DOI:10.2214/AJR.06.1004
AJR 2007; 188:1485-1494
© American Roentgen Ray Society
Multiple-Electrode Radiofrequency Ablation of Hepatic Malignancies: Initial Clinical Experience
Paul F. Laeseke1,
Tina M. Frey2,
Chris L. Brace2,
Lisa A. Sampson2,
Thomas C. Winter, III2,
Jan R. Ketzler2 and
Fred T. Lee, Jr.2
1 Department of Biomedical Engineering, University of Wisconsin, Madison,
WI.
2 Department of Radiology, University of Wisconsin, 600 Highland Ave., Madison,
WI 53792-3252.
Received July 31, 2006;
accepted after revision January 15, 2007.
Address correspondence to F. T. Lee, Jr.
Abstract
OBJECTIVE. The objective of our study was to retrospectively analyze
our initial clinical experience with percutaneous multiple-electrode
radiofrequency ablation and evaluate its safety and efficacy for treating
hepatic malignancies.
MATERIALS AND METHODS. Thirty-eight malignant hepatic tumors (mean
diameter, 2.7 cm; range, 0.7–10.0 cm) in 23 patients (12 men and 11
women; mean age, 65 years; range, 40–84 years) were treated in 26
radiofrequency ablation sessions with an impedance-based multiple-electrode
system. One, two, or three (mean, 2.4) 17-gauge electrodes were placed, and
tumors were ablated using a combination of CT and sonography for guidance and
monitoring. Electrodes were placed in close proximity (mean spacing: two
electrodes, 1.0 cm; three electrodes, 1.4 cm) to treat large tumors or were
used independently to treat several tumors simultaneously. Contrast-enhanced
CT scans were obtained immediately after ablation to determine technical
success and evaluate for complications. Follow-up CT scans at 1, 3, 6, 9, and
12 months (mean, 4 months) after ablation were obtained to assess for tumor
progression and new metastases.
RESULTS. Local control was achieved in 37 of 38 tumors, 34 of which
were treated in one session. Ablations created with closely spaced electrodes
had a mean diameter of 4.9 cm. The total ablation time was reduced by
approximately 54% compared with an equivalent number of ablations performed
with a single-electrode system (1,014 vs 2,196 minutes). Three complications
occurred: one death from a presumed postprocedure pulmonary embolus, one
pneumothorax, and one asymptomatic perihepatic hemorrhage.
CONCLUSION. Multiple-electrode radiofrequency ablation appears to be
a safe and effective means of achieving local control in large or multiple
hepatic malignancies at short-term follow-up.
Keywords: CT • hepatobiliary imaging • hepatocellular carcinoma • interventional radiology • liver cancer • MRI • oncologic imaging • radiofrequency ablation
Introduction
Radiofrequency ablation is an effective means of achieving local control of
malignancies in several organs, including the liver, kidneys, bones, and lungs
[1–6].
The most extensive experience with radiofrequency ablation is in the liver,
where it is commonly used to treat colorectal metastases in nonoperative
candidates and hepatocellular carcinoma (HCC) in cirrhotic patients
[3,
6]. Radiofrequency ablation is
effective for small and favorably situated tumors, but local progression rates
are substantially higher for large tumors (
4 cm)
[7,
8]. Although many factors
contribute to high postradiofrequency local recurrence rates, the inability of
early single-electrode radiofrequency systems to create adequately large zones
of ablation has been crucial, leading to the requirement for sequential
overlapping ablations to treat even moderately sized tumors
[9]. Although overlapping
ablations can create large zones of necrosis, the technique is time consuming,
does not take advantage of the thermal synergy possible with
multiple-applicator systems, and is complicated by obscuration of target
tissue by microbubbles. Moreover, the requirement to precisely overlap
ablation zones by withdrawing and reinserting electrodes increases the
potential for incomplete treatment of large tumors.
Several techniques have been used to increase the volume of coagulation
with radiofrequency, including multielectrode arrays, saline infusion, pulsing
algorithms, internally cooled or deployable electrodes, and bipolar systems
[10–17].
Various combinations of these have been incorporated into commercially
available systems that are capable of coagulating large volumes of tissue.
However, none offers the ability to customize the shape of the ablation zone
or to treat multiple tumors simultaneously. Moreover, the greatest increase in
ablation zone size has been seen with saline infusion and deployable
electrodes; however, these systems may lead to cleft, irregular ablation
zones, and an increased risk of collateral damage
[18,
19].
Tumor ablation systems that support multiple independent applicators have
been available for many years in other non–radiofrequency ablative
techniques such as laser and cryoablation. These systems have numerous
advantages compared with single-applicator systems. The applicators can be
placed in proximity to exploit thermal synergy and reduce the need for
sequential overlapping ablations
[20–22].
Thermal synergy is the synergistic relationship between closely spaced
applicators that results in disproportionately large volumes of coagulation.
Thermal synergy is also characterized by more extreme, tumoricidal
temperatures, which can help overcome the deleterious effects of perfusion.
Finally, multiple applicators can be used independently to treat several
distinct areas of tissue or several tumors simultaneously
[23].
Development of multiple-electrode radiofrequency systems has been hindered
by the fact that little current flow (and subsequent tissue heating) occurs
between simultaneously activated electrodes in proximity
[22,
24]. Interelectrode spacing
must be minimized (
1.5 cm) or the resulting ablation zones will be
irregular with a central cool spot that can potentially preserve tumor
[10,
11]. A multiple-electrode
bipolar (multipolar) radiofrequency system that is capable of creating large
zones of necrosis has recently been described
[25,
26]. However, this system
requires precise parallel electrode placement given that current flow is
confined to the tissue between the electrodes, uses larger electrodes (1.8 mm)
than a single-electrode system, and is not currently available in the United
States.
A monopolar multiple-electrode radiofrequency system based on switching
among several 17-gauge (1.5-mm) electrodes has also been described
[20,
22,
23,
27] and is now available for
clinical use (Cool-tip radiofrequency Switching Controller, Valleylab). This
system is capable of driving up to three electrically independent electrodes
by using the off-time built into an impedance-based pulsing algorithm to power
additional electrodes [14].
Preclinical studies using an in vivo porcine liver model have established its
ability to create large confluent zones of necrosis and to simultaneously
create multiple ablation zones
[20,
23].
The purpose of this study was to retrospectively analyze our initial
clinical experience with percutaneous multiple-electrode radiofrequency
ablation and evaluate its safety and efficacy for treating hepatic
malignancies.
Materials and Methods
With approval from our institutional review board, a Health Insurance
Portability and Accountability Act–compliant (HIPAA-compliant)
retrospective analysis of medical records and imaging studies was performed
for all patients undergoing multiple-electrode radiofrequency ablation of
hepatic malignancies from November 1, 2004, to January 25, 2006. Waiver of
consent was obtained.
Patients
Twenty-three patients with 38 malignant hepatic tumors were treated with
multiple-electrode radiofrequency ablation at our institution. Written
informed consent was obtained from all the patients before they underwent
treatment. The study population consisted of 12 men (52%) and 11 women (48%)
with a mean age of 65 years (range, 40–84 years). Of the 38 tumors, 14
primary hepatic malignancies and 24 liver metastases were treated in nine and
14 patients, respectively. Tumor diameter ranged from 0.7 to 10.0 cm (mean,
2.7 cm). Twenty-eight tumors (mean diameter, 3.2 cm) were treated with
multiple electrodes placed in proximity. The remaining 10 tumors (mean
diameter, 1.4 cm) were treated by using individual electrodes to treat two or
three tumors at a time. Table 1
lists the number and size of tumors treated by tumor type.
Multiple-Electrode Radiofrequency Ablation Procedures
All ablations were performed by one of two radiologists with an average of
11 years of experience (range, 10–12 years) performing radiofrequency
ablation. Each patient was prepared using aseptic technique and draped before
the procedures. Radiofrequency ablations were performed with the patient under
general anesthesia using a commercially available monopolar multiple-electrode
radiofrequency ablation system based on switching between electrodes at
impedance spikes (Cool-tip radiofrequency Switching Controller, Valleylab)
(Fig. 1). The 200-W, 480-kHz
monopolar radiofrequency generator uses an impedance feedback loop to control
switching times and maximize energy delivery. Electrodes were 17-gauge in
diameter, with a length of 15 cm and an active tip of 3 cm (SWCT1530,
Valleylab), and were placed using either an intercostal or a subcostal
approach. Chilled sterile water (< 20°C at the electrode tip) was
circulated inside the electrodes to minimize tissue charring near the
electrode. Four return pads (DGP-HP, Valleylab) were placed on the patient's
thighs to complete the circuit.
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Fig. 1 —Multiple-electrode radiofrequency system (Cool-tip
radiofrequency Switching Controller, Valleylab) consists of 200-W monopolar
radiofrequency generator operating at 480 kHz (left, top box) and
switching system (left, bottom box) that can be used to power up to
three electrically independent electrodes (right) (SWCT1530,
Valleylab).
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The switching system is an addition to the base generator and is capable of
powering up to three electrically independent electrodes by switching to the
next electrode once the impedance reaches 30 
above baseline or at a
predetermined time interval of 30 seconds. If the impedance did not spike
after 5 minutes because the electrodes were in a cystic or highly vascular
environment, the switching algorithm was interrupted and each electrode was
powered to an impedance spike (
30–90 seconds per electrode), after
which the standard switching algorithm was resumed. Two (mean spacing, 1.0 cm;
range, 0.5–2.1 cm) or three (mean spacing, 1.4 cm; range, 0.5–3.2
cm) electrodes were placed in proximity to treat large volumes of tissue
(Figs. 2A,
2B,
3A,
3B,
3C,
4A,
4B,
4C,
4D,
4E). Interelectrode spacing
was generally kept to 2.0 cm or less to prevent coagulation zones from
becoming cleft or irregular
[20]. The rigid electrode
spacer (2.0-cm triangular configuration) enclosed with the multiple-electrode
packs was not used.
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Fig. 2B —56-year-old man with 2.8-cm hepatocellular carcinoma treated
with two electrodes. Postablation hepatic artery phase contrast-enhanced CT
scan shows successful treatment with ablation zone (arrow) completely
covering tumor.
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Fig. 3A —Multiple-electrode radiofrequency ablation performed with
three closely spaced electrodes to treat large ovarian cancer metastasis in
57-year-old woman. CT scan shows tumor with mean diameter of 5.1 cm
(arrow).
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Fig. 3B —Multiple-electrode radiofrequency ablation performed with
three closely spaced electrodes to treat large ovarian cancer metastasis in
57-year-old woman. Intraprocedural CT scan obtained to confirm placement of
three electrodes (arrow). Note that electrodes do not have to be
placed parallel to one another.
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Fig. 3C —Multiple-electrode radiofrequency ablation performed with
three closely spaced electrodes to treat large ovarian cancer metastasis in
57-year-old woman. Postprocedure CT scan shows large confluent ablation zone
(arrow) covering entire tumor.
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Fig. 4A —72-year-old man with large, irregular tumor mass formed by
three hepatocellular carcinomas treated with multiple-electrode radiofrequency
ablation. Preablation CT scan (A) and sonograms (B and C)
show two small nodules measuring 2.8 cm (arrowhead, B) and 1.9
cm (arrowhead, C) adjacent to larger 5.6-cm nodule
(arrows).
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Fig. 4B —72-year-old man with large, irregular tumor mass formed by
three hepatocellular carcinomas treated with multiple-electrode radiofrequency
ablation. Preablation CT scan (A) and sonograms (B and C)
show two small nodules measuring 2.8 cm (arrowhead, B) and 1.9
cm (arrowhead, C) adjacent to larger 5.6-cm nodule
(arrows).
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Fig. 4C —72-year-old man with large, irregular tumor mass formed by
three hepatocellular carcinomas treated with multiple-electrode radiofrequency
ablation. Preablation CT scan (A) and sonograms (B and C)
show two small nodules measuring 2.8 cm (arrowhead, B) and 1.9
cm (arrowhead, C) adjacent to larger 5.6-cm nodule
(arrows).
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Fig. 4D —72-year-old man with large, irregular tumor mass formed by
three hepatocellular carcinomas treated with multiple-electrode radiofrequency
ablation. First nodule was successfully ablated with 12-minute ablation using
two electrodes (arrows). Second 12-minute ablation with three
electrodes was used to treat distal aspect of larger tumor. Remaining portion
of that tumor and third tumor were treated simultaneously with three
electrodes (not shown).
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Fig. 4E —72-year-old man with large, irregular tumor mass formed by
three hepatocellular carcinomas treated with multiple-electrode radiofrequency
ablation. Immediate postablation CT scan shows successful ablation of tumors
with conglomerate ablation zone (arrow) measuring 5.4 x 8.9
cm.
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When electrodes were placed in proximity to create a single large zone of
ablation, electrodes were inserted through separate puncture sites and were
initially activated for 16 minutes. If subsequent ablations of the same tumor
were necessary, additional ablations were performed for 16 minutes depending
on the tissue impedance [20].
When using the electrodes independently to treat multiple tumors (Fig.
5A,
5B,
5C), ablations were performed
for 12–13 minutes [23].
If all electrodes continuously attained impedance spikes in < 10 seconds
per cycle, the ablation was terminated.
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Fig. 5A —67-year-old woman with two hepatocellular carcinomas treated
simultaneously with multiple-electrode radiofrequency ablation. Preablation CT
scans show two tumors with mean diameters of 1.4 cm (arrow, A)
and 1.6 cm (arrow, B).
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Fig. 5B —67-year-old woman with two hepatocellular carcinomas treated
simultaneously with multiple-electrode radiofrequency ablation. Preablation CT
scans show two tumors with mean diameters of 1.4 cm (arrow, A)
and 1.6 cm (arrow, B).
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Fig. 5C —67-year-old woman with two hepatocellular carcinomas treated
simultaneously with multiple-electrode radiofrequency ablation. Immediate
postablation CT scan with contrast enhancement shows ablation zones
(arrows) as areas of hypoattenuation. Mean ablation zone diameters
were 2.1 and 2.6 cm, respectively. No evidence of local tumor progression was
evident on 1-month follow-up scans (not shown).
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Tract cauterization was performed before removing individual electrodes by
disabling electrode cooling, allowing the tip temperatures to reach 80°C,
and retracting the electrode at a rate that maintained this target
temperature. Power was turned off, and the electrode was completely removed
when the proximal end of the active electrode reached the liver capsule. All
ablations were performed in a 4-MDCT suite (LightSpeed Plus, GE Healthcare)
with real-time sonographic guidance and monitoring and with confirmation by CT
fluoroscopy and conventional CT as needed.
In patients with peripheral tumors adjacent to the body wall or bowel, the
liver was isolated using 5% dextrose in water (D5W)
[28–30].
D5W was injected into the peritoneum under sonographic guidance using an 18-
or 20-gauge spinal needle until at least a 2- to 3-mm layer of fluid was
identified between the target tumor and the adjacent hemidiaphragm, bowel, or
body wall. Intermittent infusions of additional D5W were performed as
necessary to maintain an adequate fluid layer. The volume of D5W infused was
recorded. Fluid displacement with D5W was performed in 12 of 26 (46%)
sessions.
Evaluation of Treatment Success and Follow-Up
Contrast-enhanced CT was performed immediately after the procedure to
determine the effectiveness of the treatment and to check for immediate
complications. Scans were obtained using our institution's biphasic abdomen
protocol. A power injector (EnVision CT, Medrad) was used to administer a
single bolus of 300 mg of iohexol (150 mL of Omnipaque, Amersham Health)
followed by 50 mL of normal saline at a pressure of 325 psi. Patients with
impaired renal function were given only 100 mg of contrast material. The
arterial and portal venous phase scans were acquired at a delay of 35 and
70–85 seconds, respectively. Immediate assessment was performed in one
patient without using contrast material because of the patient's contrast
allergy.
Follow-up imaging consisted of CT with contrast material at 1, 3, 6, 9, and
12 months after ablation. Images were analyzed for the presence of local tumor
progression to determine local control rates. Tumor recurrence was defined as
a focal area of enhancement or as growth of new tissue in or around previously
ablated tumor on a follow-up contrast-enhanced CT scan. Follow-up scans were
interpreted by one of eight members of the abdominal imaging division at our
institution. Follow-up images were interpreted with the knowledge that the
patient had undergone percutaneous tumor ablation, but without knowledge about
this particular study. In the absence of new extrahepatic disease, patients
with local tumor progression were considered for a second radiofrequency
ablation treatment (n =2).
Statistical Analysis
The ablation zones were measured by one of the radiologists who performed
the procedures in an unblinded fashion using postablation contrast-enhanced CT
scans. The ablation zone was defined as the hypoattenuating area that did not
enhance with contrast material. The surrounding area of hyperemia was not
included in the ablation zone measurements. Descriptive statistics were
performed (InStat, version 3.06, GraphPad Software) to characterize the
treatment sessions (number, ablation time, number of ablations, number of
electrodes, and volume of D5W used), ablation zones (number and size),
complications, and outcomes (local progression and control rates).
Results
Treatment Sessions
Twenty-two of 23 patients are alive at the time of article preparation.
Seventy-seven ablations were used to treat 38 tumors during 26 patient visits.
The mean volume of D5W instilled into the peritoneal cavity to protect
perihepatic structures was approximately 850 mL (range, 240–3,000 mL).
The mean number of ablations needed to treat a single tumor was 2.0 (range,
1–7) and, on average, 2.4 (range, 1–3) electrodes were used per
ablation. The mean ablation times (actual time the generator was active) were
27 minutes (range, 12–80 minutes) and 39 minutes (range, 12–109
minutes) for one tumor and session, respectively. The total ablation time of
all cases was 1,014 minutes. This represents an approximately 54%
(1,014/2,196) reduction in time when compared with an equivalent number of
12-minute ablations performed with a single-electrode system (183 electrode
placements x 12 minutes = 2,196 minutes).
Table 2 summarizes the subset
of tumors treated with closely spaced electrodes—that is, it excludes
the 10 tumors in which multiple tumors were simultaneously treated with single
electrodes.
Complications
Three complications occurred during the 26 treatment sessions: one death
from a presumed pulmonary embolus days after ablation in a patient with
chronic obstructive pulmonary disease, coronary artery disease, and cirrhosis
(the family declined an autopsy); one pneumothorax; and one asymptomatic small
perihepatic hemorrhage. The pneumothorax was treated by inserting a chest tube
into the right pleural space under CT fluoroscopic guidance with almost
complete resolution before transferring the patient to the recovery room. The
chest tube was removed and the patient was discharged the next day. The
perihepatic hemorrhage was detected on the immediate postprocedure CT scan,
but was asymptomatic and clinically insignificant.
Treatment Outcome and Follow-Up
When electrodes were placed in proximity, ablation zones had a mean
diameter of 4.9 cm with a minimum and maximum diameter of 4.2 cm (range,
2.2–8.1 cm) and 5.5 cm (range, 2.8–12.0 cm), respectively. Note
that these measurements include some ablation zones created by sequentially
overlapping multiple-electrode ablations. The mean diameter of ablation zones
when the electrodes were used independently to simultaneously treat multiple
tumors was 2.9 cm with a minimum and maximum diameter of 2.6 cm (range,
1.4–3.7 cm) and 3.1 cm (range, 1.8–4.0 cm), respectively.
Follow-up ranged from less than 1 month to 12 months (mean, 4 months).
Local control was achieved in 22 of 23 patients (96%) and 37 of 38 tumors
(97%), of which 34 (92%) were successfully ablated in one treatment session.
Local control was achieved in 96% (27/28) of large tumors for which closely
spaced electrodes were used and in 100% (10/10) of tumors for which individual
electrodes were used to simultaneously treat several small tumors.
Table 3 summarizes the cases
with local tumor progression. The mean diameter of tumors with local
progression was 5.3 cm versus 2.5 cm for tumors that were successfully treated
in one session. Two tumors with local progression were seen in one patient
with ovarian cancer metastases. In this patient, one tumor progressed once,
requiring a total of two treatment sessions, and another progressed twice,
necessitating three treatment sessions. Percutaneous radiofrequency ablation
was used to achieve local control in another patient with local progression.
One patient with local progression of an incompletely treated 10-cm colorectal
metastasis detected 1 month after ablation declined further treatment (Fig.
6A,
6B,
6C,
6D,
6E). Finally, in addition to
the four documented cases of local progression, one patient with multiple HCCs
had a questionable margin that was treated a second time with intraoperative
radiofrequency ablation during ablation of an additional tumor.
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Fig. 6A —Multiple-electrode radiofrequency ablation of previously
treated (radiofrequency ablation and cryoablation) 10-cm colorectal metastasis
in 65-year-old woman. Preablation CT scan (A) and sonogram (B)
show large tumor (arrow) abutting inferior vena cava (IVC,
B).
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Fig. 6B —Multiple-electrode radiofrequency ablation of previously
treated (radiofrequency ablation and cryoablation) 10-cm colorectal metastasis
in 65-year-old woman. Preablation CT scan (A) and sonogram (B)
show large tumor (arrow) abutting inferior vena cava (IVC,
B).
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Fig. 6C —Multiple-electrode radiofrequency ablation of previously
treated (radiofrequency ablation and cryoablation) 10-cm colorectal metastasis
in 65-year-old woman. Intraprocedural sonograms show three electrodes
(small arrows, C) placed in tumor. Microbubbles
(large arrows) forming during ablation approximate
developing conglomerate zone of ablation.
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Fig. 6D —Multiple-electrode radiofrequency ablation of previously
treated (radiofrequency ablation and cryoablation) 10-cm colorectal metastasis
in 65-year-old woman. Intraprocedural sonograms show three electrodes
(small arrows, C) placed in tumor. Microbubbles
(large arrows) forming during ablation approximate
developing conglomerate zone of ablation.
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Fig. 6E — Multiple-electrode radiofrequency ablation of previously
treated (radiofrequency ablation and cryoablation) 10-cm colorectal metastasis
in 65-year-old woman. Postablation CT scan shows large hypoattenuating
ablation zone (arrow) that covers tumor, indicative of successful
treatment. However, 1-month follow-up CT scan (not shown) revealed persistent
tumor (not shown) and patient declined further treatment.
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Discussion
To our knowledge, this report is the first clinical case series describing
the use of a new multiple-electrode monopolar radiofrequency ablation system.
Our preliminary experience indicates that multiple-electrode radiofrequency
ablation is a safe procedure that appears effective at short-term follow-up.
This series contains some patients with larger tumors than were previously
treated at our institution, and complication and short-term local control
(97%) rates are comparable to those achieved with single-electrode systems
[31–34].
Performing ablations with multiple electrodes simultaneously also reduced
treatment time by approximately 54% when compared with an equivalent number of
ablations performed with a single-electrode system. Finally, the need for
sequential overlapping ablations to treat large tumors is reduced with the
multiple-electrode system. This may increase the accuracy of electrode
placement and subsequent treatment success because performing overlapping
ablations is complicated by obscuration of the target tissue by microbubbles
from previous ablations.
The Cool-tip radiofrequency ablation system (Valleylab) uses a pulsing
algorithm to increase the extent of coagulation, and the multiple-electrode
system takes advantage of this by powering additional electrodes when the
generator would normally be turned off. Tissue impedance generally rises and
spikes during ablation, at which time the system switches to the next
electrode. If the electrode is situated in a cystic lesion or in extremely
vascular tissue, the tissue impedance may never spike. If this happens, the
system will prematurely switch between electrodes at a fixed time interval.
Prematurely switching between electrodes is suboptimal because tissue
dehydration and vascular thrombosis have not yet occurred, and the surrounding
tissue will be substantially cooled by flowing blood while the other
electrodes are activated. We encountered this situation several times during
the course of this study: If the tissue impedance did not spike on any
electrode within the first 5 minutes, our protocol was to interrupt the normal
switching algorithm and power each electrode individually until the impedance
did spike. After an impedance spike was achieved at each electrode, the
standard switching algorithm was resumed. Alternatively, each electrode can be
powered until a spike in impedance is encountered before starting the
ablation.
In contrast to the bipolar multiple-electrode (multipolar) system in which
current flow is confined to tissue between electrodes, the monopolar
multiple-electrode system does not require precise parallel electrode
placement because each electrode is electrically independent, can be used to
simultaneously treat several tumors, uses smaller electrodes (1.5 vs 1.8 mm),
and requires the use of ground pads. When several electrodes are placed in
proximity, their thermal effects interact synergistically (thermal synergy)
and lead to higher temperatures (routinely > 80°C immediately after the
ablation) within disproportionately large, confluent zones of ablation. This
synergistic relationship was evident in preclinical in vivo studies in which
multiple-electrode radiofrequency ablation with three electrodes increased the
volume of ablated tissue by more than 100% over three ablations with a single
electrode [20]. The higher
core temperatures led to increased thermal conduction to the periphery of the
ablation zone. Moreover, vascular thrombosis and devascularization in ablation
zones led to a decrease in perfusion-mediated cooling at nearby electrodes.
Therefore, it is possible to create large, relatively spherical ablation zones
with two or three electrodes placed in a linear or triangular array,
respectively. Because the electrodes are physically and electrically
independent of one another, they can be placed in different configurations to
treat irregular tumors or to treat two or three tumors at the same time.
One major complication and one minor complication in this series, a
pneumothorax requiring insertion of a chest tube and an asymptomatic
perihepatic hemorrhage, respectively, are known complications of hepatic
radiofrequency ablation, and the complication rates appear to be similar
between single- and multiple-electrode systems. The death from a postprocedure
pulmonary embolus occurred after a successful and uneventful procedure in a
patient who had coagulopathy corrected before the procedure. There should be
no intrinsic difference in complication rates with the multiple-electrode
system compared with conventional single-electrode systems when corrected for
the number of electrode placements. However, because the multiple-electrode
system can be used to ablate larger volumes of tissue than single-electrode
systems, the tendency may be to increase the size of tumors indicated for
percutaneous ablation. The overall complication rate for treating large tumors
may increase due to the increased number of electrode placements, large volume
of tissue that is ablated, and increased chance that a large tumor is
immediately adjacent to a critical structure that is vulnerable to collateral
damage. Larger studies are needed to determine whether treating large tumors
with multiple electrodes leads to an increase in clinically significant
complications.
Placement of multiple radiofrequency electrodes in this study was not
difficult compared with placing a single electrode, which requires precise
placement in the center of the tumor. The tumors treated in this series
required both subcostal and intercostal electrode placement, and an adequate
sonography window was available in all cases. Placement of the first electrode
generally took the longest because of the need to find a suitable window and
approach, but subsequent placements were generally faster because the first
electrode could be used as a guide needle. Therefore, our estimate of the time
saved might be conservative because it does not include the time saved during
electrode placement. On the basis of our experience with this system, we do
not anticipate that placement of multiple electrodes will be a limiting factor
in the clinical use of this or other multiple-applicator systems (e.g.,
cryoablation and microwave ablation).
Preclinical studies have established 2 cm as the maximum interelectrode
spacing that still results in a regular ablation zone without significant
clefting [20]. This spacing is
potentially conservative given the highly vascular nature of normal porcine
liver, and our clinical experience indicates that confluent volumes of
necrosis can consistently be achieved if the interelectrode spacing is kept
within this range. In this series, the mean electrode spacing (measured on
intraablation CT or sonographic images) for ablations with two or three
closely spaced electrodes was 1.0 and 1.4 cm, respectively.
Counterintuitively, it may not be necessary to maximize the interelectrode
spacing to create very large ablations because thermal synergy is greater for
closer electrode spacings. Finally, although an interelectrode spacing of 2 cm
is critical to optimal performance of this particular system, it does not
require that electrodes be placed parallel to each other, and in fact,
electrodes may even be touching. This may be particularly important when using
an intercostal approach where the percutaneous window may be small.
Many of the limitations of the monopolar multiple-electrode system are
similar to those of the conventional radiofrequency systems. For example, the
ability to detect viable tumor during and immediately after ablation is still
limited. When assessed retrospectively, the treatment failures can largely be
attributed to an inability to distinguish viable tumor from necrotic tissue on
immediate postablation imaging. This resulted in premature termination of
procedures before achieving adequate necrosis of the entire tumor and a
surrounding ablative margin. In the case of the 10-cm tumor, most the mass was
successfully treated at the initial setting, but the procedure was ended too
early because the tumor appeared to be completely treated on postprocedure
contrast-enhanced CT images. It was only at the 1-month postprocedure CT
examination that the residual low-attenuation tumor could be distinguished
from an increasingly low-attenuation necrotic tumor along the deep margin. If
this residual tumor could have been identified on CT or sonography at the time
of the procedure, the treatment would have continued. This limitation is
common to all heat-based ablation systems using the same imaging guidance.
An additional limitation of the multiple-electrode system is the complexity
of the first-generation device. For example, when using three electrodes,
there are now three power cords and three input–output water channels
for cooling, resulting in nine cables on the sterile field. The control
interface is also quite complex because of the add-on nature of the switching
controller with the radiofrequency generator. These problems should be
alleviated when an integrated second-generation device becomes available.
This study had certain limitations. First, the follow-up period was
relatively short and additional studies are needed to determine the impact of
multiple-electrode radiofrequency ablation on long-term survival and disease
progression. Also, a diverse group of tumors were treated. Although this shows
the versatility of the technique in treating a variety of tumors, it did not
allow us to establish the performance of the system in any one tumor type with
certainty. Finally, several patients had tumors that were local progressions
from previous radiofrequency ablations, making accurate measurement of the
tumor and ablation zone diameter difficult. Although those particular
measurements were likely affected, they should not negatively impact the
significance of the study. Given that the purpose of this study was to
establish the overall safety and short-term efficacy of multiple-electrode
radiofrequency ablation, these appear to be acceptable limitations.
In conclusion, multiple-electrode radiofrequency ablation is a safe
procedure and shows short-term effectiveness as a means of achieving local
control in large or irregular hepatic tumors and may be used to treat several
tumors simultaneously. Further development of multiple-electrode systems will
likely increase the effectiveness of this technique in the management of liver
tumors, and more clinical experience will help define its role in treating
tumors in other organs such as the kidney, lung, and bone.
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Abstract
Introduction
