Apparatus and method for creating small focus deep hyperthermia in tissue

A method of constructing and operating a radio frequency annular phased array hyperthermia system to provide a heated focal zone of a desired size in a selected position in a tissue mass, said hyperthermia system including a plurality of radio frequency energy radiator applicators positioned in at least one ring around and spaced by a bolus media from the tissue mass to radiate radio frequency signals of a predetermined frequency through the bolus media into said tissue mass in a manner wherein the electric field of the radiated signals from each radio frequency energy radiator applicator are superimposed in a controlled manner to produce the heated focal zone in the selected position in the tissue mass, the plurality of radio frequency energy radiator applicators having a specific number of radio frequency energy radiator applicators, each radio frequency energy radiator applicator having a center and the bolus media having a dielectric constant and an interface with the outer surface of the tissue mass, and the system having the plurality of radio frequency energy radiator applicators arranged in the at least one ring around the tissue mass to have an applicator separation distance between the centers of respective adjacent radio frequency energy radiator applicators in the at least one ring, an applicator spaced distance between the center of each radio frequency energy radiator applicator and a direct interface point located at the bolus-tissue mass intersection along the path from the center of a radio frequency energy radiator applicator to the center of the tissue mass, and a diagonal distance between the center of an adjacent radio frequency energy radiator applicator and a direct interface point, comprising the steps of:

• determining an operating frequency to provide a desired size of the heated focal zone in the tissue mass;
• determining the number of radio frequency energy radiator applicators in the plurality of radio frequency energy radiator applicators to be arranged around the tissue mass, the size of the bolus between the radio frequency energy radiator applicators and the tissue mass, and the bolus media dielectric constant so that at the determined frequency, the applicator separation distance is no greater than 0.8 of the wavelength of the radio frequency signal of the determined frequency in the bolus media and the difference in phase at the direct interface point between a radio frequency signal of the determined frequency traveling through the bolus media along the applicator spaced distance and the radio frequency signal of the determined frequency traveling along the diagonal distance is no more than 135 degrees.