RF Ablation

Mechanism

RF ablation is the most widely applied and thoroughly studied of the ablation technologies. The potential for local tumor destruction by RF was first appreciated when McGahan et al. published a paper detailing their work with in vivo porcine liver ablation using a rudimentary RF applicator based on the Bovie electrocautery system (45). Subsequently, there have been rapid improvements in the technology and it is now in widespread clinical use for the ablation of hepatic malignancies. In 1997, the first use of RF ablation was reported in the kidney (46), followed by the first percutaneous renal tumor ablation in 1998 (47), and during the last several years, multiple clinical series have been reported. These studies have established the safety and short-term efficacy of RF ablation for the treatment of renal tumors (15-18).

In RF ablation, a high-frequency alternating current (approximately 450 kHz) is applied to an electrode. Grounding pads are used to establish an electrical circuit through the body from the electrodes to the pads. The current agitates the tissue ions surrounding the electrode, thus creating molecular friction and heat. Because the surface area of the electrode is relatively small in comparison to the grounding pads, the current density around the electrode is quite high. This results in significant energy deposition and tissue heating around the electrode, but ideally not at the grounding pads. When the temperature reaches 50°C, the tissue coagulates (48). It is very important to apply the grounding pads correctly, because severe skin burns can result from incorrect pad placement. Four fundamental considerations for ground pad placement are as follows:

1. Pads should be placed equidistant from the ablation zone to avoid preferential heating of the nearest pad.

2. The long axis of the pad should be placed to receive the current.

3. Pads should never be placed superficial to a metal prosthesis or other metal object to avoid arcing.

4. The current path should not cross the heart if possible, especially if the patient has a pacemaker.

There are two primary factors that limit the size of the ablation zone when using RF. RF only creates ionic agitation, and thus only actively heats the tissues that are within a few millimeters of the electrode (49). This significantly limits the size of the ablation zone because the remainder of the tissue heating is due to thermal conduction. Utilizing a needle-shaped electrode, this dependence on thermal conduction limits the ablation zone to approximately 1.6 cm, even under ideal circumstances (45). The energy dispersion related to flowing blood at tissue vessel interfaces (''heat sink effect'') (50) further limits the size of the ablation zone and results in thermal protection of perivascular tissue and tumor (51). As a result, vessels larger than 2 mm can be spared along with the surrounding tissues. This effect at least partially explains the difficulty in obtaining complete ablation of centrally located renal tumors located adjacent to large vessels in the renal hilum (15). Also, because the active heating only occurs in the tissues directly adjacent to the electrode, these tissues can reach very high temperatures. This is problematic because when tissue temperature reaches 100° C, tissue boiling, dessication, and charring occurs. As a result, eschar forms around the active portions of the electrode. This eschar acts as an effective insulator, limiting further current deposition (52). Many modifications have been made to the electrodes and the generators to overcome these limitations, and ablation zones up to 7 cm in diameter can now be achieved in some circumstances, by using multiple-prong electrodes and saline infusion (52).

Current Ablation Technology

There are currently three companies with Food and Drug Administration (FDA)-approved devices available in the United States. The companies have modified the electrodes in different ways in an attempt to optimize the ablation zone size. The multiple-prong, expandable array is utilized by two devices (Fig. 5) (Starburst/Starburst XL, Rita Medical Systems, Mountain View, California; LeV-een needle electrode, Radiotherapeutics, Natick, Massachusetts) and involves placement of a needle cannula from which multiple electrode tines are deployed in an umbrella-shaped array. Each prong produces a discrete ablation zone and the conglomerate ablation can be quite large. An internally cooled electrode (Fig. 6) (Cool-tip single and cluster electrodes, Valleylab, Boulder, Colorado) has a cooled perfusate that flows within an internal lumen during the ablation. Active cooling of the electrode and adjacent tissues decreases the high temperatures encountered in close proximity to the RF probe, and thus decreases tissue charring. The decreased eschar formation results in decreased impedance, allowing more current deposition. The multiple-prong and triple internally cooled (cluster) electrodes have increased surface area as compared to a single electrode, which also decreases impedance and allows more current to be deposited in tissue when compared to a single electrode.

A developing technique is the use of perfused or "wet" electrodes (52). These electrodes have an opening in the active tip through which fluids are infused, usually before or during an ablation. Normal or hypertonic saline is usually used. This technique increases the size of the thermal ablation zone by multiple mechanisms. The saline acts to conduct the electricity and heat more efficiently than soft tissue. There is also direct osmotic damage to the tissues from hypertonic saline. If the infusion is

Multi Tined Expandable Electrodes

Figure 5 Multiple-prong RF electrodes. The prongs are deployed in an umbrella-like configuration after image-guided placement of the needle cannula into the target. The increased surface area and span of the multiple prongs result in an increase in the size of the ablation zone. Abbreviation: RF, radiofrequency.

Figure 5 Multiple-prong RF electrodes. The prongs are deployed in an umbrella-like configuration after image-guided placement of the needle cannula into the target. The increased surface area and span of the multiple prongs result in an increase in the size of the ablation zone. Abbreviation: RF, radiofrequency.

performed during the ablation, heated fluid is forced into the tissues (53). All of these factors lead to a larger zone of ablation. However, there is also a relative loss of control over the ablation zone because the infusate diffuses along tissue planes and can flow to unexpected locations, sometimes quite distant from the electrode itself (53,54). This lack of control has been a significant issue for hepatic ablations, but will probably not be as important for the kidney. Gerota's fascia should help contain the infusate, thus limiting collateral damage.

Cool Tip Electrode

Figure 6 Internally cooled electrodes. A cooled perfusate flows within an internal lumen, decreasing charring and eschar formation on the electrode. This decreases impedance and thus increases the size of the ablation zone. The cluster electrode gives a larger zone of ablation than the single electrode (Radionics, Medford, Massachusetts).

Figure 6 Internally cooled electrodes. A cooled perfusate flows within an internal lumen, decreasing charring and eschar formation on the electrode. This decreases impedance and thus increases the size of the ablation zone. The cluster electrode gives a larger zone of ablation than the single electrode (Radionics, Medford, Massachusetts).

During the last several years, RF generators and their current-control algorithms have improved. RF generators are capable of generating higher currents (up to 2.0 amp), leading to larger zones of ablation. The control of current deposition is based upon a feedback loop that modulates the current based upon impedance, current, or temperature. Fine-tuning the rate of current deposition allows maximum energy deposition while minimizing the amount of tissue charring that occurs. In addition, a multiple electrode system has now become clinically available. This system (Switching Controller Valleylab, Boulder, Colorado) can power up to three electrodes by switching at each impedance spike. As our understanding of the electrical and thermal properties of specific tissues increases, it is likely that different ablation algorithms will become available for different tissues and tumors (55).

Methods of Application

RF ablation can be performed during conventional open surgery, at laparoscopy, or as an image-guided percutaneous technique. Small-gauge RF probes and the inherent cautery associated with heat-based ablation decreases the likelihood of significant postprocedure hemorrhage. Applying the knowledge acquired during the development of hepatic ablation technology should speed the growth of renal RF ablation and ablation in general as an image-guided, percutaneous technique. Surgical or laparoscopic techniques should be reserved for cases with confounding variables that would make a percutaneous approach difficult or risky.

Results

The treatment of RCC with ablative techniques is a rapidly evolving, relatively young field, and as a result, only short-term outcome data are available. The data to this point are extremely promising with excellent primary and secondary local control and an exceptional safety profile (Table 1). However, because of the indolent course of many small RCCs, several more years of follow-up will be required to provide data that can be meaningfully compared with the results of radical and partial nephrectomy. Interpretation of the data, particularly when it comes to evaluating the limitations of ablation, is also confounded by the rapid improvements in the technology and techniques of ablation. Because of these improvements, the indications have been rapidly expanding and will continue to do so. Tumors that were considered unapproachable by ablation several years ago are now routinely treated in clinical practice. Also, as with many developing therapies, many of the patients included in clinical trials are poor candidates for traditional treatments and often have significant comorbidities. This may bias survival data to some extent.

Overall, the treatment of small (<3 cm) parenchymal tumors and frankly exo-phytic tumors has been very successful with minimal side effects and excellent local control. The ablation of larger (particularly >5 cm) or central tumors has been more difficult with a higher local recurrence and incomplete treatment rate (15-18). This led to at least one additional ablation session for approximately 25% of the central tumors (15). Results should continue to improve during the next several years as patient selection, RF technology, and the experience of practitioners progresses.

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