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A graded-mesh finite-difference time-domain (FDTD) code, together with an alternate-direction-implicit finite-difference (ADI-FD) solution of the bioheat equation, are used for studying arrays of sleeved-slot antennas imbedded in a brain-equivalent phantom. The FDTD code allows efficient and accurate modeling of the fine structure of each antenna and of a sufficiently wide surrounding region. The ADI-FD solution of the bioheat equation allows evaluation of transient and steady-state temperature distributions in the brain-equivalent phantom with acceptable computational costs. The solution of the dosimetric-thermal problem in the volume irradiated by the antenna array permits the assessment of dimensions of the region where the temperature increase is above 43°C (the threshold for an effective hyperthermia treatment) as a function of the array input power. Arrays made of three identical antennas placed at the vertices of equilateral triangles of 10-, 15-, and 20-mm sides have been studied. The temperature of 43°C is reached in approximately 3 min in a deep-seated tumor region, from 10 to 40 mm in diameter, by applying input power levels between 2-32 W.