Applications of microwave, ultrasound, and radiofrequency heating

Different needs must be considered in the control of localized tumors or disseminated disease when deep heating techniques are used. For localized disease, various electromagnetic microwaves, radiofrequency currents and high-powered ultrasound sources are being used. At the present time, each approach has its own strengths and limitations. Microwave and inductive approaches appear to be useful only for surface lesions. Radiofrequency currents, generated by pairs of plates, give more deeper heating in the body, but excessive heating occurs in fatty layers related to high resistivity of the tissue and sometimes near the edge of the applicators. In an invasive approach, needles are implanted into the tumor and excited by a radiofrequency source. Considerable clinical successes are claimed for local heating of the lesion with this approach, but it is only useful for specific sites of the body. Ultrasound has special advantages for both surface or deep-seated soft tissues due to its flexibility and its ability to focus the beam onto the lesion. However, ultrasound cannot be used in the chest cavity or near bone, and the dosimetry is not defined at present. For disseminated disease, much larger regions, particularly in the chest cavity, can be heated, with the hope that the reduced blood circulation in some tumors can achieve a preferential heating in the lesion. Several microwave and inductive heating approaches are being used or considered in the achievement of this goal.