A catheter for the treatment of tumors and other affections using hyperthermia induced by radio frequency includes a tubular body with or without an anchoring balloon at its distal end. An active electrode is positioned around the body and connected to a radio frequency generator. At least one thermistor needle measures the temperature around the electrode. The active electrode is flexible and connectable to a supply source of a fluid for cooling the electrode. It includes a helical conduit, which can be metal, conductive ceramic or graphite.