Influence of a transverse magnetic field on the response of different detectors in a high energy photon beam near the surface

Purpose: The characteristics of radiation detectors have to be assessed for dosimetry in the presence of magnetic fields, i.e. in conditions found in combined machines for magnetic resonance imaging and radiotherapy. While a lot of attention is directed toward correction factors for absolute dosimetry in magnetic fields, relative dose measurements are an equally important task to be performed. There is a need to experimentally analyze detector response differences in the build-up region in the presence of a transverse magnetic field.

Methods: Depth dose curves with different detectors (microDiamond PTW 60019, unshielded diode PTW 60012, ionization chamber PTW Semiflex 31010 and EBT3 film) were acquired for a beam quality of 6MV in an 8×10cm² field at SSD 110cm with and without a transverse magnetic field of up to 1.1T. For these experiments, an electromagnet was placed in front of a conventional linear accelerator of the type Elekta Precise. The detectors were positioned in a water phantom fitting between the poles of the electromagnet. The beam entered through a 0.3mm thin PMMA foil window, which enabled measurements even close to the surface. Ratios of the response with and without the magnetic field for different detectors were investigated. The film served as a reference.

Results: Changes in the depth dose curve near the surface due to the magnetic field were not correctly reproduced by all detectors. EBT3 film and the microDiamond detector agreed up to the surface. The diode showed up to 2% deviation from the film in the build-up region, but it could still be considered within the uncertainties. However, the curves obtained with the ionization chamber showed up to 6% deviation from the film and even completely different trends in the surface-near region. At depths larger than 2cm, there were no noticeable differences between the different detectors for relative depth dose curves.

Conclusions: At the descending part of the depth dose, the tested detectors did not show artifacts within the magnetic field. However, air-filled ionization chambers cannot be recommended for relative dosimetry in magnetic fields near the surface. Diamond detectors might be a suitable alternative and future investigations should concentrate on the performance of such detectors.