On the use of ferromagnetic microparticles in microwave and radio frequency hyperthermia

Small magnetic particles, e.g. of magnetite (Fe3O4) or ferrites, are reasonably well tolerated by the body. These particles can be introduced to the body either in a `free¿ form or attached to macroscopic carriers, e.g. carbohydrate particles. If small enough, the particles will be transported by the lymphatic system to the nodes and they may end in the spleen and liver, or remain buried in the lymphatic nodes. If they are of a larger size or attached to microcarriers the particles will most likely end in the reticulo-endothelial cells. When such magnetic particles are placed in an electromagnetic field, they exhibit losses arising from the presence of a hysteresis loop or from ferromagnetic resonance. The effect is a localized heating of the particles and the surrounding tissue. The radio frequency magnetic field also heats tissues directly by inducing eddy currents and the same mechanism is operating when heating large implants made of ferromagnetic material. In the case of ferrites, which are good insulators, the dominant mechanism of heating is due to hysteresis. A possibility exists to increase the discrimination between the implant heating and surrounding tissue heating by utilization of dimensional resonances. It is also worth remembering that strong magnetic field gradients may find use in moving the ferrite powders into a tumour region.