br Material and Methods br

Material and Methods
Results
Discussion Radiofrequency ablation has a long history of use. It has been used especially in the neurosurgical field [8,9]. Rosenthal described the first ablations in bone 20 years ago [5]. Technically the radiofrequency generator forms an electric current leading to an ionic agitation in the tissue creating heat. The phosphodiesterase inhibitors leads to cell death and coagulation necrosis. Radiofrequency ablation has several advantages: it is a minimal invasive therapy, major complications are unknown [6]. The energy application can be controlled by the applicator and generator design. Impedance measurements allow an exact thermal ablation of pathologic tissue. However radiofrequency ablation is used as a palliative therapy in bone metastases/ bone tumor patients [6,10]. Actually osteoid osteoma is the only bone tumor which can be treated by ablation following a curative approach [11,12]. The ablation instrument which was used in this study contained a bipolar, articulated extendable electrode with multiple thermocouples and a proprietary generator (Fig. 1). The electrode provides a site specific and controllable ablation. The possibility to articulate and extend the electrode enables to follow a preoperatively planned ablation scheme. In this study all ablation procedures were carried out sufficiently and without complications. The electrodes were positioned CT-guided. After introducing into the bone, the active length of the electrode was positioned in the center of the tumor tissue using articulation and extension of the instrument. A possible disadvantage of the electrode is that there is no feedback regarding the extent and the shape of the heat necrosis. In the CT or X-ray scan during the intervention the change of the heat reaction cannot be displayed [13]. Therefore the precision of the electrode is not sufficient to enable a curative treatment of the metastases. The setup of the procedure with the preoperative planning and the intraoperative guided positioning of the electrode is complex. But the design of the instrument is a step in the right direction offering variable opportunities in the treatment of bone metastases. Especially automation might be possible with this electrode using a CT data set for planning and enabling a complete ablation of the metastases. The animal model used in this trial enabled a reproducible induction of local bone tumor. The spheroids were stable and big enough to be inserted in the rabbit femur. They additionally prevented the tumor cells from being washed out into the body inducing a systemic tumor spread. All procedures were carried out without complications. A CT scan of the whole body showed no systemic spread. The mean tumor size was 26mm2 and big enough for testing radiofrequency ablation sufficiently. MRI scans before and after the radiofrequency ablation are essential to evaluate tissue changes of the thermal ablation before and after the intervention. In this study MRI after the ablation showed a heat necrosis of 23±6mm in size with remaining tumor tissue around the necrosis. MRI during the intervention is not possible due to the design of the RF system. Changes of the tissue cannot be displayed during the ablation. The study presented here has two disadvantages: the results of this animal trial cannot be transferred to clinical conditions readily and the animals were sacrificed after the radiofrequency ablation. Data would have been more detailed if the animals would have survived the procedure and another MRI scan would have been taken e.g. 1 week after the ablation. This was not possible due to the standards of our Ethical Committee. Rabbits were used in this study because the bone composition and density is nearly similar to human bone [14]. They are the most commonly used animals for medical research [15]. Bones of small animals like rats or mice are not optimal to sufficiently evaluate surgical techniques due to limited size and different microstructure [16]. VX-2 carcinoma was used in this trial because it is the only bone tumor available for rabbits.