A medical probe assembly, tissue treatment system, and method are provided for ablating tissue. The probe assembly comprises an elongated member and electrode elements mechanically coupled to the distal end of the elongated member. The electrode elements are configurable as two bipolar electrode pairs with a common electrode element. At least one of the electrode elements comprises a plurality of electrodes (such as, e.g., needle electrodes) radially extendable from the elongated member. An ablation source, such as a radio frequency source, can be connected to the probe assembly in order to convey ablation energy to the electrode pairs, either simultaneously or sequentially.