Principles of Radiofrequency Thermal Ablation (RFTA)

A New Use of RF Energy

While electricity has been used in surgery for many years, RFTA presents a relatively new surgical alternative. RFTA differs significantly from traditional electrocautery, which employs a self-contained electrical device, and from surgical methods that use radiofrequency energy to cut tissue or coagulate blood (also sometimes referred to as electrocautery or RF Cautery).

Electrocautery, one of the earliest uses of electricity in surgery, was originally defined as the application of an electrically heated probe to tissue. In this approach, the probe itself is heated by a current flowing through a heater element in its tip. The heat is transferred by conduction from the probe to the tissue, and no current flows into the patient from the probe.¹

Later developments led to the use of radiofrequency (RF) energy to pass high frequency electrical current through the patient, making the patient part of a complete electrical circuit. Applied at very high power levels (one hundred to several hundred watts) and high voltage (up to 800 volts), RF energy can be used to cut through tissue (RF Cautery). With this method, the electrode is brought close to, or in actual contact with, the target tissue. The current forms an electrical arc ahead of the electrode and volatilizes the tissues, separating them as though they were cut.² This process is sometimes referred to as fulguration. By modifying the waveform of this current, the physician can use it to coagulate or dehydrate the edges of the severed tissues, thereby arresting bleeding in certain surgical operations. This type of electrosurgery generates tissue temperatures of many hundred degrees Centigrade (up to 800 degrees) most RF generators in common use today are intended to achieve both RF cautery and blood coagulation.

Radiofrequency Thermal Ablation (RFTA) differs fundamentally from the methods described above: it uses a low power level (as low as 2 to 10 watts), a low voltage (around 80 volts) and generates relatively low tissue temperatures (less than 100 degrees Centigrade); the electrode is put in direct contact with the target tissue, where, through resistive heating of the tissue itself, RF current creates a small lesion of necrotic tissue, which is later replaced by scar tissue and removed by the body, resulting in the shrinkage of the treated structure. This process is known as tissue coagulation (distinct from the above mentioned blood coagulation) and results in tissue ablation and tissue volume reduction.

How RFTA Works

Like other treatment methods using radiofrequency energy RFTA works by making the patient a part of a complete electrical circuit. An active electrode connected generator is inserted into the targeted area in the patient's body, with the electrode tip at the center of the area ablated. A return electrode is placed on a large muscle in the patient's body (generally on the back), and connected to the generator to complete the circuit.

When the generator delivers power, an RF current approximately 460 kilohertz and regular sine wave from the electrode into the surrounding tissue, towards the return electrode. The current causes ionic agitation in the tissue near the electrode tip, where the current density is highest, as the ions attempt to follow the direct variations of the alternating current.³ This agitation results in frictional (also called resistive) heating so that the surrounding the needle, rather than the needle itself, primary source of heat.

Irreversible damage to tissue (i.e., denaturation of tissue proteins) occurs at temperatures greater than 47 degrees Centigrade. Heat generated near the tip of the electrode with the RFTA method typically ranges from 40 to 90 degrees Centigrade, and drops off rapidly with increasing distance from the needle tip. This results in a small, controlled, oval-shaped lesion of necrotic tissue surrounding the electrode tip.

RF Heat Generation

Heat generated by passing an RF current through tissue increases proportionally to the square of current intensity and to duration of application, and in inverse proportion to distance from the needle tip, elevated to the fourth power.

By far the most influential of these factors is the distance from the needle tip. The heat generated at a given distance r (radius) from the needle tip varies as 1/r⁴. Tissue heating decreases very quickly with distance from the needle tip, so that the lesion remains circumscribed to an area close to the tip of the needle. This accounts for the precise, controllable lesion size characteristic of the RFTA method.

Factors Affecting Lesion Size

• Duration of RF energy application
  Within a small radius (a few mm) from the electrode tip, tissue heats up resistively under the influence of RF current. This heat is transported by conduction to tissue planes further away from the electrode, and in so doing extends the size of the lesion. The longer RF power is applied, the more the lesion grows in this fashion. However, as the outer limits of the lesion extend further away from the electrode tip, conduction dissipates heat to larger tissue areas, eventually failing to raise tissue temperatures over 47 degrees Centigrade, the threshold for tissue coagulation.

  In summary, lesion size increases with duration of power application until an equilibrium is achieved between RF heat generation and heat loss through conduction, at which point lesion size stabilizes, irrespective of longer power application.

• Current intensity
  The frictional heat produced from ionic agitator proportional to current density, which itself decreases with increasing distance from the needle tip. Therefore all other things being equal, and assuming homogeneous tissue, a given constant current intensity from the needle will result in a given lesion size.

  For a given tissue nature and needle configuration, there is an upper limit to the lesion size that can be achieved and there is one current intensity which will achieve maximum lesion size. Below that point, increasing current intensity results in increasing RF heat generation and larger lesion creation. Beyond that point, greater current intensities tend to heat up tissue too much, elevating tissue temperature to 100 degrees Centigrade and above and thereby causing carbonization of the tissue in the immediate vicinity of the electrode tip. This carbonization in turn increases tissue resistance to further passage of current (i.e. tissue impedance), which limits the extension of the lesion to a thin tissue shell surrounding the needle tip.

• Probe size
  Keeping all other factors equal, increasing the length of the electrode tip produces a larger lesion. However, as the length and the surface area of the exposed tip increase current intensity must be increased to maintain a current density adequate to generate sufficient heat to obtain the desired thermal tissue effects.

Histological Results of RFTA

On a cellular level, the tissue damage and recovery from RFTA demonstrates the body's typical pattern of tissue injury, followed by scar formation and retraction of tissue.

The lesion site one hour after the treatment shows the typical effects of tissue coagulation: the destroyed cells have a structureless, homogenous appearance, and there is edema and congestion within the tissues. At 24 hours, the lesion is well defined, with a rim of hyperemic tissue surrounding the white damaged tissue. A mild to moderate acute inflammatory response can be seen.

At 72 hours microscopic examination shows well-developed cell necrosis, with extensive loss of cell nuclei. Ten days after treatment, white tissue surrounds the lesion, indicating fibrosis, with collagen deposition replacing the dead muscle tissue. There is minimal edema, and the chronic inflammation present is associated with fibrosis. At three weeks, the lesion is white and glossy, with well formed scar tissue.

Small blood vessels within the lesion and on its periphery are destroyed during the RFTA procedure, but are reformed as the scar develops. The vessels surrounding the lesion remain intact and viable.

Clinical Effects

Most RFTA-based procedures can be performed under local anesthesia on an outpatient basis. The application of energy can be performed in a matter of minutes, depending upon the application in the body. Following the procedure, the patient may experience some edema and some pain, which should be mild enough to be treated with nonprescription medication for one or two days. Patients can typically return to work or to normal activities shortly after treatment.

The proximal end of the electrode, i.e. the part that is in direct contact with the mucosa during the procedure, is insulated in order to prevent mucosal thermal damage. When the distal end of the electrode is inserted correctly within the targeted tissue, the insulation protects the surface skin and the mucosal layer from burns or irritation.

The point of entry for the electrode is completed healed after 24 to 48 hours, after the initial swelling decreases.

The internal healing process for the lesion generally takes three to eight weeks, during which time the necrotic tissue is replaced by scar tissue, which has less volume than healthy cells and thereby creates retraction in the tissue. Over time, scar tissue is removed by the body, creating a further reduction in overall tissue volume.

In a recent study of the application of RFTA for the possible treatment of upper airway obstructions, this biological process has been shown to achieve tissue volume reduction of the targeted area of 26% in pig tongues, as measured days after RF current application.⁴

Summary

RFTA is a minimally-invasive surgical technique that uses radiofrequency current to reduce tissue volume in a precise targeted manner. The technique creates small, controlled lesions and has minimal effect on surrounding tissue making it appropriate for areas in proximity to vital organs. The necrotic tissue in the lesions is gradually replaced by scar tissue, which is then removed as part of the body's natural process.

Low-power radiofrequency energy can be harnessed to provide a relatively quick and painless procedure for tissue ablation. This energy form has numerous potential applications in the body: it has been used for two decades, first for cranial nerve problems, and then for cancer, cardiology and enlarged prostates. It is currently being investigated for use in the upper airway.