Bipolar Radiofrequency Ablation: an Evolution From Monopolar Radiofrequency Ablation
Monopolar radiofrequency ablation (MRFA) was approved by the US Food and Drug Administration in 1996 for the management of hepatic tumors. At present the technology has evolved to be used as a bipolar system. To readers who are not familiar with the difference between the two, this blog will be useful. Limitations to MRFA include long ablation times (10 minutes to 25 minutes per ablation) and the “heat-sink effect.” Convective heat loss occurs when tumors are located near large blood vessels in the liver or “heat sinks” (similar to radiators in the automobiles) which draw the energy toward the path of least resistance. During treatment, this path flows from the electrode (which is the needle placed for ablation), through the highly conductive liquid contained within the vessels, and to the grounding pad attached to the patient, bypassing the tumor. Burns have been reported at grounding pad sites. (Click images to enlarge.)
There are a few alternative ablation modalities that are more time efficient and circumvent the heat-sink effect, and these include microwave ablation (MWA) and irreversible electroporation (IRE). Currently, IRE is not in mainstream use, remains investigational, and is only available in selected centers. Microwave ablation requires less time than MRFA, with ablation times in the range of 7 minutes to 10 minutes, and it has been used with increasing frequency. At present, there is more data in the literature on MWA than IRE, but further studies are needed to confirm the effectiveness of this modality of ablation.
Some clinical outcomes as well as safety and technical information have been reported for bipolar radiofrequency ablation (BRFA), with results similar to early studies in MRFA. Bipolar radiofrequency ablation has been supported by few studies, but it offers the advantage of using available RFA generators, which in this day and age of economic constraints may be a big advantage. It can be performed with relatively fast ablations of 5 minutes to 7 minutes, and ablates all tissue between the two electrodes, preventing the heat-sink effect. The use of energy flow only between the two electrodes eliminates the need for grounding pads. The dissipation of energy between the two electrodes makes it very effective and this line of sight energy transmission makes it a very efficient technology. Potential disadvantages are that a new technology always has a learning curve and also introduction of two electrodes (needles). Although introducting two large electrodes is cumbersome, there is also an advantage to this. The needles can be less precisely placed than in MRFA where it has to be placed in the center of the tumor. In BRFA the electrodes can be placed on either side of the tumor to encompass it. The rigor in placing the needles is that they should be placed parallel to each other without any contact between the two.
In our experience all ablations were performed laparoscopically. We used the InCircle (RFA Medical Inc.) device. Intraoperatively, the goal was to place the two electrodes approximately 1 cm away from the borders of the tumor under laparoscopic ultrasound guidance. Either a 3.5 or 5.0 cm device was used, depending on the size of the tumor, with maximal distance between the electrodes being 4.5 and 6.0 cm, respectively, to avoid generating independent monopolar electrodes. Under laparoscopic guidance the needles were placed percutaneously. Initially, a spinal needle was introduced so that the trajectory of the needle could be checked with a much smaller gauge needle. Under ultrasound guidance, the point of entry on the liver is marked and then needles are placed. Initial ablation was achieved with either 60–80 W for 3 minutes for the 3.5 cm device or 100–135 W for 5 minutes when using the 5 cm device. The power was increased by 20 W after each minute until maximal impedance was reached. After ablation, an expedited confirmation ablation was performed until maximal impedance was again reached, usually taking about 30 sec. Tract ablations were performed at 100 W. Ablation time was recorded as the total time for ablation plus confirmation for each tumor.
Click the modalities and specialties below to see more related content:
