Observations on the Use of Ferromagnetic Implants for Inducing Hyperthermia

Magnetic induction heating of ferromagnetic implants can be used to produce highly localized hyperthermia in deep seated tumors. We discuss the physical parameters which characterize this method and give illustrations from initial clinical investigations in animals. The physical parameters studied include magnetic field strength, frequency, load size, field uniformity, coil designs, and the heating potential of implant materials and configurations. Calculations consistent with our experimental results predict a maximum heating frequency of the order of 500 kHz for large cross-sectional loads, such as the human abdomen, and 1.9 MHz for smaller loads, such as the human brain. An experlhnental technique is introduced for accurate quantitative evaluation of the heating potentials of ferromagnetic materials in a gelled phantom medium. These data are analyzed in terms of heating efficiency per unit implant length (η
_L
), which is itself a function of implant length, diameter, annealed state, and orientation with respect to the magnetic field. A spiral sheet coil design is described and recommendations are given for proper E-Field shielding of induction coils for clinical applications. A brief discussion of techniques of implanting the ferromagnetic materials is also given. Finally, several in vivo animal studies are presented to illustrate the use of the technique for treating tumors in pelvis, thorax, oral-pharynx, and brain.