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Temperature distributions are computed for tissue models assumed to be heated by constant power seeds, and from that, the heating power which the implants have to produce to achieve clinically acceptable temperatures in the tumor are obtained. Calculations of the heat produced by thermal seeds exposed to an electromagnetic induction field showed it to be strongly dependent on the permeability of the material, on the field frequency, on the seed diameter, and on the orientation of the implants with respect to the field. It is recommended that, other parameters permitting, the implants be oriented parallel to the induction field and that the field frequency be approximately 200 kHz or lower. Under these conditions, implants with diameters as small as 0.25 mm produce sufflcient heat for any clinical application without undue heating by eddy currents flowing within the patient. The use of frequencies above the recommended range puts certain restrictions on the implant geometry and on the magnetic properties of their material. Needles oriented perpendicular to the field produce enough heat to reach therapeutic temperatures only within a narrow range of parameters.