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

This work studies the idea of using strong transverse magnetic (B) fields w ith high-energy photon beams to enhance dose distributions for conformal ra diotherapy. EGS4 Monte Carlo code is modified to incorporate charged partic le transport in B fields and is used to calculate effects of B fields on do se distributions for a variety of high-energy photon beams. Two types of hy pothetical B fields, curl-free linear fields and dipole fields, are used to demonstrate the idea. The major results from the calculation for the linea r B fields are: (I) strong transverse B fields (>1 T) with high longitudina l gradients (G) (>0.5 T/cm) can produce dramatic dose enhancement as well a s dose reduction in localized regions for high-energy photon beams; (2) the magnitude of the enhancement (reduction) and the geometric extension and t he location of this enhancement (reduction) depend on the strength and grad ient of the B field, and photon-beam energy; (3) for a given B field, the d ose 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 centr al axis of a 15 MV photon beam; and (5) a 60% of dose enhancement was obser ved 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 confi gurations show that, by employing a well-designed B field in photon-beam ra diotherapy, it is possible to achieve a significant dose enhancement within the target, while obtaining a substantial dose reduction over critical str uctures. (C) 2001 American Association of Physicists in Medicine.