AbstractID: 12092 Title: Magnetic field effects on radiation dose distribution
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 MRI magnetic 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 MRI tissue 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 electron dose 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.