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 along the length of the catheter and is electrically insulated except for a small length thereof which is adapted to be received within the tumor volume. By placing at least a pair of such catheters within the tumor mass and by connecting the conductors to a high frequency power source a heat producing current can be generated through the tumor tissue between the exposed portions of the catheter to damage the tumor cells. In addition, each catheter includes an axial passage permitting radioactive seeds or other tumor treating materials to be inserted within the tumor mass to cause further damage to and ultimately necrosis of the tumor without significantly affecting the adjacent healthy tissue of the body.