Radiofrequency Ablation of Hepatic Lesions: A Review

Venkataramu N. Krishnamurthy, MD; V. Javier Casillas, MD; Lina Latorre, MD

Disclosures

Appl Radiol. 2003;32(10) 

In This Article

Ablation of Lesions Under Special Circumstances

When the tumor is >=3 cm, single ablation is not sufficient, thus increasing the concern for under treatment and tumor recurrence. The difficulty in achieving complete ablation of larger tumors is not only because of the tumor volume, but also because of poor local electrical and heat conductivity within the tumor.[29] The latter is attributed to the heterogeneity of tumor caused by fibrosis and calcifications. Given these factors, the strategy to obtain larger ablation includes overlapping of ablations, improving electrical conductivity within the tumor by intratumoral saline injection, and modifications of electrode design.

Overlapping ablation technique. It is logical to expect that overlapping ablations can produce larger ablation volumes. After a single-ablation sphere, 6 overlapping ablations (4 in x-y plane and 2 in the z-axis) produce, geometrically, the next largest ablation. This approach will produce a composite sphere with internal diameter of only 3.75 cm (just 1.25 times the single-ablation sphere). This is suitable for tumors <1.75 cm in diameter (subtracting 2 cm from the 3.75 cm diameter for the 360° 1-cm tumor-free margin). Fourteen overlapping ablations is the next step, and the composite sphere produced will have a diameter of 5 cm (1.7 times that of the single-ablation sphere). This is suitable for a 3-cm diameter tumor (subtracting 2 cm from the 5-cm diameter for the 360° 1-cm tumor-free margin). In actual practice, it is very difficult (almost impossible) to reproduce this model. To overcome this problem, the concept of overlapping cylinder strategy has been advocated.[30] In this technique, spheres of ablation are overlapped linearly to create cylinders, and the cylinders are then overlapped. Geometrically, this model may produce imperfect coverage, but it can be performed practically with greater ease and success.

Improving electrical conductivity within the tumor. Intratumoral saline injection prior to or during RF ablation improves electrical conductivity within the tumor. This causes greater deposition of the RF energy, leading to increased tumor coagulation.[31] It is of interest that neither the concentration nor the volume of the saline solution correlates linearly with the size of the RF thermal injury. Therefore, optimal saline concentration and volume need to be titrated carefully for each of the different devices and different tumors.

Modifications of electrode design. Electrode modifications to ablate larger tumors include clustered array electrodes, bipolar electrodes, and pulsed RF deposition systems.[24,32,33] Clustered array electrodes (such as the Radionics model) represent an extension of the same concept as the single internally cooled electrode described earlier. Goldberg et al[24] demonstrated that the sphere of coagulation necrosis produced by a cluster of electrodes placed <1 cm apart is greater than that created by a composite of individual electrodes. Their in vivo experiments on liver ablation demonstrated that cluster electrodes placed 0.5 cm apart produced an ablation sphere of 3.1 cm versus 1.8 cm produced by conventional single electrodes under otherwise similar conditions.[24] Their initial clinical experience in 10 patients with colorectal metastasis showed ablation injury of 5.3 cm ± 0.6 using a clustered electrode with a single 12- to 15-minute ablation.

A bipolar electrode has a different design. There is no grounding pad; instead, there is an active electrode and a closely placed grounding electrode. The heat is generated not only around the active electrode, but also around the grounding electrode and in the space between the two. This is in contrast to the monopolar electrode, where heat is generated only at the active electrode. Early clinical experience demonstrates that bipolar needles produce a larger coagulation volume of 3-cm diameter by a single application alone.[32] Absence of a grounding pad eliminates the risk of grounding pad burns also.

Pulse RF ablation is another strategy aimed at increasing volume of coagulation by increasing the RF energy deposition.[33] In this technique, higher energy deposition is alternated with lower energy deposition. During periods of low energy deposition, the tissue around the electrode cools down, allowing for even higher energy deposition during the next cycle of ablation. This method allows for deeper heat penetration, creating a larger ablation zone. The experience is limited, but seems a promising technique for treating larger lesions.

Tumors that are subcapsular or at the liver hilum; and those adjacent to the gallbladder, colon, intestines, and diaphragm are of special concern because of the difficulty in achieving complete ablation with a good tumor-free margin and increased risk of complication. In the case of subcapsular tumors, if the tumor involves the capsule, the latter should also be ablated with the tumor (Figure 2). Careful selection of the needle configuration is important. If the tumor is small and rounded, an expandable needle system (such as the RITA and Radiotherapeutics models) is useful because they can produce more spherical ablation injury compared with the straight Cool-tip needles (Radionics). For tumors that are stranded at the angles of liver, such as in the inferior angle of right lobe or in lateral segment of left lobe, a straight cooled-tip needle would be more useful since it can be fully deployed for ablation. The prongs of the expandable needle systems often penetrate the liver capsule, increasing the risk of bleeding when they are fully deployed. If the expandable needle has to be used, then the prongs should be partially deployed and ablation has to be performed multiple times to cover the whole tumor.

(A) Hepatocellular carcinoma ablation. CT scan of the liver shows a well-circumscribed hypodense mass in the right lobe abutting the capsule. (B) One-month follow-up CT scan in the same patient shows complete ablation of the tumor. The lesion shows low-attenuation with thin smooth margins. Note lack of contrast-enhancement or nodules in the wall.

(A) Hepatocellular carcinoma ablation. CT scan of the liver shows a well-circumscribed hypodense mass in the right lobe abutting the capsule. (B) One-month follow-up CT scan in the same patient shows complete ablation of the tumor. The lesion shows low-attenuation with thin smooth margins. Note lack of contrast-enhancement or nodules in the wall.

Tumors at the hepatic hilum present another distinct problem because of their proximity to main vessels, eg, the portal vein and hepatic artery. These large vessels produce what is known as perfusion-mediated tissue cooling or the heat-sink effect.[34] The rapid flow of blood in these vessels washes out the heat quickly, preventing build-up of optimal temperature to induce coagulation necrosis in the periphery of tumor adjacent to the vessel. In pig liver models, Hansen et al[35] showed that vessels >3 mm in diameter prevent complete ablation of liver tissue. Studies have shown that reducing hepatic perfusion by mechanical or pharmacologic methods improves coagulation in the tumor.[34,36] This can be produced at laparotomy or during the laparoscopic-guided approach by clamping the hepatic artery and portal vein in the hepatoduodenal ligament (Pringle maneuver). Although clamping can be applied safely for up to 1 hour, there is increased risk of liver ischemia and related complications, such as liver failure and biliary strictures. Also, resorting to open procedures would offset the potential benefits of a minimally invasive percutaneous approach. Alternatively, angiographic balloon occlusion of a hepatic artery has been used during percutaneous procedures.[37] Hepatic artery embolization with temporary embolic agents, such as gel foam has also been reported. However, the practical utility and the efficacy of these additional maneuvers is questionable because of the invasive nature of the techniques and the dual blood supply of the liver. Other techniques for reducing blood flow, eg, pharmacologic modulation and antiangiogenesis therapy, are largely experimental at this time.[36]

There is another difficulty in targeting tumors at the hepatic hilum, since the tumor is located in the space between large vessels, which is small and narrow. The expandable needle electrodes should be partially deployed; otherwise they may penetrate the large vessels, resulting in internal bleeding or pseudoaneurysm formation. This usually necessitates multiple ablations. Straight-tip internally cooled needles are advantageous in this location because they can be deployed fully and there is little concern for vessel penetration. They also have better tumor coagulation due to internal cooling design, as discussed earlier.

The tumors located close to the diaphragm or other viscera, such as the gallbladder and intestines, are also difficult to treat. Diaphragmatic thermal injury can cause severe, persistent pain and breathing difficulty, leading to lower lung atelectasis and pleural effusion. The chronic pain may require long-term analgesics. Damage to viscera can cause cholecystitis or ischemic bowel injury. Therefore, treatment of tumors located at these critical locations may be better performed using open surgical procedures, as will be discussed in the following section.

Tumors located adjacent to the dia-phragm, gallbladder, or bowel can be treated more easily during an open procedure because the liver and the adjacent organs can be separated or mobilized, thus avoiding potential injury.[38,39] These techniques also allow for more accurate tumor staging, like detection of peritoneal or surface implants and lymphadenopathy. They provide an opportunity to perform intraoperative US that can detect unsuspected tumor deposits (Figure 3). These findings can avoid an unnecessary local tumor treatment.[39,40] Both of these approaches also allow for performance of the Pringle maneuver, which can increase the extent of tumor coagulation. But the open techniques are invasive and more costly, and are associated with higher morbidity and mortality than percutaneous approaches.

Intraoperative radiofrequency ablation. (A) Preoperative CT scan shows an ill-defined single hepatoma lesion in the segment 5 of right lobe of liver. (B) Intraoperative ultrasound shows tumor abutting the transjugular intrahepatic shunt (TIPS) (bright curvilinear parallel ehoes). Additional tumors (not shown) were also seen in segments 4 and 5. Intraoperative radiofrequency ablation of all three lesions was performed. (C) One month follow-up CT scan shows complete ablation of all three tumors and a patent TIPS.

Intraoperative radiofrequency ablation. (A) Preoperative CT scan shows an ill-defined single hepatoma lesion in the segment 5 of right lobe of liver. (B) Intraoperative ultrasound shows tumor abutting the transjugular intrahepatic shunt (TIPS) (bright curvilinear parallel ehoes). Additional tumors (not shown) were also seen in segments 4 and 5. Intraoperative radiofrequency ablation of all three lesions was performed. (C) One month follow-up CT scan shows complete ablation of all three tumors and a patent TIPS.

Intraoperative radiofrequency ablation. (A) Preoperative CT scan shows an ill-defined single hepatoma lesion in the segment 5 of right lobe of liver. (B) Intraoperative ultrasound shows tumor abutting the transjugular intrahepatic shunt (TIPS) (bright curvilinear parallel ehoes). Additional tumors (not shown) were also seen in segments 4 and 5. Intraoperative radiofrequency ablation of all three lesions was performed. (C) One month follow-up CT scan shows complete ablation of all three tumors and a patent TIPS.

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