An apparatus for applying a therapeutic electric field to a target region of a patient, the apparatus comprising: first and second pairs of electrodes (11, 12) configured for placement against the patient's body at different positions about the target region; a control signal generator (43, 44) configured to generate a periodic control signal (21, 22; 23, 24) with first and second output states, the first output state having a first period of time ranging from 10 ms to 1000 ms and the second output state having a second period of time ranging from 10 ms to 1000 ms; and an AC signal generator configured to generate a first AC signal having a field strength of at least 1 V/cm and a frequency between 50 and 500 kHz across the first pair of electrodes when the control signal (21; 23) is in the first output state and a second AC signal having a field strength of at least 1 V/cm and a frequency between 50 and 500 kHz across the second pair of electrodes when the control signal (22; 24) is in the second output state; wherein switching between the generation of the first AC signal across the first pair of electrodes and the second AC signal across the second pair of electrodes is provided by switching between the first and second output states and the first and second AC signals have a rising amplitude in a third period of time when switched on and a falling amplitude in a fourth period of time when switched off, whereby the control signal generator (43, 44) cyclically changes a direction of the electric field between a first direction for the first period of time and a second direction for the second period of time when the first and second pairs of electrodes (11, 12) are placed at the different positions about the target region; wherein the first and second AC signals are turned on and off at a rate that is slow relative to the reciprocal of the frequency of the first and second AC signals and fast relative to the first and second periods of time (t1, t2). first and second pairs of electrodes (11, 12) configured for placement against the patient's body at different positions about the target region; a control signal generator (43, 44) configured to generate a periodic control signal (21, 22; 23, 24) with first and second output states, the first output state having a first period of time ranging from 10 ms to 1000 ms and the second output state having a second period of time ranging from 10 ms to 1000 ms; and an AC signal generator configured to generate a first AC signal having a field strength of at least 1 V/cm and a frequency between 50 and 500 kHz across the first pair of electrodes when the control signal (21; 23) is in the first output state and a second AC signal having a field strength of at least 1 V/cm and a frequency between 50 and 500 kHz across the second pair of electrodes when the control signal (22; 24) is in the second output state; wherein switching between the generation of the first AC signal across the first pair of electrodes and the second AC signal across the second pair of electrodes is provided by switching between the first and second output states and the first and second AC signals have a rising amplitude in a third period of time when switched on and a falling amplitude in a fourth period of time when switched off, whereby the control signal generator (43, 44) cyclically changes a direction of the electric field between a first direction for the first period of time and a second direction for the second period of time when the first and second pairs of electrodes (11, 12) are placed at the different positions about the target region; wherein the first and second AC signals are turned on and off at a rate that is slow relative to the reciprocal of the frequency of the first and second AC signals and fast relative to the first and second periods of time (t1, t2).