[Ultrasound--mechanisms of action and application in sonodynamic therapy].

Ultrasound is a mechanical wave at a frequency beyond human hearing. In nature, ultrasound waves of low frequency are emitted and received by many animal species. However, people are interested in ultrasound which is artificially produced because of its possible practical applications. The wide spectrum of ultrasound (US) application in industrial technology and devices of general use poses a risk to human health because ultrasound in high doses can be harmful. People can also be exposed to ultrasound generated during medical treatment, mainly in ultrasound diagnostics and therapy as well as surgery. The possible risk to human health creates the need for investigation of the effects and mechanisms of its biological activity. The mechanisms of ultrasound action on biological material can be divided into thermal and nonthermal. Thermal effects occur when acoustic energy is absorbed and transformed to heat and depends on the absorption and dissipation of ultrasound energy. Nonthermal mechanisms can be classified as cavitational and shear stress. Shear stress includes the effects of the radiation pressure, radiation force, and acoustic streaming. Cavitation leading to the formation of reactive oxygen species and its consequences are of primary interest. To support photodynamic cancer therapy in the 1970s, the idea of sonodynamic cancer therapy was formulated. Sonodynamic therapy is a promising new technique for killing cancer cells based on the synergistic interactions of ultrasound and certain chemical compounds called "sonosensitizers". In spite of the experimental proof of the existence of the sonodynamic effect, attempts of a clear qualification of the mechanism of this process have been unsuccessful.