Reducing underdosing near low-density media irradiated with X-rays by using longitudinal magnetic fields

The underdosing of lesions surrounding upper respiratory air-cavities, and those within the lung, is well known in photon therapy. In both cases, lateral disequilibrium exists because more electrons leave the central regions than those that enter laterally. The underdosage, which is not predicted by traditional treatment planning systems, worsens as the held size becomes small compared to the lateral extent of electrons. The use of strong longitudinal magnetic fields to solve problems associated with lateral electronic disequilibrium was suggested by Bielajew (1993). Here, the authors study how relatively weak magnetic fields (/spl sim/0.5 T) can reduce the underdosing near low-density regions. The degree of improvement was estimated with a simple water-air-water slab geometry using EGS4 Monte Carlo simulations. For a 6 MV beam irradiating a 3 cm thick air-gap located between 5 cm water slabs, the ratio of doses between the distal wall and the proximal wall is 0.6 for a beam 5 cm wide. When a longitudinal magnetic field of 0.5 T is applied, the ratio increases to 0.9. The longitudinal magnetic field enforces equilibrium by restricting the lateral range of electrons to approximately a gyroradius, proportional to the transverse momentum of electrons and inversely proportional to the magnetic field intensity. The field therefore sets a bound on the lateral excursion of the electrons, which is virtually independent of the density variations in the medium. The lateral confinement of secondary electrons by sub-MRI strength longitudinal fields therefore has potential in reducing underdosing of lesions surrounded by low-density tissues in photon radiotherapy.