抄録
The development of new less-invasive method for cancer therapy is needed to improve the quality of life of cancer patients. For this purpose, appropriate treatment of the exact localized area of cancer is very important. The ultrasonic hyperthermia method has been proposed as a new method for this purpose; however, many difficult problems, such as pain generation due to hot spots produced at the muscle-bone interface, remain to be solved. An alternative method is short-term irradiation of high intensity focused ultrasound (HIFU). This method is effective for treating deep-seated tumors, as a sufficiently high thermal dose can easily be applied to the target area without any severe damage to the normal tissue. However, many problems must still be solved for clinical application. In this paper, the effects of the HIFU method are theoretically and experimentally investigated. The relationship between the thermal dose and thermal denaturalization of tissues is theoretically obtained. The nonlinear behavior of high-intensity ultrasound and the generation of harmonic components are discussed. The relationship between the dose of ultrasonic irradiation and the area of thermal denaturalization are also theoretically discussed, based on the result of computer simulations for various irradiation periods. Distributions of temperature elevation and thermal dose are calculated from the simulations and are visualized using a thermal-sensitive phantom. The results showed that irradiation with short pulses of high intensity ultrasound can produce a sharp gradient of thermal dose at the boundary between cancerous and normal tissues. This means that a sharp boundary of thermal denaturalization can be obtained due to the small influence of heat conduction during a short irradiation period. The effect of changes in SAR and thermal conductivity of tissue on the spreading of the necrosed area is also discussed. The accuracy of computer simulation was validated by comparing the results of thermal phantom experiments and computer simulation.
