An interstitial hyperthermia system at 27 MHz

Abstract

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.

Vorteile der wassergefilterten gegenüber herkömmlicher Infrarot-Strahlung in der Neonatologie

“…Darüber hinaus wurden Untersuchungen an künst-lichen, auf Agar-Basis hergestellten Gewebephantomen vorgenommen. Die aus der experimentellen Strahlenphysik entlehnte Agar-Rezeptur (930 ml H 2 O + 4,3 g NaCl, 37 ml Formaldehyd, 30 g Agar) dient im Prinzip dazu, Wasser ± woraus der Organismus bekanntlich zum groûen Teil besteht ± in eine ¹feste Formª zu bringen und so gallertartige Körper variabler Gröûe und Gestalt herzustellen, die in ihren thermischen Eigenschaften lebendem Gewebe nahekommen [20,21]. Aufgrund ihres überwiegenden Wassergehaltes weisen sie auûer-dem eine hohe Evaporationsrate auf, so dass sie zur Simulation des neonatalen Wärmeaustausches besonders geeignet sind.…”

Heating technology for malignant tumors: a review

“…Extensive research has been performed on interstitial heating techniques, resulting in several pre-clinical/research prototypes and commercially available products [227][228][229][230][231]. Investigations include hot sources [170,[232][233][234], laser sources [203,235], monopole, dipole, slot or helical coil microwave antennas [171,[236][237][238], resistively-coupled radiofrequency local current field electrodes [239], capacitively coupled radiofrequency catheter-based electrodes [240][241][242] and ultrasound transducers [243]. More detailed reviews of interstitial heating systems, including detailed system specifications, can be found in the literature [227][228][229][230][231].…”

“…When interstitial radiofrequency electrodes are used, the heating generated by the RF currents between electrodes is strongly influenced by local electrical tissue properties and the location and alignment of the other electrodes [244]. These drawbacks can be overcome by using electrodes operating from within low-loss catheters (e.g., Nylon or Teflon), creating high impedance capacitive coupling across the catheter wall, as in the 27 MHz multiple electrode current source (MECS) system [240][241][242]. A single probe consists of multiple independent electrodes, each 10-20 mm long.…”

Dose uniformity in MECS interstitial hyperthermia: the impact of longitudinal control in model anatomies

“…In this paper the influence of the (bulk) tissue inhomogeneity will be considered in the context of our 27 MHz multi-electrode current source (MECS) interstitial hyperthermia (IHT) system (Lagendijk et al 1995). The MECS technique is designed to deposit HF energy in tissue through electrodes inserted in plastic catheters (Visser et al 1989). Several independent electrodes can be operated in a single catheter.…”