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.

Unlike electron beams, scant attention has been paid in the literature to possible magnetic field effects on therapeutic photon beams. Generally, dose profiles are considered to be fully determined by beam shape, photon spectrum and the substances in the beam path. Here we show that small superconducting magnets can exercise potentially useful control over photon dose profiles. The magnet produces a locally strong transverse field with large gradients and is applied to the tissue surface below which the photon beam is passing. For one practical magnet design, our simulations, which use the EGS-4 Monte Carlo code modified to include magnetic field effects, show significant intensification and shielding effects. In water phantoms, the effects extend to 3-4 cm or more beyond the warm face of the cryostat and greater distances are achieved in phantoms simulating lung (density approximately 0.3). Advances in applying the concept and in superconducting materials and magnet design hold promise for extending these ranges.

SUMMARY The purpose of this study was to explore the potential advantages of using strong magnetic fields to increase tumor dose and to decrease normal tissue dose in radiation therapy. Strong magnetic fields are capable of altering the trajectories of charged particles. A magnetic field applied perpendicularly to the X-ray beam forces the secondary electrons and positrons to spiral and produces a dose peak. The same magnetic field also prevents the electrons and positrons from traveling downstream and produces a lower dose region distal to the dose peak. The locations of these high-and low-dose regions are potentially adjustable to enhance the dose to the target volume and decrease the dose to normal tissues. We studied this effect using the Monte Carlo simulation technique. The EGS4 code was used to simulate the effect produced by a coil magnet currently under construction. The coil magnet is designed to support up to 350 A operating current and 15 T peak field on windings. Dose calculations in a water phantom show that the transverse magnetic field produces significant dose effects along the beam direction of radiation therapy X-rays. Depending on the beam orientation, the radiation dose at different depths along the beam can be increased or reduced. This dose effect varies with photon energy, field size, magnetic field strength, and relative magnet/beam geometry. The off-axis beam profiles also show considerable skewness under the influence of the magnetic field. The magnetic field-induced dose shift may result in high dose regions outside the geometrical boundary of the initial radiation beam. We have demonstrated that current or near-term magnet technology is capable of producing significant dose enhancement and reduction in radiation therapy photon beams. This technology should be further developed to improve our ability to deliver higher doses to the tumor and lower doses to normal tissues in radiation therapy.

Factors affecting the performance of filamentary scintillators are considered. Experimental data on light attenuation for both plastic scintillator filaments and thin-wall glass tubing liquid scintillators are presented. A theoretical interpretation which satisfactorily accounts for the performance of luminescent filaments in terms of bulk properties and surface reflection losses is given and permits quantitative evaluation of surface losses for various materials. While plastic scintillator fibers are mechanically convenient, it is suggested that liquid-filled fibers will prove more consistent and stable in their properties. Comments on the utility of arrays of fibers as particle track imaging devices and as image intensifier screens are included.

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. We 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 down stream; 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. Our 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.