An apparatus is disclosed for adaptively treating a tumor with an alternating electric field, the apparatus comprising an electrical impedance tomography device for measuring an impedance of the tumor, such that a size of cells in the tumor can be determined (1110) from the measured impedance of the tumor, an AC signal generator having a controllable output frequency, a processor for estimating the size of cells in the tumor based on the measured impedance of the tumor and for setting (1120) the frequency of the AC signal generator based on the estimated size of cells in the tumor and at least one pair of electrodes operatively connected to the AC signal generator for applying an alternating electric field to the tumor to selectively destroy cells in the tumor.
