Clinical studies with ultrasound-induced hyperthermia

Results obtained in clinical studies in which ultrasound was used for hyperthermia induction are reviewed. High intensity ultrasound has certain advantages for the induction of local hyperthermia: Well-collimated beams can be produced, which makes good localization possible; ultrasound can be focused; and absorption in tissue is proportional to frequency, hence the depth of penetration can be controlled. Limitations include the fact that ultrasound is reflected from tissue-air interfaces and thus cannot be used to heat tumors in the lung. Local "hot spots" due to impedance mismatches that result in standing wave patterns may occur near bone. During clinical treatments, this latter problem is frequently associated with a patient's complaint of pain. The temperature generated depends both on ultrasound absorption coefficients, which vary among different tissues, and dissipation of heat, which varies due to local differences in blood flow rates. Thus inhomogeneities of heating in tissue may be observed despite uniform ultrasound fields. Clinical results with ultrasound-induced hyperthermia have been reported by groups at Stanford, M.D. Anderson Hospital, and Massachusetts Institute of Technology-Harvard. Approximately 40-50% of the superficial tumors showed an objective response to ultrasound hyperthermia in the range of 43 degrees -50 degrees C. Responses to heat alone were partial and transitory. Some radiation responses appeared to be improved by concomitant ultrasound hyperthermia.