Eric Pepin PhD, Presenter: Nothing to Disclose

Olga Green PhD, Abstract Co-Author: Research Consultant, ViewRay, Inc

To evaluate the impact of a 0.32-Tesla magnetic field present in the first commercial MRI-IGRT system on various ionization chamber dosimetry correction factors and to discover any directional dependence thereon.

Various ionization chambers were evaluated to determine polarity, ion recombination, and stem effect correction factors using a Cobalt-60 MRI-IGRT system. Correction factors were determined using techniques outlined in the AAPM Task Group 51 report and various other publications. Correction factors were determined as a function of ion chamber orientation in the magnetic field and compared to readings taken on a linear accelerator in the absence of a magnetic field.

The various ionization chambers with a central anode were within prescribed tolerance limits for Ppol and Pion in the presence of a magnetic field. Parallel plate ionization chambers exceeded tolerance limits for Ppol, but were within tolerance for Pion in the presence of a magnetic field. There are no established tolerance limits for a stem correction factor, but several ionization chamber of both design types showed significantly different stem correction factors in the presence of a magnetic field compared to when not in a magnetic field. All ionization chambers showed a directional dependency in the stem correction factor in the presence of a magnetic field.

The ability of central anode ionization chambers to satisfy TG-51 correction factor tolerance limits seems uninhibited by the presence of a magnetic field, however parallel plate chambers were strongly affected by the presence of a magnetic field. Chambers of all design types may require bi-directional commissioning due to variations in the stem corrrection factor when in the presence of a magnetic field.

Certain designs of ionization chambers seem unaffected by a magnetic field and are suitable for use with the first MRI-IGRT machine.

Pepin, E, Green, O, Effect of a Low-strength Magnetic Field on Ionization Chamber Reference Dosimetry.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13025824.html