Thermal Ablation of Liver Lesions

Radiofrequency Ablation

The goal of RF ablation is to induce thermal injury and tissue coagulation through electromagnetic energy deposition. For tumor ablation purposes, the frequency of the electromagnetic energy sources is usually in the range of 375–500 kHz. RF applicators are named electrodes.

The patient is part of a closed-loop circuit that includes an RF generator, an electrode needle, and a large dispersive electrode (ground pads). An alternating electric field is created within the patient’s tissue. Because of the relatively high electrical resistance of tissue in comparison with the metal electrodes, there is marked agitation of the tissue ions surrounding the electrode attempting to follow the changes in the direction of alternating electric current. The agitation results in frictional heat around the electrode. The discrepancy between the small surface area of the needle electrode and the large area of the ground pads causes the generated heat to be focused and concentrated around the needle electrode.

One or multiple electrodes are inserted directly into the tumor to deliver RF energy current. Several electrode types are available for clinical RF ablation, which can be monopolar or bipolar and have different designs (multitined expandable, internally cooled, perfused).

All these features enable a substantial and reproducible enlargement of the volume of thermal necrosis produced with a single-needle insertion. This has resulted in the widespread clinical application of RF ablation, which has been the most widely used thermal ablative modality for a long time, especially in hepatocellular carcinoma (HCC).

Tumor size is of utmost importance to determine the outcome of ablation. Because RF ablation produces in vivo ablation spheres of 5.5–5.6 cm in diameter, the tumor should not exceed 3.5 cm in longest axis to obtain a safety margin of 1 cm all around the lesion.

RF ablation of lesions adjacent to the gallbladder or the liver hilum is associated with a risk of thermal injury to the biliary tract. Lesions located along the surface of the liver can be considered for RF ablation, although their treatment requires experienced hands and may be associated with a higher risk of complications, while superficial lesions adjacent to any part of the gastrointestinal tract must be avoided because of the risk of thermal injury of the gastric or bowel wall. The mobility of the small bowel may also provide the bowel with greater protection compared with the relatively fixed colon. In contrast, RF ablation of lesions located in the vicinity of hepatic vessels is possible because flowing blood usually “refrigerates” the vascular wall, protecting it from thermal injury. In these cases, however, the risk of incomplete ablation of the neoplastic tissue adjacent to the vessel may increase because of the heat loss caused by the vessel itself.

Microwave Ablation

In MW ablation, tumor destruction is induced from electromagnetic energy sources by using devices with frequencies from 300 MHz to 300 GHz. Currently available MW ablation devices function at the 915-MHz or 2.45-GHz frequencies designated for industrial, scientific, and medical use. MW applicators are named antennas.

The passage of MW into cells or other materials containing water results in rapid molecular rotation, which generates and uniformly distributes heat continuously until the radiation is stopped. In general, the greatest heating from any system occurs within 1 cm of the antenna, and the mechanism of cell death caused by MW is similar to RF.

However, MW offers additional advantages to thermal ablation. In particular, MWs are able to radiate through all biological tissues, even in those with high electrical impedance (such as bone, lung, and charred or desiccated tissues) with higher intratumoral temperatures reached in less time and larger ablation zones with respect to RF.

In addition, MW ablation is less influenced by “heat sinks” compared with RF (i.e., the presence of a large vessel—more than 3 mm—near the target lesion, which can potentially remove heat before complete tumor ablation is achieved).

Several different MW devices are commercially available, with different combinations of technical elements mainly related to the energy source and the antenna design. Among the most relevant are:

• • MW frequency emission, maximum available generator power (which determines the maximum dimension of ablation, in most cases 60–195W),

  • antenna caliber and shape (affecting the spatial distribution of the electromagnetic field and, as a consequence, the final shape of the ablation zone),

  • the number of antennas that can be used simultaneously with a single generator (determining a more uniform, larger and more spherical ablation zones), and

  • methods of energy delivery (manual or automatic, continuous or pulsed).

System performance can vary widely. Therefore, it is critical that physicians understand the ablation-zone shapes and sizes created by different time and power combinations with a particular system.

Multiple factors contribute to determining the final volume and extension of the overall ablation zone. Beyond technical elements related to the energy source and the antenna design, local tissue properties, (e.g., specific heat, electrical conductivity, and water content), vascular perfusion and large-vessel heat-sink effects may deeply affect the overall energy absorption and influence the resulting dimensions of the ablation zone.

Indications

The indication for ablation of a liver tumor should come from a multidisciplinary tumor board discussion and be clearly recorded in a concurrent manner by the interventional radiologist, oncologist, hepatologist, and liver surgeon. Multidisciplinary evaluation will take into consideration the clinical specificities beyond liver tumor burden, such as comorbidities, compliance to treatment, general performance status, and history of the disease to select the best approach for the individual patient, following the principles of the precision medicine.