In addition to the laser, microwave or other energy sources, interstitial thermotherapy with radio-frequency current (RFITT) in bipolar technique has already been shown in vitro to be a safe and economical alternative energy source with a comparable operating performance. The bipolar technique is, from the technical point of view, completely without risk whereas with monopolar devices, where a neutral electrode has to be applied, an uncontrolled current flow passes through the patient's body. The therapeutical application efficiency of these bipolar RF-needle applicators was evaluated using newly designed high performance flushed and cooled probes (qq 3 mm). These can be used to create large coagulation volumes in tissue such as for the palliative treatment of liver metastases or the therapy of the benign prostate hyperplasia. As a result, the achievable lesion size resulting from these flushed and internally cooled RF- probes could be increased by a factor of three compared to a standard bipolar probe. With these bipolar power RF- applicators, coagulation dimensions of 5 cm length and 4 cm diameter with a power input of 40 watt could be achieved within 20 minutes. No carbonization and electrode tissue adherence was found. Investigations in vitro with adapted RFITT-probes using paramagnetic materials such as titanium alloys and high performance plastic have shown that monitoring under MRI (Siemens Magnetom, 1.5 Tesla), allows visualization of the development of the spatial temperature distribution in tissue using an intermittent diagnostic and therapeutical application. This does not lead to a loss in performance compared to continuous application. A ratio of 1:4 (15 s Thermo Flash MRI, 60 s RF-energy) has shown to be feasible.