Optimal Source Distribution for Hyperthermia at the Center of a Sphere of Muscle Tissue

In noninvasive electromagnetic hyperthermia it is important that applicators deposit as least as much power at the tumor as anywhere else in the intervening tissue. This study determines the ideal penetration limits for heating the center of a volume of muscle tissue according to this constraint. First, it is shown that the most efficiently heated volume is a sphere. Then, using both a diffraction integral formulation and a spherical harmonic modal approach, field distributions within the sphere are derived. It is shown that combining odd harmonics can generate a nearly uniform surface power distribution, and that this distribution is optimal. The power deposition patterns for these distributions for various standard frequencies are presented. It is concluded that 915 MHz is the best frequency for heating spheres less than 9.5 cm in radius.