Total implants for hyperthermia application and thermometry

Tests have been performed in saline phantoms on a 6.78 MHz hyperthermia applicator designed for complete implantation; no conductors would penetrate the skin following the implant procedure. Energy is coupled across the skin from an external to an internal loop antenna: the latter is connected by transmission line to conventional r.f. tissue electrodes which are typically either parallel plates or arrays of cylindrical pins. In phantoms the technique produces deep, localized heating at a selected site without significant hearing at the 'subdermal' receiving antenna near the surface. In addition, the hyperthermia applicator may have potential as an indicator of temperature for the tissue volume at the deep site, since complex impedance at the external antenna is a function of tissue resistivity/temperature at the electrodes. Specifically, temperature at the deep site can be controlled effectively by regulation of the phase angle between voltage and current at the external antenna. A separate passive implantable electromagnetic-reflectance thermometer, energized by external electromagnetic fields, has been designed. This device, also tested in gel and water phantoms, is energized and interrogated by external r.f. fields and tracks a thermocouple to within +/- 0.1 degree C over the range of interest in clinical hyperthermia. The intended application for these devices is repeated, long-term hyperthermic treatment and thermometry of deep-seated malignant tumours following a single invasive procedure.