Technical Note: Ion chamber angular dependence in a magnetic field.

Purpose: There have been several studies investigating dose deposition effects within radiation detectors in the presence of a magnetic field. However, to date there has only been a passing investigation which explicitly investigates detector dose-response as a function of detector orientation. Herein we will investigate the dose-response as a function angular orientation of a PR06C ionization chamber. We will also benchmark the Monte Carlo code PENELOPE with the newly developed magnetic field Fano test. Methods: The PENELOPE Monte Carlo package was used to simulate a PR06C ionization chamber in 0.35 T through 1.5 T magnetic fields oriented either parallel or orthogonal to an incident 6 MV radiation beam. The ionization chamber was rotated through a number of polar and azimuthal angles. The dose deposited within the chamber at each angular position and magnetic field strength was scored then normalized to that deposited in the same orientation with no magnetic field. The simulation was also benchmarked via a Fano test in magnetic field. Results: The Fano test yielded a 0.4% difference between simulation and expected result, which is similar to previous findings and sufficient for the purposes of this study. The angular dose-response map in all cases where the magnetic field is oriented orthogonal to the radiation beam is quite varied and can range from 0.89 to 1.08. Angular deviations as small as 3 degrees can lead to dose-response changes in excess of 1%. When the magnetic field is parallel to the photon beam, the angular dose-response map is homogeneous and less than 1% below 1.0 T. Conclusions: Within a magnetic field-oriented orthogonal to the radiation beam, the ionization chamber dose-response fluctuates greatly as a function of polar and azimuthal angle, where a parallel field yields a more homogeneous dose-response.(C) 2017 American Association of Physicists in Medicine