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Technical Developments |
1 From the Department of Radiology and Nuclear Medicine (B.B.F., K.J.W., T.A.) and Department of Surgery (J.P.R.), Campus Benjamin Franklin-Charité-University Medicine Berlin, Hindenburgdamm 30, 12200 Berlin, Germany; and Celon Medical Instruments, Teltow, Germany (A.R.). Received June 23, 2004; revision requested September 1; revision received January 14, 2005; accepted February 16. Address correspondence to B.B.F. (e-mail: bernd.frericks{at}charite.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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All RF ablation devices currently in use are monopolar. With these devices, an ablation electrode is positioned in the tumor and a neutral electrode is placed on the thigh (grounding pad) to close the electrical circuit. Existing RF ablation systems have some limitations. The size of potentially treatable tumors is limited because the diameter of the ablation zones usually does not exceed 4 cm (7) unless the ablation probe is repositioned for a second ablation. Unpredictable electrical current paths between the ablation electrode and the grounding pad may lead to heterogeneous energy deposition and, thus, to eccentric ablation zones or even collateral damage. Skin burns at the grounding pad have been reported in a few instances (8).
Several innovations, such as pulsed energy deposition (9), internally cooled or open perfused probes (10), and umbrella-shaped or multiprong electrodes (2,11) have been introduced to improve the effectiveness of RF ablation devices (12). A further innovation is the recently developed multipolar RF ablation device (13), which enables up to three bipolar RF probes to be placed within or closely around the tumor. No grounding pad is required, and the electrical current runs between up to six electrodes, with up to 15 possible electrode combinations within and closely around the tumor. Because the energy is focused on the target zone, more homogeneous energy deposition and larger ablation zones should be possible.
The purpose of our study was to prospectively evaluate a multipolar RF ablation device in patients with hepatic malignancy with regard to feasibility, achieved ablation zone size and shape, technical effectiveness, and complications.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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Patient Population
The study was approved by our institutional review board, and written informed consent was obtained from all patients before therapy and after the nature of the procedure had been fully explained. From July 2003 to April 2004, 12 consecutive patients (six men, six women; mean age, 69 years ± 9 [standard deviation]) with 19 liver tumors (15 metastases from colorectal cancer in nine patients, three hepatocellular carcinomas in two patients, and one metastasis from breast cancer in one patient) were treated. At the beginning of the study, the exclusion criterion was the same as that generally recommended for conventional RF ablation—that is, the tumor diameter could not exceed 4 cm (14). After achieving encouraging results with regard to ablation size, we included two patients with tumors larger than 4 cm in diameter (a 45-mm solitary hepatocellular carcinoma treated percutaneously and a 60-mm colorectal metastasis ablated intraoperatively).
Preinterventional hepatic imaging was not standardized for the purpose of the study and was performed as part of the routine clinical work-up. Imaging was performed with biphasic 16–detector row computed tomography (CT) (Somatom Sensation; Siemens Medical Solutions, Erlangen, Germany) in four patients and with contrast material–enhanced MR imaging (Somatom Vision; Siemens Medical Solutions) in eight. At MR imaging, seven patients received ferucarbotran (Resovist; Schering, Berlin, Germany), and one patient received gadopentetate dimeglumine (Magnevist; Schering). Seven patients had previously been treated with chemotherapy (n = 3), interstitial laser ablation (n = 3), or immunotherapy (n = 1). The tumors had a mean diameter of 26 mm ± 13 (range, 5–60 mm). Each patient was discussed in an interdisciplinary meeting before RF ablation. Eleven patients were considered unsuitable for tumor resection, and one patient refused surgery.
RF Ablation Device
For ablation, a recently developed multipolar RF ablation system (CelonLab POWER; Celon Medical Instruments, Teltow, Germany) was used in all patients. This system has an operating frequency of 470 kHz and a maximum power output of 250 W. It can simultaneously operate up to three internally cooled (with 30 mL/min of normal saline solution at room temperature) bipolar coagulation probes with an active tip length of 20, 30, or 40 mm. The bipolar probes have a diameter of 1.8 mm (15 F) and a shaft length of 10–25 cm. Depending on tumor size, shape, location, and accessibility, RF ablation can be performed with one, two, or three probes. For tumors smaller than 1.5 cm, it is usually adequate to place one probe centrally within the tumor.
Sometimes, however, it may be difficult to place one probe centrally into small and hard metastases because they have a tendency to "escape" from the needle. Therefore, an alternative approach with two or three probes placed closely around the tumor is possible with the multipolar system. For larger tumors, two or three probes are placed in the outer third of the tumor. If necessary, this can be followed by a second ablation after the probes are partially retracted or repositioned to achieve an adequate safety margin. If two or three probes are used simultaneously, the probes should run parallel and the distance between the probes should not be less than 5 or greater than 30 mm.
The number of connected probes is automatically detected by the system. If one bipolar probe is used, the generator operates in bipolar mode and the high-frequency current flows between the two electrodes at the tip of the bipolar coagulation probe (Fig 1). If two or three probes are connected, the unit works in the multipolar mode. In the latter case, a total of 15 possible electrode pairs are automatically activated alternately for up to 2 seconds, depending on the actual local tissue resistance. As a consequence, the current passes between two electrodes that may, for example, be positioned at opposite tumor margins, and hence crosses the tumor repetitively.
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) three times or when the power output is inadequate (less than one-third of the preset power output), this electrode pair is excluded from further ablation cycles. Power output is stopped automatically if the resistance of all possible electrode pairs exceeds the limit three times, which indicates that the dehydration has extended along the electrodes completely and that the coagulation process has ended. After a short interval (5 minutes) for tissue rehydration, a second cycle may be started.
RF Ablation Procedure
Twenty-two ablations were performed in 17 ablation sessions. Ablations were performed with either a percutaneous or an intraoperative approach, depending on tumor size, location, and percutaneous accessibility. Three patients with six tumors were treated intraoperatively while receiving general anesthesia by one radiologist (T.A., with 4 years of experience with thermal ablation) and one surgeon (J.P.R., with 7 years of experience with thermal ablation). Intraoperative ablations were performed with ultrasonographic (US) guidance. A Pringle maneuver (temporary occlusion of the hepatic artery and portal vein with soft ligation of the falciform ligament in the liver hilum) was performed during the ablation of four of these six tumors in two patients to reduce perfusion-mediated cooling and, thus, to increase the volume of ablated tissue.
Nine patients with 13 tumors were treated percutaneously in 14 sessions by one of two radiologists (B.B.F., with 4 years of experience with RF ablation, and T.A.) while receiving conscious sedation. Five patients underwent repeat sessions for the following reasons: The patient had two or more tumors (n = 2), there was minor tumor residue after the initial treatment (n = 2), or there was an inadequate ablative safety margin (n = 1) after the initial treatment. One prophylactic dose of 2 g of ceftriaxone sodium (Rocephin; Roche, Grenzach-Wyhlen, Germany) was intravenously administered in all patients immediately before initial insertion of the probe. Percutaneous ablation was performed with either US or CT guidance, depending on the visibility of the tumor at unenhanced US. In seven percutaneous sessions, tumor visualization and access were good at unenhanced US and ablation was performed with US guidance in real-time B mode by using an Acuson Sequoia unit (Siemens Ultrasound, Moutain View, Calif) and a 3.5-MHz curved-array transducer. In the remaining seven percutaneous sessions, tumor visualization and access were poor at unenhanced US but good at CT. In these seven sessions, all nine ablations were performed with CT guidance (Somatom Plus 4P; Siemens). CT guidance was intermittent in seven ablations; CT fluoroscopy was used in two ablations of deeply located tumors that were difficult to access.
One to three probes were positioned in or around the tumor by using imaging guidance. If more than one probe was used, the distance between the probes in their final position was measured. The puncture time, which was defined as the time from the beginning of local anesthesia administration to the final evaluation of probe position before ablation, was documented. The initial power output ranged from 15 to 90 W, depending on tumor size and location, number of and distance between probes, and active probe tip length. The power output was gradually increased to values between 90 and 130 W.
Postinterventional treatment response was assessed with contrast-enhanced US performed approximately 10 minutes after ablation by using 2.4 mL of BR1 (SonoVue; Bracco, Milan, Italy) and Contrast Pulse Sequencing (Siemens) or with contrast-enhanced CT immediately after ablation by using 120 mL of iohexol (Ultravist 300; Schering). A nonenhancing area was considered to represent the ablation zone. If an inadequate ablation zone was revealed, further ablation was performed, if necessary, after probe retraction or repositioning. Ablation time, which was defined as the time of energy deposition into the tumor tissue, was documented.
At the end of ablation, track ablation was performed in the bipolar mode for each probe separately with a power setting of 25 W and deactivated internal water circulation.
Postinterventional Follow-up and Image Analysis
Postinterventional follow-up was performed in all patients with a 1.5-T MR unit (Somatom Vision; Siemens) and a dedicated body-array coil 24–48 hours after ablation and was repeated every 3 months. The imaging protocol included an unenhanced T1-weighted spin-echo sequence (repetition time msec/echo time msec, 700/12), a T2-weighted fast spin-echo sequence (5200/120), and a dynamic T1-weighted three-dimensional gradient-echo sequence (three-dimensional volumetric interpolated breath-hold examination; 5.2/2.6; flip angle, 20°) that was enhanced with gadobenate dimeglumine (Multihance; Bracco, 0.1 mL per kilogram of body weight). The following parameters were used with the latter sequence: 256 x 134 matrix, 300—360-mm field of view, 160-mm-thick slabs, and 4-mm-thick sections. In addition, contrast-enhanced biphasic 16–detector row CT was performed 1 week after ablation.
On the basis of the portal venous phase images obtained at the initial dynamic MR examination (images of 19 ablation zones in 11 patients) or the portal venous phase images obtained at biphasic CT (images of three ablation zones in one patient whose MR images had artifacts), ablation zone sizes and shapes were evaluated quantitatively with semiautomatic planimetry by using established noncommercial software (HepaVision; MeVis-Center for Medical Diagnostic Systems and Visualization, Bremen, Germany) (15,16). Data processing was performed by one radiologist (B.B.F.), and each data set was analyzed once. The software employs an algorithm with real-time computation of optimal boundary paths between a few user-defined contour points: A connection between the last point and the current position of the cursor is calculated in real time, with consideration of signal intensities at the boundary between untreated liver parenchyma and the ablation zone (17,18). After segmentation, three maximum diameters and the ablation zone volume were calculated automatically with the sum-of-area technique (19), and the ablation zones were visualized as three-dimensional objects for better shape perception. All ablation zones were evaluated qualitatively by two observers (B.B.F. and T.A.) in consensus. By using the three-dimensional objects, the shape of the ablation zone was classified as (a) concentric if it was round or ellipsoid with smooth margins and had no substantial shape deformities, with maximum diameter variations of less than 10%; (b) moderately eccentric if moderate deformities, with at least one diameter varying between 10% and 30%, were noted; and (c) severely eccentric if major deformities, with at least one diameter varying by more than 30%, were observed.
Probable causes of eccentric zones were classified as either technical or perfusion mediated (ie, related to the presence of large vessels in close contact). Small extensions along the probe track were documented and included in planimetry for ablation zone volumetry; they were, however, disregarded in the description of the shape of the ablation zone because they were always outside the primary target volume.
In evaluating the effectiveness of the technique (20), tumor ablation was classified as complete or incomplete on the basis of previously described signal intensity characteristics (21) at MR imaging performed 1 day after RF ablation and through a comparison of postinterventional with preinterventional imaging findings. Follow-up MR images were evaluated by two radiologists (B.B.F. and T.A.) for local tumor progression (20,21) and new tumors.
Postinterventional clinical follow-up was performed by one surgeon (J.P.R.) and two radiologists (B.B.F. and T.A.) until patients were discharged from the hospital (2 days after uncomplicated percutaneous ablations) and at follow-up imaging. Any complications and treatment side effects were documented (20).
Statistical Analysis
Computer software (GraphPad Prism for Windows, version 4.01; GraphPad Software, San Diego, Calif) was used for statistical analysis. Descriptive statistical measures, including mean values, standard deviations, and ranges were calculated for the ablation zone sizes and volumes. Ablation zone volumes were correlated with the number of coagulation probes used. The relationship between ablation zone volume and applied energy was analyzed by using Pearson correlation analysis. A P value of less than .05 was considered to indicate a statistically significant difference. In addition, ablation zone volumes and applied energy were displayed in a scatterplot with an adjustment line to enable estimation of the expected ablation zone volume as a function of the applied energy.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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The mean total ablation time was 32 minutes ± 14 (range, 5–68 minutes), and a mean total energy of 93 kJ ± 61 (range, 8–251 kJ) was applied per tumor.
Ablation Zone Size and Shape
In two patients treated with the percutaneous approach, four of the 22 RF ablation zones fused with zones from previous thermal ablations so that the margins of the new zones could not be defined accurately. These ablation zones were therefore excluded from the analysis of size and shape; thus, 18 of the 22 ablation zones were used for full evaluation. Fourteen of these 18 fully evaluable ablation zones were achieved without the Pringle maneuver.
The ablation zone sizes are presented in the Table. Ablation zone size increased with the number of probes used (Fig 2). When three probes were used percutaneously (n = 9), a mean ablation zone volume of 56 mL ± 32 was achieved with a mean energy of 139.8 kJ ± 59. The maximum ablation zone size achieved was 91 x 62 x 79 mm (volume, 110 mL) with the percutaneous approach (in a procedure that involved repositioning of one of three probes) and 73 x 98 x 74 mm (volume, 195 mL) with the intraoperative approach (in a procedure that involved repositioning of two of three probes and performance of the Pringle maneuver for 15 minutes). In 11 of the 14 ablations performed without the Pringle maneuver, the ablation zone volume showed close correlation with the applied energy (R2 = 0.8816) (Fig 3). Three ablation zones were smaller than expected on the basis of the applied energy: In the first patient, who was treated with a percutaneous approach, some of the energy was applied to the abdominal wall. In another patient treated with the percutaneous approach, the newly created ablation zone lay directly below an older ablation zone from a previously performed interstitial laser ablation; therefore, some of the energy might have been applied to the previous laser ablation zone. A third ablation zone created intraoperatively without the Pringle maneuver lay immediately between the inferior vena cava and the right portal vein.
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Follow-up Results
The mean follow-up period was 10 months (range, 4–15 months). The incompletely ablated tumor showed progression after 9 months. In one patient, two additional local tumor progressions were detected 9 months after ablation. These two local tumor progressions occurred in the two ablation zones (described earlier) that were severely eccentric with incomplete fusion of the single ablation zones. In both cases, repeat ablation was technically effective (data not shown).
Three patients developed new hepatic tumors: Breast cancer was diagnosed in one patient 1 month after ablation, and metastases from colorectal cancer were diagnosed in two patients 3 months after ablation.
Three patients died 7, 10, and 12 months after ablation: One patient developed new colorectal metastases in a cirrhotic liver, one died of a recurrent primary tumor (rectal cancer), and one had local progression of colorectal metastases, as mentioned earlier, and died 1 year after RF ablation.
Complications and Side Effects
There was one major complication: In a patient with a single subcapsular metastasis in the anterior aspect of segment IVa, RF-induced thermal necrosis of the abdominal wall and jejunum—with subsequent perforation—occurred. This necessitated surgical treatment, and the patient had a prolonged but complete recovery. This patient had a history of multiple laparotomies. Pre- and postinterventional imaging revealed that the bowel loop was not in direct contact with the liver at the level of the metastasis. One minor complication, a small intraabdominal hemorrhage that necessitated no treatment, occurred.
Ten patients experienced side effects after ablation. Two patients had small subcapsular hepatic blood collections, and five had small pleural effusions; these patients were asymptomatic and did not require treatment. For up to 72 hours, three patients experienced postinterventional pain, which was controlled with nonsteroidal anti-inflammatory drugs.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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The new system presented in this study is the first and, to our knowledge, the only commercially available multipolar RF ablation device, and it combines the benefits of the bipolar technique with the well-established improvements in monopolar systems such as multiprobe insertions, internally cooled probes, and resistance-controlled pulsed-energy deposition. It enables the simultaneous operation of up to three bipolar probes, which results in a multipolar system with increased efficacy. Results of previous investigations have shown that the effectiveness of simultaneous operation of multiple energy sources (probes) for tissue coagulation is significantly greater than that with the same number of probes applied sequentially with the same amount of energy (25). Consequently, with simultaneous (rather than sequential) use of several probes, larger tumors can be treated (26). By using an experimental in vivo porcine model in which four monopolar RF ablation devices were compared, Brieger et al (7) observed a markedly more homogeneous energy deposition with the internally cooled RF ablation devices than with other monopolar devices. In addition, among the four devices, the internally cooled devices showed the second-highest energy efficiency in creating thermal ablation zones.
The new multipolar RF ablation device was easy to use, and placement of the probes was straightforward with both US and CT guidance. Although three probes were used in most ablations, resulting in more than one initial probe insertion per tumor, the mean puncture time per tumor of 24 minutes was acceptable, and no clinically relevant probe insertion–related complications occurred. Unfortunately, one major complication involving thermal necrosis of the abdominal wall and a small-bowel loop occurred, and surgical treatment was necessary. Critical review of the pre- and peri-interventional images did not reveal direct contact between the inserted probes and the small-bowel wall. We believe, however, that the active tip of one of the three probes was in contact with the abdominal wall for a short period of time. This may have allowed for energy deposition in the abdominal wall, which usually results only in minor abdominal wall edema. In this case, however, a small-bowel loop was stuck to the abdominal wall by adhesions that were secondary to multiple prior laparotomies. Therefore, some amount of the applied energy, potentially in addition to hot gas escaping from the liver at the probe insertion site and entrapped between the bowel and abdominal wall, may have heated the wall of the adherent small-bowel segment. This theory is supported by the finding that the volume of the wedge-shaped hepatic ablation zone (13 mL) was markedly less than that expected to result from the applied energy.
Our early results suggest good technique effectiveness, and, compared with values reported in the literature for standard RF ablation, large ablation zones were achieved: When three probes were used percutaneously, a mean ablation zone volume of 56 mL ± 32 was achieved. Denys et al (27) compared four different monopolar RF ablation devices in an in vivo porcine model and explanted calf livers. All RF ablation probes were placed away from large vessels. Depending on the system used, mean ablation zone volumes ranged between 29 mL ± 11 and 42 mL ± 10.
To our knowledge, this is the first clinical report concerning thermal ablation therapy in which ablation zone volume and shape were assessed with planimetry on the basis of data from portal venous phase MR imaging. Although most ablation zones had a typical concentric configuration without major shape deviations, two (11%) were severely eccentric, with incomplete fusion of the individual ablation zones. Both cases were regarded as technical failures. As a consequence, we are now using higher amounts of energy if more than one probe is used and the distance between individual probes exceeds 15 mm.
This study had several limitations. Preinterventional imaging was performed with different imaging modalities as part of the routine clinical work-up and not for the purpose of this study. Our study focused on the clinical feasibility and technical effectiveness of the multipolar RF ablation device, as well as the rate of complications resulting from its use. We had a limited number of patients and a short follow-up period. Further larger studies are required to evaluate the clinical benefit of multipolar RF ablation in comparison with other local treatment options.
In summary, our data indicate that this multipolar RF ablation device has the potential to achieve larger maximum ablation sizes than standard monopolar RF ablation devices and offers more flexible tumor treatment by allowing the use of one, two, or three bipolar probes.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Abbreviations: RF = radiofrequency
See Materials and Methods for pertinent disclosures
Author contributions: Guarantor of integrity of entire study, B.B.F.; study concepts, B.B.F., T.A.; study design, T.A.; literature research, B.B.F.; clinical studies, B.B.F., J.P.R., T.A.; data acquisition, B.B.F., T.A.; data analysis/interpretation, B.B.F., J.P.R., A.R., T.A.; statistical analysis, T.A.; manuscript preparation, B.B.F.; manuscript definition of intellectual content, B.B.F., T.A., K.J.W.; manuscript editing, J.P.R., A.R.; manuscript revision/review, J.P.R., A.R., T.A.; manuscript final version approval, K.J.W., T.A.
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References |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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