Experimental characterization of helical coils as hyperthermia applicators

A series of helical-coil hyperthermia applicators have been designed for treating human limbs. Several experiments to determine their operating characteristics were conducted using muscle-equivalent, cylindrical, and lower-body-shaped phantoms. It was found that this kind of applicator has to be operated at resonances which are both sharp and load-dependent. This can have significant clinical implications, since changes in the position of the patient and/or the tissue dielectric properties with temperature can produce a severe mismatch. Moreover, even though the patterns of energy deposition were found to be relative transversely uniform and axially belt-shaped within the cylindrical phantoms, they were strongly dependent on the shape of the phantom and of the coil for the more realistic human-shaped phantom. Intense local heating was observed whenever the winding of the helical coil was within a few millimeters of the surface of the human-shaped phantom. The tests with the human-shaped phantom showed that there can be significant energy deposition outside of the region intended for treatment.