Dose and Position Quality Assurance Using the RADPOS System for 4D Radiotherapy with CyberKnife R Marants 1 , E. Vandervoort 2 and J.E. Cygler 1,2,3 1 Department of Physics, Carleton University, Ottawa, Canada 2 Department of Medical Physics, The Ottawa Hospital Cancer Centre, Ottawa, Canada 3 Department of Radiology, University of Ottawa, Ottawa, Canada Abstract— The CyberKnife system consists of a compact LINAC mounted on a mobile robotic arm and the Synchrony Respiratory Motion Tracking System. This complex radio- therapy system needs independent performance verification to assure safe treatments. In this work, we use the RADPOS 4D dosimetry system to verify CyberKnife’s motion tracking and delivered dose. RADPOS motion measurements are compared with internal metal fiducial and external LED marker log files. Dose measurements are compared with film and treatment planning system (TPS) calculations. RADPOS and EBT3 Gaf- Chromic films were calibrated in Solid Water (5 cm depth, 80 cm source-detector distance, 60 mm cone, Exradin ion cham- ber, Fluke electrometer). A CT-based treatment plan was created for a Solid Water breast phantom containing fiducials and RADPOS. Dose calculations were performed using Multi- Plan TPS, Monte Carlo (MC) and ray tracing (RT) algorithms. Before treatment, film was inserted inside the breast phantom adjacent to RADPOS. The breast phantom and LED markers were positioned on the chest platform of a Quasar Motion Phantom. Position logging began for RADPOS and Synchrony, Quasar motion started, and irradiation commenced. A coordi- nate alignment algorithm was implemented, allowing position tracking modalities to be compared in a common coordinate system. The average standard deviation of the differences between LED and RADPOS position measurements was 0.33, 0.39, and 0.56 mm along the left/right, superior/inferior, and anterior/posterior directions, respectively. Dose percent differ- ence values during static phantom irradiations were 0.3% (RADPOS/RT), 1.3% ( RADPOS/MC), 0.5% (Film/RT), 1.5% (Film/MC), and -0.2% (RADPOS/Film), while values during dynamic phantom irradiations were 2.0% (RADPOS/RT), 2.9% (RADPOS/MC), -1.0% (Film/RT), 0.0% (Film/MC), and 3.0% (RADPOS/Film). Average gamma results were greater than 96% for MC and RT dose calculation algorithms, and for dynamic and static treatments. Our work demonstrates that RADPOS is a useful tool for independent QA of CyberKnife treatments with Synchrony respiratory compensation. Keywords— CyberKnife, RADPOS, Dosimetry, Film, QA. I. INTRODUCTION Radiotherapy treatment becomes more difficult in cases where patient motion, such as breathing, causes movement of the treatment target or organs-at-risk. Several methods to deal with this problem are currently in use, such as increas- ing the treatment margins, implementing breath-hold tech- niques or other active breathing control approaches, and real-time tumor motion tracking[1], such as the motion compensation used by the CyberKnife system. This last solution has motivated this research and is the focus of this report. The Accuray CyberKnife Robotic Radiosurgery System consists of a 6 MV compact linear accelerator mounted on a robotic arm, which gives it up to six degrees-of-freedom of motion. It is able to deliver small, high-intensity x-ray radi- otherapy beams from many different non-coplanar direc- tions to the tumor, resulting in highly conformal dose distri- butions. In addition, the CyberKnife system implements tumor tracking through the use of internally implanted metal fiducials and motion tracking through the use of external LED optical markers. The CyberKnife system adjusts the direction of the beams during treatment (i.e. while beam is on) to correct for tumor motion due to breathing. The Synchrony Respiratory Motion Tracking System employs external optical LED markers and the imaging of internally implanted fiducials to build a predictive correlation model, which dynamically corrects beam delivery based on a patient’s breathing pat- tern. Throughout a treatment, the model is continuously updated with the latest tracking data, correcting for drifts in the patient breathing pattern and tumor position. With such a complex delivery system, there is a need for thorough quality assurance which can look at different components of this system independently. This can potentially be accomplished with the use of RADPOS. RADPOS is a 4D dosimetry system consisting of a microMOSFET dosimeter combined with an electromag- netic positioning sensor. RADPOS has the ability to per- form real-time dose and position measurements simultane- ously[2], making it an excellent candidate for acting as an independent QA tool for the Cyberknife Synchrony tracking algorithm. The RADPOS system is composed of the MOSFET reader, which is responsible for dose measurements, and the transmitter, pre-amplifier and 3D-guidance-tracker, which are responsible for position measurements. The RADPOS © Springer International Publishing Switzerland 2015 D.A. Jaffray (ed.), World Congress on Medical Physics and Biomedical Engineering, June 7-12, 2015, Toronto, Canada, IFMBE Proceedings 51, DOI: 10.1007/978-3-319-19387-8_147 599