A 27 MHz capacitive-coupling interstitial hyperthermia system has been developed. It uses thin flexible applicators which can easily be inserted in standard brachytherapy catheters. The system can be operated in two different configurations. In the external ground return configuration tissue is heated by currents passing from the catheters to external ground returns; in the balanced configuration, by currents passing between applicators with a phase difference of 180 degrees. The purpose of this study was to find out which configuration is preferable, in terms of temperature homogeneity and clinical usefulness. Model calculations show that, due to the high impedance associated with capacitive coupling, the applicators can be represented as current sources, in contrast to local current field electrodes which are voltage sources. SAR measurements in muscle-equivalent phantoms illustrate that homogeneous heating patterns along the catheters can be expected in both configurations in regular as well as in irregular implants. Using the external ground return configuration the power of each applicator can be controlled individually. Calculations and measurements show that cross-talk cannot be avoided completely in this configuration, but that it can be minimized by using applicators operating in phase.

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