A method and apparatus for the treatment of tumors by heating with ultrasonic energy. A transducer assembly is provided in a housing having a refractive assembly disposed between the tumor and the transducer. The refractive assembly moves with respect to the transducer assembly. The refractive assembly includes one or more refractive elements. The refractive assembly is interchangeable to provide treatment of different types of tumors using the same transducer assembly and housing. The refractive assembly may focus, deflect or diffuse the ultrasonic energy depending on the arrangement of elements. Motion of the refractive assembly relative to the tumor results in a time-averaging spatial deposition pattern which is more highly uniform due to the averaging of maxima and minima which occurs in the near field.