Session Item

Integration of linear accelerator and magnetic resonance tomography is a great advantage for radiotherapy treatment, the MR-linacs provide a high-contrast imaging in real-time irradiation without exposing extra doses to the patient, but the strong magnetic fields have an influence of the trajectories of the secondary electrons. Due to the Lorentz force the trajectories of these secondary electrons become helical. As a result, the dose distribution in water and the dose response of ionization chambers will change. Therefore, new correction factors are required in clinical dosimetry for MR-linacs. In this work, the Monte Carlo code EGSnrc was applied to calculate the correction factors k_B of two ionization chambers (SNC 125c and SNC 600c, Sun Nuclear Corp., Melbourne, USA) for different strengths and directions of the magnetic field B. 

The cylindrical chambers were modeled in detail according to the information given by the manufacturer and placed in a water phantom. They were irradiated under reference conditions according to the TRS-398 and DIN 6800-2 Codes of Practice. Both codes differ regarding the positioning of the detector. A 6 MV spectrum of an ELEKTA linac was used as photon source.

In this study the magnetic field correction factors were calculated in different directions of magnetic field relative to the beam axis and the chamber’s symmetry axis and was varied between 0 and 2 T in steps of 0.2 T.

Additionally, the effective point of measurement for the chamber SNC 125c was determined, by comparing the depth dose curve in a small water voxel and the depth dose curve in the chamber in absence and presence of magnetic field.

In case, where the magnetic field is parallel to the chamber axis (B_x), k_B of the SNC 600c and the SNC 125c changes in dependence of the magnetic field strength B_x up to 1% and 0.5% respectively. In this case the Lorenz force directs the secondary electrons perpendicular to the chamber axis, as a result the correction factor k_B is symmetrical around B_x = 0 T.

If the magnetic field is perpendicular to the chamber axis (B_y), the change is up to 7% and 2.3% respectively, and the Lorenz force directs the secondary electrons in the direction of the stem or the chamber tip. The variation of k_B as a function of B is somewhat larger applying the TRS-398 protocol than applying the German DIN protocol.

In this study, the k_B values for two different ion chambers and two different Codes of Practice in different magnetic field strengths were determined. In case where the external magnetic field B is parallel the chamber axis, the variation of k_B as a function of the magnetic field is within 1.5% even for the large-volume SNC 600c chamber.