Radiofrequency ablation: simultaneous application of multiple electrodes via switching creates larger, more confluent ablations than sequential application in a large animal model

Abstract

Purpose: To compare radiofrequency (RF) ablations created by using a sequential technique to those created simultaneously by using a switching algorithm in ex vivo and in vivo liver models.

Materials and methods: RF ablation was performed by using either sequential or switched application of three cooled electrodes in a 2-cm triangular array in ex vivo bovine liver (28 total ablations) and in vivo swine liver (12 total ablations) models. For sequential ablations, electrodes were powered for 12 minutes each with a 5-minute rest interval between activations to simulate electrode repositioning. Switched ablations were created by using a multiple-electrode switching system for 12 minutes. Temperatures were measured during ex vivo experiments at four points in the ablation zone. Ablation zones were measured for minimum and maximum diameter, cross-sectional area, and isoperimetric ratio. Mann-Whitney and Wilcoxon matched pairs tests were used to identify differences between groups.

Results: The switched application created larger and more circular zones of ablation than did the sequential application, with mean (+/-standard deviation) ex vivo cross-sectional areas of 25.4 cm(2) +/- 5 .3 and 18.8 cm(2) +/- 6.6 (P = .001), respectively, and mean in vivo areas of 17.1 cm(2) +/- 5.1 and 13.2 cm(2) +/- 4.2 (P < .05). Higher temperatures and more rapid heating occurred with the switched application; switched treatments were 74% faster than sequential treatments (12 vs 46 minutes). In the sequential group, subsequent ablations grew progressively larger due to local ischemia.

Conclusions: Switched application of three electrodes creates larger, more confluent ablations in less time than sequential application. Thermal synergy and ablation-induced ischemia both substantially influence multiple-electrode ablations.

Figure 1

Experimental setup for both groups. A spacer was used to ensure parallel insertion and maintain the 2.0 cm spacing between electrodes. All in vivo procedures were performed at open surgery.

Figure 2

Representative ablations created in ex vivo bovine liver tissue using sequential (left) and simultaneous (right) application of three electrodes. Ablations created simultaneously were consistently larger and more circular in cross section, had better temperature profiles and could be created much faster than those created sequentially.

Figure 3

Mean temperature elevations recorded at each time point for sequential and simultaneous application of three electrodes ex vivo. Temperatures were consistently higher and rose faster at each measurement point when using simultaneous application.

Figure 4

Representative ablations created in vivo using sequential (left) and simultaneous (right) application of three electrodes. As in the ex vivo study, ablations created simultaneously were again larger and more circular in cross section than those created sequentially. Note that each subsequent ablation in the sequential image is larger than previous ablations. This effect is most likely a result of local ischemia created by previous ablations.

Figure 5

Local ischemia induced after the first ablation of a sequential ablation (edges marked with arrows). The reduced perfusion caused by this ablation allowed subsequent ablations to grow larger in size, due to the lack of perfusion-mediated cooling. Ischemia is more pronounced in areas peripheral to the ablation.

Figure 6

Simplified example comparing a static-size model (left) to a more realistic model (right) that accounts for changes in ablation size to predict coverage of a 3 cm tumor using the overlapping ablation technique. The initial ablation size was equivalent in both models (1.8 cm diameter) but in the dynamic-size model the ablation diameter grew by 13, 25 and 25 percent, based on the results of the present study. The static-size model requires seven ablations to cover the tumor with a small margin whereas the model based on in vivo results requires only four.