The use of magnetic fields to improve photon dose distributions for radiation therapy-a possible approach to "poor man's proton" beam properties

Several researchers have proposed the use of magnetic fields to modify the dose distributions for electron beams over the past 50 years. However, the potential use of magnetic fields to improve the dose distributions produced by X-ray beams has been largely ignored due to the conventional paradigm that magnetic fields are not expected to affect the behavior of uncharged radiations like X-rays and neutrons. This assumption is not necessarily valid. The authors have studied the effect of intense locally applied transverse magnetic fields on X-ray dose distributions for various beam energies, field sizes, and magnetic field strengths using Monte Carlo simulation. Significant dose enhancement is found close to the proximal magnetic field gradient region due to the prevention of secondary electrons from traveling downstream; whereas significant dose reduction is observed close to the distal gradient due to the need for re-establishment of the secondary electronic build-up region. The degree of dose enhancement and reduction along the central axis of the beam can be up to 100% and 60% respectively under idealized conditions. The isodose distributions for realistic magnetic fields also show lateral shifts of the electron dose deposition patterns. The authors' preliminary data show that a transversely applied magnetic field is capable of producing controllable and almost proton-like dose peaks in addition to dose dips in X-ray beams. This magnetic field induced dose modulation has the potential under certain circumstances of increasing the dose to the target volume and reducing the dose to the adjacent normal tissues in photon radiation therapy.