Switching Monopolar Radiofrequency Ablation Technique Using Multiple, Internally Cooled Electrodes and a Multichannel Generator: Ex Vivo and In Vivo Pilot Study

Objective:We sought to determine the optimal switching time and interprobe distance for creating a large coagulation zone in the liver by switching monopolar radiofrequency ablation (RFA) technique using a prototype multichannel radiofrequency generator and multiple electrodes. Materials and Methods:Using 3, 17-gauge, internally cooled electrodes and a prototype multichannel radiofrequency (RF) generator to allow automatic switching of RF energy among 3 electrodes according to their impedance changes, RF energy was applied in a switching monopolar mode to explanted bovine livers or to the livers of 7 dogs. In the ex vivo study, we evaluated the effect of the switching time (5, 15, 30, and 60 seconds) and the interprobe distance (3 cm and 4 cm) on the mean volume of the ablation zone using the switching RFA; we then compared the ablation volume of the switching RFA group (12 minutes) with that of the overlapping RFA group (12 minutes × 3 = 36 minutes). In our preliminary in vivo experiments using 2 dogs, the electrodes were placed in a triangular array (spacing 2 cm, 3 cm, 3.5 cm, or 4 cm) and were activated for 12 minutes or 20 minutes in a monopolar mode (power output 200 W). In our primary experiments using 5 dogs, RFA in a switching mode created 10 coagulation zones at a 2- to 3-cm interprobe distance. The duration of the RFA was 12 minutes. The size and geometry of the coagulation zone also were assessed. Results:In this ex vivo study, switching monopolar RFA at a 3-cm interprobe distance and at a 30-second switching time allowed the creation of a large, confluent ablation zone. There was no significant difference in the mean ablation volume between the overlapping (72.9 ± 12.6 cm3) and the switching RFA groups (65.7 ± 12.6 cm3; P > 0.05). In the preliminary in vivo experiments, RFA created spherical coagulations at interprobe distances of 2 cm and 3 cm but at larger distances than 3 cm, RFA created partially confluent coagulation zones. In our principal experiments, switching RFA created areas of well-defined coagulation, ie, at a 2-cm interprobe distance, the volume and short-axis coagulation diameter were 35.5± 5.7 cm3 and 4.6 ± 0.5 cm, respectively, whereas at 3 cm, they were 40.7 ± 12.8 cm3 and 4.8 ± 0.8 cm, respectively. The mean values of the circularity (isometric ratio) of the coagulation at the 2- and 3-cm interprobe distances were 0.95 ± 0.02, and 0.85± 0.06, respectively. Conclusions:Our study demonstrated that switching monopolar RFA using the multichannel RF system at a 2- or 3-cm interprobe distance and at a 30-second switching time can create a large, confluent coagulation zone in the liver within a clinically acceptable time frame. We believe that this technology will provide a useful tool for the treatment of large liver tumors.