An interstitial hyperthermia system using thin, flexible wires operating at a frequency of 27 MHz has been evaluated for possible use in combination with (iridium) brachytherapy applications employing flexible nylon afterloading catheters. This method of interstitial heating is argued to be a variation of local-current-field heating, however with the direct galvanic contact between electrode and tissue replaced by a capacitive coupling between wire and tissue. Through a special design of the wire the length of the heated area along the catheter can be chosen freely. Standard non-prepared implantation catheters can be used, so that no change from the established implantation techniques is required. Possible advantages of these 27 MHz wires over coaxial dipole antennas at higher frequencies are simplicity and low cost of the heating system, free choice of effective heating length for each wire and uniformity of heating along this effective heating length up to the tip of the wire. Flexibility of the wires is maintained, enabling application in curved (even U-shaped) catheters, e.g. for head and neck implantations. Impedance matching of the wires to the generator, necessary for an optimal transfer of power, appears possible by means of a variable air coil. Care should be taken to avoid stray capacitances of the connecting cables with respect to ground or among the different cables used. Measurements in muscle-equivalent split phantoms with infrared thermography have been performed with both single and multiple wires. The resulting SAR distributions confirm the working principle of these low-frequency wires (resistive heating) and illustrate the difference with radiative antennas at higher frequencies. The system is being applied both in experimental animal studies and in a clinical pilot study.

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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