WE-D-BRC-02: Magnetic Field Effects On Radiation Dose Distribution

Abstract

The high magnetic field associated with magnetic resonance imaging complicates the design and engineering of combined MRI‐radiation therapy systems due to the potential interactions between the magnetic field and the electron beam of the accelerator, together with the converse problem of perturbation of the MRImagnetic field by ferromagnetic materials in the structure of the accelerator system. Addressing these issues increases the cost and complexity of a combined system. Hence the need for a high magnetic field is generally viewed as a disadvantage by radiation physicists or, at best, as a necessary price to pay for imaging with MRItissue contrast. However, the effects of the magnetic field on the radiation therapy delivery system are not uniformly negative and it has been proposed that the magnetic field may be turned to advantage in reducing the secondary electrondose to the skin. It has also been hypothesized that it may be practical to utilize an appropriately oriented magnetic field to improve the dose distribution within the patient by reducing the penumbra of secondary electrons around the beam and, possibly, to provide a sharper cut‐off of the deposited dose downstream of the treatment region. If this can be done effectively it could furnish a proton‐like Bragg peak from an x‐ray treatment beam If a magnetic field is present in the volume of the lesion during treatment, the trajectories of the secondary electrons and positrons generated by the interaction of the high energy X‐ray beam with tissue, are known to be perturbed. J.J.W. Langendijk and coworkers have modeled this in terms of its effect on treatment planning. This paper reports work carried out using the EGS 5 Monte Carlo code, (which can incorporate both electric and magnetic fields), to model the dose distributions obtained in tissue with various magnetic field orientations and strengths, using typical Clinac X‐ray spectra. The following sub topics are addressed: 1. The magnitude of the effect of the magnetic field in different tissue types is computed. 2. The practicality of this approach for modifying dose distribution is assessed, based upon the magnetic field strength and magnetic field conformations that are required. A portion of the research reported here was funded by Varian Medical Systems.