Abstract: A catheter for use in treating tumors is flexible and relatively small in diameter so that it can be inserted interstitially into the tumor mass. A conductor is provided about a length of the catheter and is electrically insulated except for a small length or small lengths thereof which are adapted to be received within the tumor volume. By connecting the conductor to a high frequency power source a heat producing current can be generated through the tumor tissue adjacent the exposed portions of the catheter to damage the tumor cells. One or more temperature sensing devices are included in the catheter and are located adjacent to the exposed conductor or conductors for monitoring the temperature of the adjacent tumor tissue. The signals from these devices, which are picked up from a connector at one end of the catheter, can then be used to control the current to the conductor or conductors so as to maintain the tumor volume at a constant and uniform desired temperature.