Program Information
The Effective Point of Measurement of Farmer Ionization Chambers in Magnetic Fields
C K Spindeldreier1,2*, O Schrenk1,2 , C P Karger1,2 , S Greilich1,2 , A Pfaffenberger1,2 , (1) German Cancer Research Center (DKFZ), Heidelberg, Germany, (2) Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany
Presentations
SU-H3-GePD-J(A)-6 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Joint Imaging-Therapy ePoster Lounge - A
Purpose: To perform commissioning and quality assurance measurements in hybrid magnetic resonance guided radiation therapy devices (MRgRT), dosimetry in high magnetic fields needs to be performed. In this work, we investigate the effective point of measurement (EPOM) of a Farmer ionization chamber under the influence of magnetic fields.
Methods: A PTW Farmer chamber (PTW 30013) was modeled with the egs++ library of the EGSnrc Monte Carlo code system. The shift of the EPOM along the central beam axis was determined by comparing calculated depth dose curves in water to those generated with the chamber model. Magnetic fields of 0.0 T, 0.35 T, 0.5 T, 1.0 T and 1.5 T were applied either perpendicular to beam and chamber, parallel to the chamber or parallel to the beam. Chamber and beam were always kept perpendicular to each other.
Results: When applying a 1.5 T magnetic field perpendicular to the beam and to the chamber, the EPOM was located at 0.35*r (r = chamber radius) in the direction of the source, in contrast to 0.53*r without magnetic field. For a 1.5 T field parallel to the chamber, the EPOM was located at 0.30*r and for a 1.5 T field parallel to the beam, it was found at 0.59*r.
Conclusion: The magnetic field of hybrid MRgRT devices has a significant influence on the EPOM of the PTW 30013 Farmer ionization chamber. This has to be considered in future dosimetry protocols. The lateral shift of the EPOM [O’Brien AAPM 2016] as well as the dependence on the chamber radius will be analyzed in future simulations. Acknowledgments: We thank Dr. E. Schuele and R. Kranzer (PTW) for detailed information on the ionization chamber geometry and Dr. I. Kawrakow for providing a magnetic field macro for the EGSnrc code.
Funding Support, Disclosures, and Conflict of Interest: This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.
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