The modification of dose distributions from secondary electrons produced by accelerator-generated photon or electron beams in the presence of strong magnetic fields was studied. A need exists to predict the action of magnetic fields on X-ray tissue doses and to identify those combinations of X-ray energies, magnetic field strengths, and tissue factors where the dose might be changed significantly from an exposure without the presence of a magnetic field. Modification of X-ray produced tissue dose arises from the ability of a strong magnetic field to induce deflections on the path of secondary electrons. In order to demonstrate the existence of this deflection, measurements were made of the relative dose distributions present within a tissue -equivalent phantom produced by exposures to X-rays and in the presence of strong magnetic fields. The dose measurements were made using radiographic film detectors, sandwiched within a polystyrene target phantom irradiated in the presence of different magnetic field intensities. The fields were oriented transversely to the direction of the incident X-ray beam. Optical densities of the film exposures were converted to equivalent tissue doses for comparison to the predictions of a semi-analytical relative dose model for the process. This model was a combination of the electron Continuous Slowing Down Approximation, modified to account for multiple scattering, and a exponential photon dose model. As a result of this work, it was found that: (1) strong magnetic fields in the range of 1.2 to 5 T can induce changes in the tissue distribution of X-ray produced dose to small volumes in excess of 10%, (2) the region of maximum dose may be displaced significantly from the undeflected target volume, and (3) a reasonable estimate of the magnitude of these changes can be predicted, if the X-ray energy distribution and magnetic flux density are known. Such changes in deposited doses may be clinically significant and should be taken into account in treatment planning, when appropriate. They may also provide a simple means of dose enhancement and reduction of deep dose to healthy tissues in certain circumstances.
- Pub Date:
- January 1990