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1 All authors: Department of Radiology, University Hospitals of Cleveland, Case Western Reserve University, 11100 Euclid Ave., Cleveland, OH 44106.
Received July 2, 2002;
accepted after revision October 28, 2002.
Address correspondence to J. S. Lewin.
Abstract
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MATERIALS AND METHODS. An ex vivo porcine hepatic model was used. Three clips were placed around a radiofrequency electrode at 10, 20, and 30 mm from the point of insertion. Clips were arranged in a plane either perpendicular or parallel to the electrode track. After placement of the liver specimen on a grounding pad, radiofrequency energy was applied in a standardized manner for 5 min. Lesion growth and morphology were documented for each minute.
RESULTS. Radiofrequency lesions appeared circular and homogeneous after 5 min. Lesion diameter perpendicular to the radiofrequency electrode averaged 30 mm. However, lesion formation was irregular during the early phase of the radiofrequency ablation. The lesion extended irregularly toward the 1-cm clip after 60 sec of ablation. During the second minute, a distinct lesion was observed around the clip 1 cm from the electrode; the primary lesion had not yet reached the clip. During the final 3 min, the primary lesion reached the 1-cm clip and ultimately incorporated the satellite lesion. No lesions were detected surrounding the more distant clips.
CONCLUSION. Our data suggest that with the parameters applied in our study, radiofrequency ablation can be safely performed in patients with implanted clips. No aberrant conduction is observed around surgical clips that are located 20 mm and further from the radiofrequency electrode.
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The radiofrequency generator applies an oscillating radiofrequency current that flows between the active electrode and the grounding pad. The current is distributed within the conductive medium of the body; the current density is typically highest at the ablation probe and decreases with distance. Variations in local conductivity due to different tissues and materials give rise to local variations in the current density. According to Maxwell's equations [4], the spatial variation in the current density, the distance from the probe, and the time-varying current create an electric field that is induced within the patient's body. This electric field creates the ohmic power density that gives rise to local heating. The ohmic power density is equal to the tissue conductivity times the square of the magnitude of the local electric field. Therefore, ligating clips from previous liver surgery or cholecystectomy will have a different conductivity than surrounding tissue [4].
This difference in electric conductivity will create an altered distribution of the electric field during the radiofrequency ablation. This altered distribution can conceivably adversely affect the ablation by involving tissue adjacent to the electrode that would normally be considered outside the ablation zone during nearby radiofrequency thermal ablation. The metal clips essentially function as secondary antennas during the ablation [5].
The objective of our study was to determine whether the differential conductivity between the surgical clips and the tissue alters the distribution of the electric field, thereby altering the expected size or shape of the thermal lesion. In this study, the distribution of the electric field is assessed by examination of the size, shape, and rate of change of the zone of radiofrequency ablation in an ex vivo tissue model of percutaneous radiofrequency thermal ablation. This information is indispensable when establishing whether minimally invasive radiofrequency thermal therapy adjacent to previous surgical clips increases patient risk in the clinical setting.
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In successive experimental settings, the triple-cluster radiofrequency thermal ablation electrodes with an active tip length of 25 mm (Radionics) were inserted perpendicular and parallel to the conductive grounding pad into the ex vivo liver specimen. Three surgical titanium alloy clips (Micro Surgical Components, Somerset, NJ) were positioned in planes perpendicular and parallel to the radiofrequency thermal ablation electrode at predefined locations 10, 20, and 30 mm from the point of insertion. These predefined distances were based on the maximal expected diameter of thermal lesions that can be achieved with a triple-cluster radiofrequency thermal ablation electrode [6, 7, 8]. The distance from the surgical clip plane to the beginning of the active thermal ablation electrode tip was uniformly 15 mm.
Radiofrequency Ablation
Radiofrequency energy was applied via the 17-gauge triple-cluster
radiofrequency electrodes for 5 min in each experimental setup. Ablation was
performed using a 200-W radiofrequency generator (RFG-3C; Radionics) operating
at 500 kHz.
Quantitative Data Analysis
Each experimental setup was repeated three times. Surface growth of the
primary radiofrequency-induced thermal lesion originating from the thermal
ablation electrode was determined by visual inspection every 20 sec without
interruption of the ablation procedure. A macroscopic change in hepatic tissue
color of the ex vivo liver specimen was used as the criterion for the induced
thermal lesion. Of the measured diameters for all three repeated ablations,
the arithmetic mean was calculated for 20 sec. Formation of any satellite
radiofrequency lesion surrounding the surgical clips was also documented.
During a final ablation process, growth of the radiofrequency-induced lesions was documented macroscopically on cut slices parallel to the ablation electrode at 1, 3, and 5 min after the beginning of thermal energy delivery. This cross-sectional pathologic examination was performed by creating thermal ablation lesions by applying energy for 1, 2, and 3 min at different locations. This procedure was also repeated three times in one liver sample. Cross-sectional pathologic evaluation was subsequently performed, and the lesion size was determined.
Ablation parameters, such as impedance and current, were continuously monitored, and the mean values and SDs were calculated.
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The radiofrequency-induced lesion size increased in all three experimental setups within the first 3 min. No further increase in lesion size was observed during the remaining 2 min of ablation.
All radiofrequency lesions appeared almost circular and homogeneous after an application of thermal energy for a total of 5 min. However, a wedge-shaped 45° area always located at the base of the triangular electrode cluster showed slower growth than the surrounding lesion. The maximal lesion diameter that was measured perpendicular to the radiofrequency probe ranged from 28 to 31 mm in all experimental setups.
Lesion Growth
In the experimental setups characterized by the placement of a
radiofrequency ablation electrode parallel and perpendicular to the grounding
pad and the placement of a surgical clip perpendicular to the ablation
electrode, symmetric round thermal lesions that did not reach the titanium
surgical clips were observed after 60 sec of radiofrequency energy delivery.
In addition, no satellite radiofrequency thermal lesions were noted
surrounding any surgical clips. The diameter of the radiofrequency-induced
lesion was 14 mm after 1 min of energy delivery.
Within the second minute of radiofrequency thermal ablation, the thermal lesion shape became irregular and extended toward the surgical clip 10 mm away. After 120 sec, an additional satellite lesion was observed surrounding the 10-mm surgical clip (Fig. 2A). After 3 min of ablation, the original radiofrequency thermal lesion reached the surgical clip 10 mm away and ultimately incorporated the satellite radiofrequency thermal lesion. After 5 min of radiofrequency thermal ablation, the lesion reached the maximal diameter (average, 30 mm), a symmetric configuration was observed, and no extensive carbonization occurred around the 10-mm clip (Fig. 2B).
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Neither formation of the satellite radiofrequency thermal lesion nor the irregular shape of the original radiofrequency thermal lesion was noted at any time during thermal energy deposition in the experimental setup. This finding was characterized by a parallel alignment of the radiofrequency ablation electrode in regard to the grounding pad and a parallel placement of the surgical clip plane to the ablation electrode. The original size of the radiofrequency thermal lesion increased within 3 min of energy deposition to a final average diameter of 30 mm. No further increase was noted within the remaining 2 min of radiofrequency energy deposition.
In neither of the experimental setups did radiofrequency-induced lesions reach the farther clips at 20 and 30 mm.
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At the low radiofrequency (500 kHz) used in this procedure, the induced oscillating electric field leads to an oscillatory movement of the ions in the tissue surrounding the electrode. The velocity of this movement is proportional to the density and frequency of the electric field. Tissue heating occurs because of the frictional energy loss of the oscillating ions [9, 10]. The symmetry of tissue heating is, however, also dependent on the arrangement of electrodes in the electrode cluster. The appearance of a wedge-shaped area of slower lesion growth always located at the base (adjacent to the most widely spaced pair) of the triangular electrode arrangement in our investigation was caused by the positioning of an asymmetric electrode in the triple cluster.
Substances of higher electric conductivity, such as metallic surgical clips placed within the electric field, may lead to a distortion of the normally symmetric electric field and permit accumulation of charge on the clip. Surgical clips are most often made of titanium, chromium, cobalt, and nickel alloys; the conductivity may vary in different alloys depending on the composition [11]. The accumulation and motion of induced charges lead to significantly greater heating of the implant compared with the surrounding tissue if clips are not present. Furthermore, maximal heating will occur at sharp edges or points when one considers the morphology of the implants [12].
The appearance of irregular primary lesion growth and the formation of a satellite lesion during radiofrequency ablation observed in our investigation are consistent with these underlying electric field models of radiofrequency ablation [13]. Therefore, the substantially greater heating of the metallic implant leads to the formation of a surrounding thermal lesion. Because of the reduced electric field densities at increasing distances from the electrode tip, the formation of a satellite lesion takes longer than the generation of the original lesion.
The surgical clip that was located 20 and 30 mm from the point of insertion of the cluster electrode did not show any sign of heating or surrounding lesion formation. Therefore, those surgical clips did not encounter a strong enough electric field or enough distortion of the low-amplitude local field to induce sufficient movement of the electric charge carriers to produce local heating. Under the conditions of our experimental setups, no radiofrequency-induced lesions were detected surrounding the surgical clips placed 20 and 30 mm from the ablation electrode. However, it remains likely that even those clips may induce satellite lesions if higher current levels and hence higher electric fields are applied.
Because of the radial dispersion of the electric field, its density—at the clips implanted at a 90° angle in the experimental setup characterized by a parallel alignment of the radiofrequency ablation electrode in regard to the grounding pad and a parallel placement of the surgical clip plane to the ablation electrode—was already too weak to show any thermoelectric effect.
Our ex vivo findings may be directly applicable to radiofrequency thermal ablation procedures in humans. Even though the irregular growth of lesion and the formation of a satellite lesion around the closest surgical clip 10 mm away are subsequently incorporated into the final thermal ablation, and surgical clips therefore do not present a contraindication to radiofrequency procedures, the growth of a homogeneous, singular, and symmetric lesion significantly facilitates the monitoring of the radiofrequency ablation by cross-sectional imaging. Therefore, during the initial planning phase of the radiofrequency thermal ablation, cross-sectional imaging can be used to define a thermal ablation electrode insertion path that does not position the electrode close to any metallic implant. Choosing an electrode insertion path that does not position the electrode close to any surgical clips may minimize the patient's risk of developing an irregularly shaped primary lesion. Furthermore, by placing the ablation electrode away from the ligating vascular clips in surgical patients who have undergone previous liver surgery or cholecystectomy, our findings may prevent the formation of early satellite lesions during the ablation process.
This study was conducted using an ex vivo liver specimen without perfusion of hepatic vessels that might provide additional cooling and thereby increase the heat dispersion and without the surrounding conducting fluid or tissue through which current could flow under normal conditions. Additionally, tissue edema and cellular coagulation necrosis will not occur in ex vivo liver specimens. The applied wattage and the amount of radiofrequency energy deposited in the tissue comply with the standards for minimally invasive radiofrequency procedures of the abdomen in our institution, whereas ablation parameters such as impedance depend on the tissue itself. Therefore, this ex vivo study describes the most extensive interactions that might occur. We anticipate less severe radiofrequency effects for in vivo radiofrequency thermal ablations performed near ligating clips. However, higher levels of applied current and hence higher electric fields might subsequently compensate for the known differences in tissue response between our ex vivo studies and future in vivo trials.
In conclusion, surgical clips located close to the radiofrequency probe may have an impact on the formation of the primary thermal lesion and may lead to irregular lesion growth during the early stages of energy delivery. Furthermore, satellite lesions may form around surgical clips close to the percutaneous electrode. However, over time these satellite lesions may become incorporated into the primary lesion. Our ex vivo data suggest that with the radiofrequency parameters used in this investigation, radiofrequency ablation can be performed safely in patients with implanted hepatic surgical clips. No aberrant conduction is observed around surgical clips 20 mm and farther from the radiofrequency electrode.
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