Temperature measurement errors with thermocouples inside 27 MHz current source interstitial hyperthermia applicators

The multielectrode current source (MECS) interstitial hyperthermia (MT) sys tem uses thermocouple thermometry. To obtain a homogeneous temperature dist ribution and to limit the number of traumas due to the implanted catheters, most catheters are used for both heating and thermometry. Implications of temperature measurement inside applicators are discussed. In particular, th e impact of self-heating of both the applicator and the afterloading cathet er were investigated. A one-dimensional cylindrical model was used to compute the difference betw een the temperature rise inside the applicators (Delta T-in) and in the tis sue just outside the afterloading catheter (Delta T-out) as a function of p ower absorption in the afterloading catheter, self-heating of the applicato r and the effective thermal conductivity of the surrounding tissue. Further more, the relative artefact (ERR), i.e. (Delta T-in - Delta T-out) / Delta T-in, was measured in a muscle equivalent agar phantom at different positio ns in a dual-electrode applicator and for different catheter materials. A m ethod to estimate the tissue temperature by power-off temperature decay mea surement inside the applicator was investigated. Using clinical dual-electr ode applicators in standard brachytherapy catheters in a muscle-equivalent phantom, Delta T-in, is typically twice as high as Delta T-out. The main re ason for this difference is self-heating of the thin Feeder wires in the ce ntre of the applicator. The measurement error caused by energy absorption i n the afterloading catheter is small, i.e. even for materials with a high d ielectric loss factor it is less than 5%. About 5 s after power has been sw itched off, T-in in the electrodes represents the maximum tissue temperatur e just before power-off. This delay time (t(delay)) and ERR are independent of T-in. However, they do depend on the thermal properties of the tissue. Therefore, ERR and t(delay) and their stability in perfused tissues have to be investigated to enable a reliable estimation of the tissue temperatures around electrodes in clinical practice.