A radio frequency annular phased array hyperthermia system to provide a heated focal zone in a selected position in a tissue mass, said hyperthermia system comprising: a bolus adapted to be positioned around the tissue mass in which the heated focal zone is to be provided to form an interface with the outer surface of the tissue mass, said bolus having a size and having a bolus media therein with a bolus media dielectric constant; a plurality of radio frequency energy radiator applicators positioned in at least one ring around the tissue mass in which the heated focal zone is to be provided and spaced by the bolus media a distance from the surface of the tissue mass to radiate radio frequency signals of a predetermined frequency through the bolus media into the tissue mass 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, each radio frequency energy radiator applicator having a center and spaced substantially equally from respective adjacent radio frequency energy radiator applicators circumferentially around the at least one ring, wherein the radio frequency energy radiator applicators are spaced, the bolus is sized, and the bolus media dielectric constant is selected so that the distance between centers of respective adjacent radio frequency energy radiator applicators around the at least one ring is no greater than 0.8 of the wavelength of the radio frequency signal of the predetermined frequency in the bolus media and the difference in phase at a bolus-tissue mass interface point between a radiated signal traveling through the bolus between the center of a radio frequency energy radiator applicator and the center of the tissue mass and a signal traveling through the bolus to that point from the center of an adjacent radio frequency energy radiator applicator is no more than 135 degrees. a bolus adapted to be positioned around the tissue mass in which the heated focal zone is to be provided to form an interface with the outer surface of the tissue mass, said bolus having a size and having a bolus media therein with a bolus media dielectric constant; a plurality of radio frequency energy radiator applicators positioned in at least one ring around the tissue mass in which the heated focal zone is to be provided and spaced by the bolus media a distance from the surface of the tissue mass to radiate radio frequency signals of a predetermined frequency through the bolus media into the tissue mass 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, each radio frequency energy radiator applicator having a center and spaced substantially equally from respective adjacent radio frequency energy radiator applicators circumferentially around the at least one ring, wherein the radio frequency energy radiator applicators are spaced, the bolus is sized, and the bolus media dielectric constant is selected so that the distance between centers of respective adjacent radio frequency energy radiator applicators around the at least one ring is no greater than 0.8 of the wavelength of the radio frequency signal of the predetermined frequency in the bolus media and the difference in phase at a bolus-tissue mass interface point between a radiated signal traveling through the bolus between the center of a radio frequency energy radiator applicator and the center of the tissue mass and a signal traveling through the bolus to that point from the center of an adjacent radio frequency energy radiator applicator is no more than 135 degrees. <EOS>