DESIGN OF APPLICATORS FOR A 27-MHZ MULTIELECTRODE CURRENT SOURCE INTERSTITIAL HYPERTHERMIA SYSTEM - IMPEDANCE MATCHING AND EFFECTIVE POWER

In interstitial heating one of the main requirements for achieving a c ertain elevated temperature in a tumour is that the effective power pe r applicator (P-eff), i.e. the power which is actually deposited in th e tissue, is sufficiently high. In this paper this requirement is disc ussed for the applicators of the 27 MHz multielectrode current source (MECS) interstitial hyperthermia (IHT) system. To minimize power refle ction, the applicator impedance was matched with the generator impedan ce by adjusting the length of the coaxial cable in between. Transmissi on line losses, applicator efficiency and subsequently P-eff were comp uted for several applicator types. The actual P-eff per electrode was obtained from calorimetric measurements. Experiments with RC loads, wh ich can be seen as perfect applicators, were performed to investigate the effect of mismatching on P-eff. Applicator losses were measured fo r clinically used applicators, both single- and dual-electrode, utiliz ing saline phantoms. A simple spherical tumour model, using the effect ive heat conductivity (k(eff)) to account for heat transport, was used to estimate P-eff for a given tumour size, implant size and applicato r density. Computations of P-eff of various MECS-MT electrodes were in close agreement with the phantom measurements. Most of the initial ge nerator power was absorbed in the transmission line (60-65%). The effi ciency of the applicators was about 65%. For both single- and dual-ele ctrode applicators the effective electrode power was found to be about 1 W. Model calculations show that P-eff Of 1 W is sufficient to reach a minimum tumour temperature of 43 degrees C in well perfused tumours (k(eff) = 3 W m(-1) degrees C-1), using a typical implant with 2 cm e lectrodes and 1.5 cm spacing. Mismatching can considerably affect P-ef f. Both a reduction to almost zero and a two-fold increase are possibl e. However, because the matching theory is well understood, mismatchin g is not a serious problem in clinical practice and can even be used t o increase P-eff if necessary. We conclude that the applicator design and the impedance matching method chosen in the MECS system allow heat ing to temperatures in the therapeutic range with implants used in cli nical practice.