A radio frequency annular phased array hyperthermia system providing a relatively small heated focal zone in a relatively large tissue mass includes a plurality of radio frequency energy applicators in at least one ring adapted to surround the relatively large tissue mass. A bolus having a dielectric constant is positioned between the energy applicators and the tissue mass. The energy applicators operate at a frequency high enough to create the relatively small heated focal zone. The spacing between adjacent applicators in the at least one ring is less than a critical distance and spacing between adjacent rings when the at least one ring is a plurality of side by side rings is less than a critical distance with such critical distances being interdependent on the frequency of the energy radiated, the dielectric constant of the bolus, the size of the bolus, and the size of the relatively large tissue mass.
