ESTRO 36 Abstract Book

S229 ESTRO 36 2017 _______________________________________________________________________________________________

Antwerpen, Belgium 3 Brainlab AG- Feldkirchen- Germany, R&D RT Motion Management, Munich, Germany 4 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium 5 Babes Bolyai University, Faculty of Physics, Cluj- Napoca, Romania Purpose or Objective Dynamic Wave Arc (DWA) is a system-specific non- coplanar arc technique that combines synchronized gantry-ring rotation with D-MLC optimization. This paper presents the clinical workflow, quality assurance program, and reports the geometric and dosimetric results of the first patient cohort treated with DWA. Material and Methods The RayStation TPS was clinically integrated on the Vero SBRT platform for DWA treatments. The main difference in the optimization modules of VMAT and DWA relates to angular spacing, where the DWA optimization algorithm does not consider the gantry spacing, but only the Euclidian norm of the ring and gantry angle. To support DWA deliveries, the Vero system required some additional upgrades: an MLC secondary feedback unit upgrade allowing faster dynamic MLC leaf movement of up to 4 cm/s at isocenter level, and a machine controller offering gantry-ring synchronous rotations. The first 15 patients treated with DWA represent a broad range of treatment sites: breast boost, prostate, lung SBRT and bone metastases, which allowed us to explore the potentials and assess the limitations of the current site-specific DWA template solution. Table 1 provides further information for each patient case including the corresponding DWA plan information, while Figure 1 presents the most common used DWA trajectories. For the DWA verification a variety of QA equipment was used, from 3D diode array to an anthropomorphic end-to-end phantom. The geometric accuracy of each arc was verified with an in-house developed method using fluoroscopy images.

Results The average beam-on delivery time was 3min, ranging from 1.22min to 8.82min. The average ɣ (3%,3mm) passing rate for film measurements was 97.0 ±1.6% (range from 93.3 to 98.8%), while the Delta 4 measurements presented an average ɣ (2%,2mm) of 97.7±1.4% (range from 95.3 to 99.6%) respectively. The average isocentre point dose ratio was 99.9±1.2% (range from 98.0 to 102.8%). For the lung SBRT verifications with the CIRS phantom, an average local ɣ of 97.0±1,0% and 93.1±2.0% was obtained during the coronal and sagittal film analysis, whereas the average isocentre point dose ratio was 100.0±1.4%. An overall mean gantry-ring geometric deviation of 0.04° ± 0.46° and 0.19° ± 0.26° was obtained, respectively. Conclusion DWA has been successfully added to the non-coplanar rotational IMRT techniques’ arsenal, allowing additional flexibility in dose shaping while preserving dosimetrically robust delivery. In a short period of time, it has become a standard treatment technique on the Vero system in our department. OC-0440 Characterization and clinical evaluation of a novel CT reconstruction to derive electron densities B. Van der Heyden 1 , M. Ollers 1 , C. Loon Ong 1 , F. Verhaegen 1 , W. Van Elmpt 1 1 School for Oncology and Developmental Biology- Maastricht University Medical Centre, Department of Radiation Oncology MAASTRO- GROW, Maastricht, The Netherlands Purpose or Objective Radiotherapy dose calculations are almost exclusively performed on CT images. In a clinical workflow, the Hounsfield Units (HU) are converted into electron density (ED) by using a CT to ED conversion curve calibrated for a typically fixed tube potential (e.g. 120 kV). Recently, a novel CT image reconstruction algorithm (DirectDensity TM , Siemens Healthcare GmbH, Germany) was developed that directly reconstructs the ED, independent of the tube potential of the CT scanner. This allows the elimination of a conversion curve for each tube potential. Our study describes the accuracy in terms of dose calculation of the reconstruction algorithm based on phantom measurements and shows the application in a clinical radiation therapy workflow for different tube potentials. Material and Methods The accuracy of the novel reconstruction algorithm to derive ED was validated in a Gammex RMI 467 phantom (Gammex, USA) using different tissue mimicking inserts.