Conformal photon‐beam therapy with transverse magnetic fields: A Monte Carlo study

This work studies the idea of using strong transverse magnetic ( B ) fields with high‐energy photon beams to enhance dose distributions for conformal radiotherapy. EGS4 Monte Carlo code is modified to incorporate charged particle transport in B fields and is used to calculate effects of B fields on dose distributions for a variety of high‐energy photon beams. Two types of hypothetical B fields, curl‐free linear fields and dipole fields, are used to demonstrate the idea. The major results from the calculation for the linear B fields are: (1) strong transverse B fields ( > 1   T ) with high longitudinal gradients ( G )( > 0.5   T / cm ) can produce dramatic dose enhancement as well as dose reduction in localized regions for high‐energy photon beams; (2) the magnitude of the enhancement (reduction) and the geometric extension and the location of this enhancement (reduction) depend on the strength and gradient of the B field, and photon‐beam energy; (3) for a given B field, the dose enhancement generally increases with photon‐beam energy; (4) for a 5 T B field with infinite longitudinal gradient (solenoidal field), up to 200% of dose enhancement and 40% of dose reduction were obtained along the central axis of a 15 MV photon beam; and (5) a 60% of dose enhancement was observed over a 2 cm depth region for the 15 MV beam when B = 5   T and G = 2.5   T / cm . These results are also observed, qualitatively, in the calculation with the dipole B fields. Calculations for a variety of B fields and beam configurations show that, by employing a well‐designed B field in photon‐beam radiotherapy, it is possible to achieve a significant dose enhancement within the target, while obtaining a substantial dose reduction over critical structures.