Microwave thermotherapy: Hot-spots induced by surface EM waves

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This paper is a contribution to theory of microwave applicators used in cancer treatment by so called hyperthermia. It deals with a study of excitations of surface waves around of the treated area — one kind of parasitic effects, which can significantly complicate real treatment of patients (surface waves can create superficial hot spots outside the treated area). Appropriate treatment planning by aid of numerical simulations of SAR distribution can prevent such cases. In our contribution we would like to demonstrate a possibility to eliminate excitations of the surface electromagnetic (EM) waves by optimization of water bolus shape and dimensions. According to the theory of EM field and waves, either interface between two different dielectric media or interface between dielectric media and conductor can support propagation of the so called surface waves. Except of the EM wave going directly into area of agar phantom surface waves will travel around agar phantom either inside water bolus or even outside of it, i.e. surface wave propagates along the interface between agar phantom and water bolus. For our study and discussion we have chosen microwave stripline type applicator with TEM mode, which can be used for treatment of cancer patients at frequency 434 MHz (Fig.1). It was designed at Department of Electromagnetic Field of CTU in Prague for our research purposes. To understand well the discussed problem we work both on its analytical and on its numerical solutions. We will describe basic approach to analytical solutions of this case. Our approach to develop analytical solution of the discussed problem comes from idea of the EM resonances in water bolus. If the central circle of the water bolus is approximately equal to any of whole number multiple wavelength, then under certain conditions surface waves can create hot spots on the surface of the treated area or on the surface of agar phantom, mostly right on the opposite side with respect to position of the TEM mode applicator in agar phantom. Simulations of several SAR patterns have been done by the FDTD method — see please example of the SAR distribution given in the Fig.1.