Percutaneous Radiofrequency Ablation of Liver Tumors Technique

Updated: Oct 23, 2015
  • Author: Badar Bin Bilal Shafi, MBBS, MRCP, FRCR, CCT, EBIR; Chief Editor: Kurt E Roberts, MD  more...
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Technique

Approach Considerations

In radiofrequency (RF) ablation (RFA), the heat is created from electrical energy. The heat is generated at a specific target from the frictional heat created by rapidly vibrating adjacent cells. The end point that defines adequate necrosis can be based on either temperature or impedance, depending on the needle manufacturer.

During an RFA procedure, an ablation needle is placed directly into the target tissue. One or more electrodes are deployed from the end of the needle into the tissue. The generator is turned on, and a target temperature is set. The RF energy flows through the electrodes and causes ionic agitation. This agitation and friction of ions creates heat, and once sufficient temperatures have been reached, the heat kills the target tissue.

Tiny thermometers (thermocouples) incorporated into the tips of the electrodes allow continuous monitoring of tissue temperatures. Power is automatically adjusted so that the target temperatures remain constant. As tissue temperature increases above 50ºC, cell protein is permanently damaged and coagulation necrosis starts. Above 60ºC, cell death occurs almost instantly. Approximately 15-30 minutes are required to perform a 3- to 5-cm ablation.

Ultrasonography is typically used to monitor the treatment process for increased echogenicity. This increase in echogenicity corresponds to the formation of tissue and water vapor bubbles from the treated tissue and is used as a rough estimate of the size of the ablation site. Multiple ablations can be overlapped to decrease the chance of local tumor recurrence.

The size of the ablated area is determined largely by the size of the electrode needle, the temperature of the tissue, and the duration of time the energy is applied. A sharp boundary separates dead tissue and unaffected surrounding tissue.

The above principles of RFA are common to most needle systems. Different types of needles and techniques are detailed below.

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Percutaneous Radiofrequency Ablation of Tumor

Multiple array electrode technique

Multiple curved, retractable electrodes are kept inside the needle until its tip is positioned within a tumor. When the needle tip is properly positioned, a plunger on the hub of the needle is advanced so that the electrodes extend from the tip. When fully extended, these electrodes resemble an open umbrella.

Multiple electrode tips of an expanding electrode are active. This results in more homogenous heat distribution within the tumor and creates a reproducible sphere of ablation every time. In addition to the more even distribution of heat, the expanding electrodes have other safety features. The hooks of the electrode are fixed within the liver tissue when deployed, so that no needle movement is possible during RFA. Also, no electrode cooling is required during ablation, making the procedure easier and quicker.

Multiple electrodes seem to be superior to single electrodes with respect to the local recurrence rate for tumors between 3 and 5 cm. [36]

An important advance in RF energy deposition was achieved with the development of internally cooled RF electrodes. These 14- to 18-gauge electrodes have an internal lumen through which chilled perfusate is circulated during RF application. [37]  By cooling the electrode tip during the application of RF energy, these electrodes prevent charring and vaporization and are capable of producing lesions of greater dimension, which assists in complete tumor destruction. This technique also enables greater generator power to be administered with less risk of tissue charring.

Bipolar electrode technique

With the bipolar electrode technique, the current returns to the generator through the electrode. This eliminates the need to have coupling pads applied to the patient’s thighs. This system was originally designed to use three separate electrodes, placed in close proximity to each other and adjacent to the tumor. Simultaneous application of three applicators is believed to achieve larger volumes of ablation than the use of three consecutively overlapped ablations; it also significantly reduces the time needed to perform the ablation.

Saline-enhanced radiofrequency ablation

RFA may be performed with saline enhancement. Hypertonic saline is injected into the tumor through a side port on the shaft of the electrode before the ablation is started. The aim is to increase ionicity and, therefore, conduction within the tumor, thereby increasing the volume of the ablation. [36] Tiny channels within the electrodes can be used to infuse very small volumes of saline into the tumor as it is being ablated. Again, the goal is not to enhance conductivity directly but to prevent desiccation and charring of the tumor that would otherwise prevent conductivity and limit the ablation volume.

The unpredictability of saline perfusion in scirrhous colorectal liver metastases makes it unsafe for these lesions. However, encapsulated hepatocellular carcinoma (HCC) can become uniformly perfused without the risk of saline escaping into surrounding tissue, thus allowing ablation diameters of 6-7 cm. RFA using multiple wet electrodes shows promise as an effective method for treating inoperable HCC, especially in cases with well-preserved liver function.

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Complications

Many studies have confirmed that RFA is a relatively low-risk procedure with low morbidity and mortality. [38] Few complications are associated with RFA. [39]

Numerous factors are thought to be related to causing major complications; these factors include tumor size, number of ablation sessions, electrode type (single or cluster), and operator experience. One large study from Italy that included 41 centers showed that an increased number of RF sessions was related to a higher rate of major complications with no significant relation to tumor size or electrode type. [40]

Patients can experience discomfort immediately after the procedure; this normally settles with simple analgesia. The patient may also experience delayed pain as a part of postablation syndrome.

Postablation syndrome is a common phenomenon after RFA of solid abdominal tumors, [41] occurring in approximately one third of patients. The symptoms of postablation syndrome are flulike and include low-grade fever, delayed pain, malaise, myalgia, nausea, and vomiting. Patients should be informed about postablation syndrome and its self-limiting nature before the procedure. Most patients should be able to resume normal activity in 7-10 days.

The incidence of other complications is lower than 5%. Possible complications include the following:

  • Shoulder pain
  • Cholecystitis (normally subsides after few weeks)
  • Damage to the bile ducts, resulting in biliary obstruction
  • Damage to the bowel
  • Bleeding
  • Capsular hematoma
  • Hemoperitoneum
  • Hemothorax/hydrothorax
  • Pleural effusion
  • Intraperitoneal bleeding or ascites
  • Hemobilia
  • Infection and portal thrombosis
  • Liver abscess
  • Needle tract seeding - This is recognized as a long-term complication of RFA; it occurs mainly in lesions close to the surface or capsule of the liver
  • Collateral damage to proximal vital organs - The predictable nature of RFA generally prevents this complication
  • Self-limiting subcutaneous cellulitis

Livraghi et al found the incidence of major complications (eg, peritoneal hemorrhage, neoplastic seeding, intrahepatic abscesses, and intestinal perforation) to be 2.2% in 3554 treated lesions. [40] The incidence of minor complications was 5%, and the mortality was 0.3%. [40] Giorgio et al reported a study of 375 ablation sessions; major complications were seen in 0.9% of patients. [42] In this study, the mortality was also reported as 0.3%. [42]

Studies have shown that life-threatening acute liver failure can be considered a rare possible complication of RFA. Postablation survival depends upon Child classification, tumor multiplicity, and etiology of the HCC. Studies show that patients with HCCs that developed after viral cirrhosis had a worse prognosis than those with HCCs that occurred after alcoholic cirrhosis. [43]

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Postprocedural Follow-up Imaging

RFA is usually performed in specialist centers. Radiologists and other clinicians who do not normally deal with such cases must be aware of the different aspects of post-RFA appearances, especially in view of likely recurrence. Follow-up imaging may include the use of computed tomography (CT), ultrasonography, or magnetic resonance imaging (MRI), or positron emission tomography (PET).

Contrast-enhanced computed tomography

After RFA, patient follow-up normally includes contrast-enhanced CT (CECT). At the authors’ center, a post-RFA CT study is followed by a additional study 3 months later. The usual protocol on a multidetector CT consists of precontrast and dual-phase postcontrast helical scanning. (See the images below.)

CT appearance of a liver lesion before radiofreque CT appearance of a liver lesion before radiofrequency ablation.
CT appearance of a liver lesion after radiofrequen CT appearance of a liver lesion after radiofrequency ablation.

Perihepatic collections and basal consolidation with pleural effusion are common, normal posttreatment changes. Intraperitoneal free fluid may also be seen; this normally resolves within a few days. [17] Iatrogenic arterioportal shunting and small intralesional air pockets are also seen frequently at immediate follow-up CT. [44] These usually settle within 1 month after the procedure.

A hypervascular area around the ablation, which represents increased arterial perfusion and possible inflammatory reaction after RFA, is the most common finding in the immediate postablation period. [45] In one study, this was noted in approximately 90% of patients in the first month and in more than 50% of cases between 1-3 months. [46]

A lesion with low attenuation and no enhancement after contrast with or without hyperattenuating rim shows a successful ablation. [45] In contrast, nodular and thick enhancement represents a tumor recurrence. [47] Another study indicated that lesion shape and non-enhanced CT attenuation are not significant parameters to differentiate between successfully and unsuccessfully treated lesions. [48] On the other hand, increase in lesion size and nodular enhancement pattern have significant relation to treatment failure.

Catalano et al described four different helical CT patterns of recurrence post RFA, as follows [49] :

  • Enhancing tissue within the edge of the ablated nodule (ingrowth)
  • Enhancing tissue around the treated nodule but continuously to its border (outgrowth)
  • Enhancing tissue within the same segment of the treated nodule on arterial phase images (spread)
  • Enhancing tissue within different segments from the treated nodule on arterial phase images (progression)

The postablation area is generally larger than the actual tumor size, for obvious reasons. An ablation area that is smaller than the tumor should be closely followed. [44] The ablated area gradually decreases in size but may also remain stable over the course of time.

Curvilinear calcification at the edge of the ablated area and fat between the ablated zone and normal parenchyma may also be seen occasionally. [17]

Ultrasonography

Grayscale

Ultrasonographic findings post RFA are variable and may show echo-poor, echogenic, or mixed appearance. (See the image below.) Grayscale ultrasonography alone is not a definite follow-up investigation and cannot differentiate between residual tumor and necrotic tissue. [44]

Ultrasound image of liver after radiofrequency abl Ultrasound image of liver after radiofrequency ablation.

Contrast-enhanced

Studies suggest that preablation use of contrast-enhanced ultrasonography (CEUS), in addition to CECT or MRI, for appropriate selection of patients improves the outcome in terms of tumor progression and efficacy of RFA therapy. [50, 51]

In images visible on ultrasonography, CEUS can be valuable post RFA as compared with CECT and MRI in terms of rapidity and cost-effectiveness. This is so because CEUS can be performed immediately after the procedure without exposing the patient to radiation and because an additional treatment can be performed in the same session. [52]

CEUS has improved patient follow-up and can provide information regarding ablation similar to that obtained with CECT. [53]

CEUS is more useful in known ablation zones because it has a low sensitivity in identifying the safety margin and provides incomplete coverage of the liver in patients who are at high risk of developing new tumor in the other parts of the liver. [54, 17]

Magnetic resonance imaging

Unenhanced or contrast-enhanced MRI can be used after RFA. A T1-weighted postcontrast study is more useful than the unenhanced study and is a valuable alternative to CECT. [55]

Successful ablation is denoted by no enhancement and a low signal on T2-weighted images, though increased signal on T2-weighted images does not always represent residual tumor. [56]

A study showed that MRI may have an advantage over CT in detecting early regrowth. [57] In this study, 2 of 9 local regrowths were depicted only on T2-weighted images, suggesting that MRI is a more useful sequence.

Although equally efficacious in comparison to a good CECT study, MRI is mostly used as a post-RFA problem-solving tool at present. Some studies have suggested possible use of diffusion-weighted MRI for evaluating effective treatment and recurrence. [58]

Positron emission tomography

Preliminary results from one study suggested that after RFA in patients with liver metastasis, fluorodeoxyglucose (FDG) PET-CT may detect relapse and treatment failure earlier than multidetector CT (MDCT). [59] In this study, 7 cases showed the presence of local recurrence earlier than MDCT.

Initial results of another study showed that PET-CT was significantly better than PET alone for detecting local tumor progression, whereas no significant difference was revealed between MRI and PET-CT. [60] Donckier et al showed in a study of 28 lesions that FDG-PET recognizes incomplete tumor ablation earlier than does CT. [61] This study found PET-CT to be accurate in monitoring the efficacy of RFA for treatment of liver metastases.

Further studies in a larger cohort of patients are required to establish the role of PET-CT in patients with liver metastasis who have undergone RFA.

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