Dose Response of Selected Radiation Detectors in a Magnetic Field

• Author / Creator

  Reynolds, Michael W

• With the advent and development of hybrid Linac-MR technology, we will be able to track and treat lesions in real time. This will permit the narrowing of radiotherapy margins and escalation of dose, augmenting the therapeutic outcome of radiotherapy. The addition of a magnetic field to the treatment volume introduces difficulties in the measurement of the output radiation of the accelerator. This work investigates the dose response of various ionisation chambers and solid state detectors within a magnetic field in various relative orientations of photon beam, magnetic field, and detector axis. The orientations of interest can be divided as per the directions of the magnetic field and detector axis, with the radiation direction static; the magnetic field direction is set either transverse or longitudinal to the incident radiation, and the detector axis can independently be parallel or perpendicular to the incident radiation. The well benchmarked Monte Carlo code PENELOPE is used to model the dose deposition in the active volumes of the various detectors in air and in a water tank as a function of magnetic field strength in the orientations of interest. When physically possible, the dose response of the detectors in the relevant orientations is measured experimentally with the aid of small transverse and longitudinal electromagnets. The dose response of the detectors is defined as the ratio of dose deposited with magnetic field, to that without magnetic field. Dose response of all detectors as a function of magnetic field strength was found to have a strong dependence on the relative orientations of incident 6 MV radiation, magnetic field, and detector axis. Specifically, a chamber specific correction factor is required when detectors are used within a transverse magnetic field; this correction factor will also be orientation specific. Longitudinal field orientations, in contrast, seem to require no correction up to near 1.0T, thereafter a small orientation independent correction is needed. Water tank simulations show that radiation penumbras cannot in general be measured accurately in transverse fields; however, longitudinal magnetic fields do not cause issues in penumbra measurement. For the purposes of dose determination, longitudinal magnetic fields are preferred.

• Subjects / Keywords

  • Magnetic fields
  • Linac-MR
  • Radiation Detection
  • Solid State Detector
  • Ion Chamber
  • Dose Response
  • Dosimetry

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R38P5VM8D

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.