NUMERICAL-ANALYSIS OF CAPACITIVELY COUPLED ELECTRODES FOR INTERSTITIAL HYPERTHERMIA

Multi electrode current source interstitial hyperthermia (MECS-IHT) em ploys individually controlled, 27 MHz radiofrequency electrodes insert ed into plastic brachytherapy catheters. In order to get a firm unders tanding of the physical behaviour of the electrodes and to verify the current source approximation in our hyperthermia treatment planning sy stem we have investigated (1) the electrical properties of the electro de-catheter-tissue system, and (2) the impact of inhomogeneity of the electrical properties of the tissue in the vicinity of the electrodes. The results validate the use of the ideal current source approximatio n in the treatment planning SAR model. The models predict the presence of a significant heat source inside the electrode wall when lossy cat heter materials are used, producing a conductive heating component in addition to the SAR in the tissue. For a given catheter spacing this c onductive component will produce a more heterogeneous temperature dist ribution. Thus, low-loss catheter materials like polyethylene and Tefl on are recommended. The SAR is highly localized near the catheter. Cal culations concerning a fat-muscle interface show that the SAR is highe r in the fatty tissue than in the muscle tissue; 3D SAR central by ind ividually controlled electrode segments is essential in such a situati on.