation Physics, Department of Clinical Science- Lund University, Lund, Sweden ; 2 Department of Hematology- Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden Purpose or Objective For most on-board imaging techniques in radiotherapy, such as Cone-Beam Computed Tomography (CBCT), verification of patient position is only possible with the couch at an angle of 0°. Furthermore, patient position is often only verified before, and not during, treatment. The purpose of this study was to investigate if an optical surface scanning (OSS) system with a novel surface algorithm for stereotactic radiotherapy (SRT), in combination with open masks, provide sufficient accuracy for positioning and real-time monitoring of coplanar and non-coplanar SRT treatments. Material and Methods The study was performed using an Alderson RANDO phantom and a costume-made three-point open mask (Orfit Industries, Wijnegem, Belgium) on a TrueBeam system (Varian, Palo Alto, California, USA) with an OSS three camera Catalyst TM system with the novel SRT solution (C-RAD Positioning AB, Uppsala, Sweden). For coplanar treatments the agreement between the isocenter shift calculated by the OSS system and the isocenter shift suggested after image-verification with CBCT was evaluated. A total of 40 measurements were carried out, after positioning the phantom using the OSS system’s auto-couch function For non-coplanar treatments the accuracy of positioning and real-time monitoring was evaluated by separating any potential couch rotation offsets from the uncertainties in the OSS system’s calculation of the isocenter shift. This was done by placing a high-Z marker on top of the phantom in line with the isocenter point in longitudinal and lateral direction and acquiring megavoltage (MV) images at different couch angles. By comparing the MV images acquired at each couch angle (45°, 90°, 270° and 315°) with images acquired at the standard position (0°), the couch rotation offset could be identified and thus the accuracy of the OSS system’s calculation of the isocenter position could be determined. Results For coplanar treatments the median agreement in the isocenter shift calculation between the OSS system and the CBCT was 0.1/-0.2/0.0 mm (range: -0.2 to 0.3/-0.5 to 0.1/-0.3 to 0.2 mm) in the vertical/longitudinal/lateral direction, respectively. For all rotational directions the agreement was within 0.9°. For non-coplanar treatments the accuracy of the OSS system’s calculation of the isocenter position was within 0.5 mm, with a median of - 0.3 and -0.4 mm (range: -0.4 to 0.4/-0.5 to 0.4 mm) in the longitudinal and lateral direction, respectively. Conclusion The OSS system with its novel surface algorithm for SRT in combination with open masks show excellent agreement with the CBCT system and has the ability to validate the position of a phantom with 0.5 mm accuracy regardless of couch angle. The results of the study show that the system could be very useful as a complementary tool for verification and real-time monitoring of coplanar and non- coplanar stereotactic radiotherapy treatments.

EP-1979 Intrafraction stability of 8526 deep inspiration breath holds in left-sided breast cancer D. Reitz 1 , S. Schönecker 1 , M. Pazos 1 , P. Freislederer 1 , M. Reiner 1 , M. Niyazi 1 , U. Ganswindt 2 , C. Belka 1 , S. Corradini 1 1 University Hospital LMU Munich, Department of Radiation Oncology, München, Germany ; 2 University of Innsbruck, Department of Radiation Therapy, Innsbruck, Austria Purpose or Objective Patients with left-sided breast cancer often receive deep inspiration breath hold (DIBH) radiotherapy to reduce the risk of cardiac side effects. Data of a large study cohort receiving radiotherapy with DIBH were analyzed regarding intrafraction breath-hold stability. Material and Methods 105 patients that underwent left-sided breast cancer radiotherapy with DIBH using the Catalyst TM optical surface scanner (C-RAD AB, Sweden) were analyzed. During each treatment session the vertical motion of the patient was continuously measured by the optical system and gating control (beam on/off) was performed by an audio-visual patient feedback system. The Catalyst TM system works through an optical surface scanning with LED light and