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Objectives: The objective of this study was to determine the in vivo efficacy of radiofrequency ablation (RFA) in porcine liver using Octopus j electrodes for creating a large coagulation compared with RFA using clustered electrodes. Methods: A total of 39 coagulations were created using a 200-W generator and clustered electrodes or Octopus electrodes during laparotomy in 19 pigs. Radiofrequency was applied to the livers using four protocols: (1) Group A-1, monopolar mode using a clustered electrode (n511); (2) Group A-2, monopolar mode using an Octopus electrode (n511); (3) Group B-1, consecutive monopolar mode using three, clustered electrodes (n58); and (4) Group B-2, switching monopolar mode using two Octopus electrodes (n59). The energy efficiency, shape, diameters (D) and volume (V) of the coagulation volume were compared in each of the two groups. Results: The mean maximum D and V of the coagulations in Group A-2 (4.7 cm and 33.1 cm 3 , respectively) were significantly larger than those in Group A-1 (4.1 cm and 20.3 cm 3 , respectively) (p,0.05). Furthermore, the mean minimum D, maximum D and V of the coagulations in Group B-2 were significantly larger than those in Group B-1, i.e. 5.3 vs 4.0 cm, 6.6 vs 4.9 cm and 66.9 vs 30.2 cm 3 , respectively (p,0.05). The energy efficiencies were also significantly higher in Groups A-2 and B-2 than in Groups A-1 and B-1 (p,0.05). Conclusion: The Octopus electrodes were more efficient for creating a large ablation zone than clustered electrodes, and the efficacy of RFA with Octopus electrodes can be amplified in the switching monopolar mode.

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