Achieving a smaller penumbra region for better planning in conformal radiotherapy by using a longitudinal magnetic field

The problem of perturbing electronic fluence and dose deposition due to the presence of air cavities is addressed in this work. A uniform longitudinal magnetic field of 0.5 T strength is used to Increase the electronic fluence and to eliminate the broadened penumbra around air cavities, thus achieving better beam conformity. Monte Carlo code was used to simulate a 6 MV X-ray beam passing through air cavities of 2, 4 and 6 cm length. A point on the central axis 1 cm before the edge of the cavity was studied for the effect of the magnetic field on the electron fluence, hence on improving the dose profile. The improvement was found to depend on both beam and cavity size, for constant beam energy and magnetic strength constant, being better for larger cavities and smaller beams. There was a drastic improvement in the penumbra region in all the cases studied. The improvement was largest for 6 cm air cavity, decreasing from 3.65 cm down to 1.13 cm. For the cases studied, penumbra reduction, 62% on the average, was found to be independent of beam size. The electronic gyroradius, hence the degree of `confinement´ of electrons within the beam, is a function of electronic momentum and magnetic field strength. By keeping these two factors constant, conformity is expected to be the same in all the cases