Abstract: Apparatus and a method for applying electromagnetic energy to the interior of a tumor, such as a breast tumor or brain tumor within the body, to heat and thereby destroy cells in the tumor while monitoring the temperature of the body tissues at the outer periphery of tumor. The energy is radiated from an antenna in the form similar to a hypodermic needle which is inserted into the body and into the tumor, the energy being supplied by a microwave generator which can be controlled as to time of operation and power output to the antenna. A temperature sensor or sensors are also inserted in the body and are located at the outer periphery of the tumor. The sensors are coupled with a temperature detector and a control unit which controls the operation of the microwave generator to actuate and deactuate it in accordance with the temperature at the outer periphery of the tumor.