An apparatus (40) according to the invention includes a radiation emitter (34) for ionizing radiation and a high voltage generator (1) for generating brief voltage pulses for voltage application of electrodes (6, 15, 16, 24) included in the apparatus. The electrodes are designed to be secured at or introduced into tissue in a restricted region of a human or an animal and to form between them an electric field in the tissue. The radiation emitter (34) is provided to emit ionizing radiation to a tumor in the tissue in that region which is to be treated, while the electrodes (6, 15, 16, 24) are disposed to be placed in or at the tumor in order that the electric field pass through the tumor.