THERMAL-PROPERTIES OF CAPACITIVELY COUPLED ELECTRODES IN INTERSTITIALHYPERTHERMIA

The multi-electrode current source (MECS) interstitial hyperthermia sy stem, which is used for treatment of cancer, employs segmented electro des inserted in plastic tubes implanted in the treatment volume. The m ean power deposition of the individual electrodes is controlled by var ying the duty cycle of the RF signal applied to the electrodes, using thermocouples inside the electrodes for thermometry. A non-zero loss a ngle results in self-heating of the catheter. The thermal influence of self-heating was investigated and an analysis of the measurement of t emperatures inside the catheter during and after heating is presented. Analytical models and a high-resolution numerical model were used for the calculation of steady state and transient distributions, respecti vely. The model results are compared with experimental data obtained i n a muscle equivalent phantom. Results indicate that there is no diffe rence between temperature inside and outside the catheter when using l ossless catheter materials (e.g. PE and PTFE). Self-heating in the cat heter wall has an adverse effect on the uniformity of the stationary t emperature distribution and the reliability of temperature measurement with internal thermometry. These problems remain within acceptable li mits for mildly lossy materials; the difference between the temperatur e inside and outside is only 6% when using low-loss Nylon. Analysis of the thermal decay after power-off shows that low-loss materials allow more time to obtain an accurate estimate of the tissue temperature at the catheter wall during power-on. This effect is enhanced by the pre sence of minute air layers in the applicator. Distortion of temperatur e gradients along the catheter was also investigated. Key factors are the thermal conduction across the catheter wall, and especially the pr esence of minute layers of air between consecutive layers of the probe . The distortion extends less than two millimetres, which is acceptabl e. The simulation results are compatible with measurements in phantoms and show that, if the proper choice of materials is made, the MECS ap plicator answers our expectations and that the temperature measurement inside the catheter can be used for direct feedback treatment control .