THINGS TO KNOW  ABOUT RF ABLATION


What is endovenous RF ablation?

Since the early RF description, large numbers of liver RF ablations have been reported to date worldwide. Acceptance into mainstream medicine has been encouraging, although widespread use and acceptance as standard of care remains elusive and will hinge on production of high-quality clinical data. On the technical front, RF ablation will need to deliver larger tumor ablation volumes in a timely manner. The options for percutaneous tumor ablation has considerably evolved, with the introduction of several other thermal modalities, including microwave, cryoablation, high intensity–focused ultrasound scan, irreversible electroporation, and interstitial laser.

These new ablative techniques hold great appeal, and it is natural to consider investigating “new” thermal treatment options. However, to date, the evidence suggests that RF ablation should remain the prototypical ablation device, particularly for lesions 3 cm and should still be the cornerstone of any ablation practice. It is the most frequently used ablative technique and it has the longest track record. Familiarity with RF ablation mechanism of action, clinical rationale, and techniques remains critical to any successful ablation practice.

How does endovenous RF ablation work?

RF refers to the part of the electromagnetic (EM) spectrum bounded by the frequencies of 3 Hz and 300 GHz (Fig 1). EM radiation includes (in addition to radio waves), infrared radiation, the visible spectrum, ultraviolet radiation, x-rays and -rays, in increasing frequency. Although the EM radiation spectrum has been divided into many other subtypes (for example, ultrahigh frequency radio waves [UHF]), these divisions are arbitrary and they differ only in the frequency of particle oscillation.

Even though all EM radiation subtypes have the same basic physical properties, their interactions with matter can be very different and depend on their frequency and the type of matter. Radiofrequency waves as applied in medicine cause thermal ablation of a defined volume of tissue. The RF ablation electrode acts as the cathode of an electrical circuit, which is closed by the application of dispersing pads on the patient’s thighs. Because of the small cross-sectional area of the electrode tip, there is a very high energy flux around it. On the other hand, the large cross-sectional area of the grounding pads disperses the energy, thus minimizing the energy flux. As a result, tissue damage is limited to the part of the circuit that surrounds the electrode tip.

RF ablation results in coagulative necrosis of tissue from high temperatures. The dipole molecules (mostly water) immediately next to the tip of the RF electrode attempt to remain aligned in the direction of the current and are forced to vibrate as rapidly as alternating current is applied. Molecules that are farther away from the electrode are set into motion by other vibrating molecules near them. The frictional energy losses between adjacent molecules result in local energy deposition and temperature increase. With greater distance from the electrode, the resultant energy and generated temperature drops exponentially. The RF electrode itself is not the source of heat and is not hot to the touch. It generates an alternating EM field that sets adjacent molecules into motion causing intense agitation. These molecules immediate next to the electrode are the source of the heat, which is transmitted farther by tissue conductivity.

Advantages and disadvantages of rf ablation

There is considerable heterogeneity of heat deposition through any RF ablation tissue volume. The extent of coagulation necrosis is dependent on the energy deposited and local tissue interaction minus the heat lost from cooling effects, such as the “heat sink” of adjacent blood vessels . The heat sink effect is a phenomenon that limits the effectiveness of all thermal ablation methods. When the target lesion abuts a blood vessel 3 mm or larger, the flowing blood prevents large temperature variations in the part of the tumor near the lesion, thereby keeping the tissue “cooler” .

This phenomenon potentially leaves behind residual unablated tumor near the vessel wall, increasing the chance of local tumor progression. Factors that influence local tumor progression after RF ablation include tumor-related factors (size, site, orientation, organ, histology, biology) and technical factors (electrodes, generator power, heat sink, time).

Data from RF ablation of liver primary tumors have shown that 3 cm or less is the optimal size, and there is a persistent effort to improve and enlarge ablation size with improvement of RF ablation techniques . Goldberg et al described the following relationship regarding development of a thermal lesion: induced coagulation necrosis (energy deposited local tissue interactions) – heat loss. Investigators have taken different approaches to solve these physical constraints by modulating tissue characteristics, increasing RF energy deposition, or modifying blood flow.

GENERATORS

Early RF generators for percutaneous application produced modest outputs of 50 W. Current generators manufactured in the United States are capable of 200- to 250-W outputs, delivering high frequency (460 –500 kHz) alternating current via RF electrodes (usually 14 –17 gauge) of varying configurations. The three systems have distinctly different electrode designs as well as varying algorithms and methods to deposit energy. They also use different end points as a measure of clinical success. No definitive data have shown one system to be superior over another. Despite the company suggested algorithms, there is great variability among users. Personal preference and familiarity still play a distinct role in the varying achievable patient outcomes.

ELECTRODES

Most commonly available systems in the world are monopolar devices, whereby the RF current travels from generator to electrode in tumor, through the patient toward the skin pads, and back to the generator to complete the full circuit. A recent advent has been the development of bipolar systems, whereby two or more bipolar electrodes are placed into the tumor, and the applied RF current runs from one electrode to another without the need for grounding pads.

This essentially ensures that all electrodes within the tumor are active with minimal energy loss allowing greater ablation volumes more efficiently. Bipolar systems also do not require grounding pads and avoid the risks of skin pad burns. This may be helpful in circumstances in which mobile or fibrotic tumors are challenging to direct penetration. In early reports this bipolar system attained quick large burns and may show promise for larger ablation volume, compared with that of monopolar RF ablation (13–14). Several other systems are available in Europe and Asia.

Why should you use endovenous RF ablation?

Follow-up of our study group over 24 months showed a significant improvement in the clinical scoring of patients in both groups using the CEAP grading. Recurrence rate at 2 years follow-up was 13.3% in group A, with a comparable rate of 10% in group B. Other studies have reported recurrence rates ranging from 10% to 25% following RFA of the varicose veins.13,21,22 A recent Italian study reported a 0% recurrence at 5 years follow-up.

According to the current work, RFA was significantly more expensive than surgical intervention. Our study calculated the cost of the procedure, operative time, postoperative stay, and lost days from work. Our financial analysis used an estimated average that was based on Egyptian prices and wages that may not apply to other countries and even not to all Egyptian sectors. Hence a more accurate financial analysis is needed; however RFA remains looked upon as a more costly procedure.

We have no long-term results to confirm whether the obvious benefits of the less invasive endovenous RFA are met with the durability of the technique. Further studies are needed to answer that question and as well to compare experiences with RFA to endovenous laser treatment (EVLT), which has recently emerged as an alternative to the traditional surgery as a minimally invasive technique for the treatment of saphenous vein incompetence. Endovenous laser treatment of the GSV has been proven to be comparable with or superior to traditional high ligation and stripping in aspects of its GSV occlusion rate at 5 years follow-up and clinical improvement of symptoms.24,25 Endovenous laser treatment has also been shown to be as good as surgery in the recent 2 randomized controlled trials.

Complications following EVLT are infrequent; however, the majority of patients require secondary treatment of residual truncal varicosities, usually with sclerotherapy, ambulatory phlebectomy (AP), or both. These requirements of additional, subsequent procedures following endovenous laser saphenous vein ablation have led to dissatisfaction of the patients.

Therefore, it may be a relative obstacle to the otherwise high acceptability of EVLT.30 Although patient satisfaction scores were not used in the current work, yet the significantly lower incidence of complications, shorter postintervention hospital stay, and earlier return to normal physical activity would be expected to contribute to the overwhelming patient acceptance rate noted by other studies.

What complications can happen?

Complications more typical after open surgery such as bleeding, bruising and superficial infection are relatively rare after RF treatment. Recognised complications of the RF procedures, similar to other endovenous thermal ablation devices, include haematoma, thrombophlebitis, venous thrombosis, vessel perforation, thermal injury to adjacent nerves, skin burns and discolouration.13 These are potentially serious or commonly occurring (>1% of cases) complications and should inform the consent process. As before, most of the available data are for and its predecessor device. Historic rates of skin burns quoted (for older generations of the VNUS catheter) at 2–9%22,31,39 following RF have more recently dropped to 0–2% or.8,9,27,40 Burns are most likely to occur when treating superficial (extra fascial) veins.13,36,39 Skin injury may be a particular risk for, rather than other RF procedures, because of its direct heating effect, 120C operating temperature and slower cooling rate. Infusing tumescent anaesthesia to a minimum separation of 10 mm between the target vessel and skin by liberal and clearly visualised infiltration of the solution (or saline),13 minimises risk of burns. However, excessive external apposite compression will dissipate the solution after separation has been achieved. Burns are less likely for due to its automatic, impedence-feedback cut-off when the treatment sheath is entered. No instances of skin burns occurred in our own series.

Where is endovenous RF ablation used?

Chronic venous insufficiency (CVI) is a major medical disease in the United States. With a total population of 300 million, it is estimated that 25 million persons in this country alone have symptoms of this disease (1 in 12). Great saphenous vein (GSV) reflux is the most common form of venous insufficiency in symptomatic patients and is most frequently responsible for varicose veins of the lower extremity. Therefore, therapy directed toward correcting superficial venous pathology is beneficial to many patients. In the United States, surgical high ligation and stripping is rapidly becoming senescent and will soon be extinct.

Endovenous thermal ablation of the GSV is safe and effective with faster recovery and better cosmesis than surgical high ligation and stripping. The two methods of thermal ablation presently in comprehensive vein centers are the procedure, which uses a catheter to direct radiofrequency (RF) energy from a dedicated generator, and endovenous laser (EVL) ablation, which employs a laser fiber and generator to produce focused heat. Both systems use electromagnetic energy to destroy the refluxing GSV. When this energy is delivered at the vein wall (RF or 1,320 nm laser), there is collagen shrinkage and venous spasm with minimal formation of thrombus.

When focused at the hemoglobin chromophore (810, 940, 980 nm lasers), heat injury of the endothelium by steam bubbles originating from boiling blood is the mechanism of action. Sonographic disappearance of the treated vein is the desired end result. There is a growing body of literature reporting excellent long-term results with RF and laser ablation of the saphenous vein. Interestingly, neovascularization, a principle cause of varicose vein recurrence after surgical high ligation and stripping, is rare after thermal ablation.

What does scientific literature say about endovenous RF ablation work?

The present randomized clinical trial showed that RFA of the GSV offers a safe and effective alternative to the traditional surgical approach in patients with primary varicose veins. Radiofrequency ablation has a lower overall complication rate and is less invasive than the surgical approach. Patients are hence more comfortable with the earlier post-intervention return to physical activity, but it remains a costly procedure. Comparative occlusion rates between RFA and surgical approach to GSV occlusion in primary varicose veins have been previously reported and are very encouraging.10 Our study showed a primary occlusion rate of 94% for RFA and 100% for surgery.

This is comparable to previous reports where Hingorani et al11 reported a 96% primary occlusion rate and Puggioni et al reported a 100% occlusion rate. None of our patients had DVT as complication to RFA. Previous studies reported comparable rates 1%, 13 0.7%, 14 and 0%. 15 A higher rate of 2.1% was reported by Mozes et al.16 A 16% incidence of DVT was reported following RFA in a previous study. The mean age of their study group was 62 þ 14 years, hence significantly older than ours. The incidence of DVT following RFA seems to rise above the age of 50 years and hence routine prophylaxis in these age groups has been suggested.

Still, we believe that routine Doppler ultrasonography following RFA is an essential part of patient assessment to rule out DVT, despite its rare incidence. None of our patients developed pulmonary embolism, the same was reported by others.14,15 However, Merchant et al13 reported an incidence of 0.3%. Minor complications were rare in group A, they were in the form of local parethesia (10%), pain (13.3%), thrombophlebitis (6.6%), and hematoma (3.3%). None developed cellullitis, edema, or skin burns. The incidence of postoperative complications was significantly less than that reported in group B. Comparable rates of complications were reported by other studies, skin burns in 2% to 7% of patients, paresthesia in 0% to 15%, 12,15,17-19 thrombophlebitis in 2% to 3%, 12,15,17-20 and hematoma in 5%.

Follow-up of our study group over 24 months showed a significant improvement in the clinical scoring of patients in both groups using the CEAP grading. Recurrence rate at 2 years follow-up was 13.3% in group A, with a comparable rate of 10% in group B. Other studies have reported recurrence rates ranging from 10% to 25% following RFA of the varicose veins. A recent Italian study reported a 0% recurrence at 5 years follow-up. According to the current work, RFA was significantly more expensive than surgical intervention. Our study calculated the cost of the procedure, operative time, postoperative stay, and lost days from work. Our financial analysis used an estimated average that was based on Egyptian prices and wages that may not apply to other countries and even not to all Egyptian sectors. Hence a more accurate financial analysis is needed; however RFA remains looked upon as a more costly procedure.

Advantages and disadvantages of rf ablation

Given the fact that the majority of experimental and clinical data for tumor ablation has been reported for RF ablation– based systems, RF ablation will remain relevant for the foreseeable future, as data are accumulating with novel ablation techniques. Much of oncologic practice is driven by evidence-based medicine. Nonetheless, RF ablation certainly has relative strengths and weaknesses, particularly when compared with other developing thermal modalities.

Overall, RF ablation has the advantage of being the first commercially viable ablation device and is currently available widely. It is relatively cost effective compared with other newer devices, such as microwave ablation and high-intensity– focused ultrasound scan. However, its main limitation is the lack of consistency when attempting to obtain larger homogenous tumor ablations (5–7cm) as well as the time needed to deposit sufficient energy with RF ablation. There has been considerable interest in combining RF ablation with adjuvant cytotoxic therapies, chemoembolization, intravenous liposomal Doxorubicin, and radiation therapy.

One of the limitations of RF ablation is that it is heavily dependent on good electrical and thermal tissue conductivity. If one pushes the generator’s power too high too quickly, the tissue around the electrode becomes desiccated (charred). The desiccated tissue acts as an insulating “sleeve” around the electrode, which limits the transmission of further electrical or thermal energy (Fig 4) and limits any further extension of desired tissue destruction. It is instructive to note that time is just as crucial in achieving a large ablation zone as the maximum temperature reached. Mammalian tissue is very sensitive to temperature changes. At 55°C, for example, tissue death results within 2 seconds. At 100°C, death is instantaneous as evaporation occurs. Micro bubbles primarily containing nitrogen are produced and released from the cells. This, however, is not desired in RF ablation because of the insulating effect of charred tissue. Thus, a slow, methodical energy deposition is more effective than a quick temperature rise for purposes of enlarging intentional tissue ablation (Fig 6) (6). The objective is to heat tissues to 50°–100°C for 4 – 6 minutes without causing charring or vaporization. If temperatures greater than 105°C are reached rapidly, boiling, vaporization, and carbonization result, all of which decrease energy transmission and consequently limit larger ablation sizes.