Clinical, physiological and anatomical determinants for radiofrequency hyperthermia

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Abstract
Temperature/time curves and corresponding CT scans of > 200 regional heat treatments with the hyperthermia system BSD-2000 in 43 patients have been analysed. In vivo variables and treatment parameters such as local specific absorption rate SAR, local relative SAR ŚR, total power P, local cooling coefficients Wb, and local steady-state temperature elevations ΔTss (above systemic temperature) have been determined. For determination of wb the well-known and accepted steady-state approach has been used, which was slightly modified for the purposes of this study. Specifically, comparison of cooling coefficients at the beginning and end of heat treatments were performed in tumours and normal tissues. Other variables are anatomical descriptors from CT scans, score of side effects Plim, and various clinical factors. A variance analysis of the dependent variables, specifically ΔTss and SAR, is performed with respect to factors which were estimated as predictive. The intratumoral steady-state temperature elevations are determined by the perfusion-related cooling coefficients and local SAR to almost the same extent. Increase of cooling coefficients in tumours during the heat treatment characterizing the thermoregulatory potential have a slight but less important influence with respect to the achieved temperature elevations. SAR is influenced by several anatomical factors which determine the relative SAR distribution and clinical factors which limit the total power P. However, options for controlling present RHT systems in order to optimize the relative SAR distribution or to avoid hot spot phenomena appear limited. Three-dimensional modelling calculations show that the spatial arrangement of electrical interfaces emerging from bone and fat structures limits SAR control in available RHT technology and is mainly responsible for local power-dependent discomfort (Wust et al. 1994b). Some conclusions are drawn, about how technological development of hyperthermia technology can contribute towards overcoming this problem.

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