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Published online before print February 21, 2008, 10.1148/radiol.2471062123
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(Radiology 2008;247:80-87.)
© RSNA, 2008

Experimental Studies

Microwave Ablation with Triaxial Antennas Tuned for Lung: Results in an in Vivo Porcine Model1

Nathan A. Durick, MD, Paul F. Laeseke, PhD, Lynn S. Broderick, MD, Fred T. Lee, Jr, MD, Lisa A. Sampson, CVT, Tina M. Frey, BS, Thomas F. Warner, MD, Jason P. Fine, ScD, Daniel W. van der Weide, PhD, and Christopher L. Brace, PhD

1 From the Departments of Radiology (N.A.D., P.F.L., L.S.B., F.T.L., L.A.S., T.M.F., D.W.v.d.W., C.L.B.), Pathology and Laboratory Science (T.F.W.), Biostatistics (J.P.F.), and Electrical and Computer Engineering (D.W.v.d.W., C.L.B.), University of Wisconsin-Madison, 600 Highland Ave, Box 3252, E3/311 CSC, Madison, WI 53792-3252. Received December 12, 2006; revision requested February 15, 2007; revision received June 13; accepted July 18; final version accepted September 11. Address correspondence to C.L.B. (e-mail: cl.brace{at}uwhealth.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To prospectively determine in swine the size and shape of coagulation zones created in normal lung tissue by using small-diameter triaxial microwave antennas and to prospectively quantify the effects of bronchial occlusion and multiple antennas on the coagulation zone.

Materials and Methods: The study was approved by the research animal care and use committee, and all husbandry and experimental studies were compliant with the National Research Council's Guide for the Care and Use of Laboratory Animals. Twenty-four coagulation zones (three per animal) were created at thoracotomy in eight female domestic swine (mean weight, 55 kg) by using a microwave ablation system with 17-gauge lung-tuned triaxial antennas. Ablations were performed for 10 minutes each by using (a) a single antenna, (b) a single antenna with bronchial occlusion, and (c) an array of three antennas powered simultaneously. The animals were sacrificed immediately after ablation. The coagulation zones were excised en bloc and sectioned into approximately 4-mm slices for measurement of size, shape, and circularity. Analysis of variance and two-sample t tests were used to identify differences between the three ablation groups.

Results: The overall mean diameters of coagulation achieved with a single antenna and bronchial occlusion (4.11 cm ± 1.09 [standard deviation]) and with multiple-antenna arrays (4.05 cm ± 0.69) were significantly greater than the overall mean diameter achieved with a single antenna alone (3.09 cm ± 0.83) (P = .016 for comparison with multiple antennas, P = .032 for comparison with bronchial occlusion). No significant differences in size were seen between the coagulation zones created with bronchial occlusion and those created with multiple antennas (P = .68). The coagulation zones in all groups were very circular (isoperimetric ratio > 0.80) at cross-sectional analysis.

Conclusion: A 17-gauge triaxial microwave ablation system tuned for lung tissue yielded large circular zones of coagulation in vivo in porcine lungs. The coagulation zones created with bronchial occlusion and multiple antennas were significantly larger than those created with one antenna.

© RSNA, 2008


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Radiofrequency (RF) is currently the most common thermal energy source used for ablation of lung tumors (1,2). Early experience with RF ablation in the lungs has shown that this procedure shows promise for the treatment of primary lung cancers and metastatic disease in select patient populations (2–4). However, findings in relatively recent investigations suggest that the success rate with RF ablation is greater in solid organs than in the lungs (5). Ablation of lung tumors larger than 3 cm in diameter is particularly problematic, with incomplete local treatment reported in 13%–60% of cases (6,7). Respiratory motion, heat sinking caused by pulmonary blood flow, and high tissue impedance may contribute to the inability to successfully ablate tumors in the lung and the resultant inadequate ablative margins, which result in high local recurrence rates (8–12). High tissue impedance is related to the presence of aerated tissue, which causes poor electrical and thermal conduction and thus limits power energy deposition at the margin of the thermal lesion (13). There have been attempts to improve the electrical conduction by using saline infusion or multiple-prong devices, but these techniques involve additional procedural complexity, larger (15-gauge) applicators, and additional risk of complications, including pneumothorax and pulmonary lobar necrosis secondary to saline infusion (14–16). In addition, there have been reports of substantial tissue trauma and difficulty removing multiple-prong systems from the lung (17,18).

Microwave ablation has several intrinsic advantages over RF ablation, including larger zones of active heating, less sensitivity to tissue type and charring, and capability to generate very high tissue temperatures (>150°C) and larger coagulation zones. Additional possible benefits of using microwaves are true multiple-antenna capability, improved performance near blood vessels, and no requirement for ground pads (19–23). Despite these potential advantages, few animal or human studies of microwave ablation in the lungs have been performed; this may be due to the large (≥14-gauge) applicators used in current microwave systems (21). Larger devices induce more tissue trauma and are associated with increased complications, including pneumothorax (16). The creation of large (>3.0 cm in diameter) coagulation zones with use of small-gauge applicators (<16 gauge) would reduce the risk of procedural complications and increase the safety and effectiveness of microwave systems for clinical use.

A triaxial microwave ablation system that can be specifically tuned to maximize energy deposition according to the dielectric properties of the targeted tissue has been developed (22–25). The small diameter of the antenna (17 gauge) makes this system very attractive for use in the lungs. Tuning the triaxial antenna results in increased power application with relatively small amounts of reflected power and has yielded large zones of coagulation when the antenna was applied to hepatic tissue in vivo (22,23). However, the properties of normal lung tissue are different from the properties of normal liver tissue. In aerated lung tissue, the theoretic values of relative permittivity and electrical conductivity constitute approximately 50% of these values for liver tissue (Table 1). Thus, theoretically, the optimal active length of a triaxial antenna in aerated lung tissue is approximately 41% longer than that in the liver (24,28). Therefore, the purpose of our study was to prospectively determine in swine the size and shape of coagulation zones created in normal lung tissue by using small-diameter triaxial microwave antennas and to prospectively quantify the effects of bronchial occlusion and multiple antennas on the zone of coagulation.


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  		Table 1. Dielectric Properties of Liver versus Lung Tissue

 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Animals, Anesthesia, and Procedures
Three authors (C.L.B., D.W.v.d.W., P.F.L.) have ownership interest in Micrablate (Madison, Wis), the manufacturer of the triaxial microwave ablation system used in this study. Two authors (T.M.F., L.A.S.) are unpaid consultants to Micrablate. D.W.v.d.W. holds a patent on this ablation system (U.S. patent 7,101,369, triaxial antenna for microwave tissue ablation).

Eight female domestic swine (mean weight, 55 kg; range, 50–60 kg) were used in our study. The study was approved by the research animal care and use committee of our institution, and all husbandry and experimental studies were compliant with the National Research Council's Guide for the Care and Use of Laboratory Animals (26). Preanesthetic sedation was induced with intramuscularly administered tiletamine hydrochloride–zolazepam hydrochloride (7 mg per kilogram of body weight, Telazol; Fort Dodge Laboratories, Fort Dodge, Iowa) and 2.2 mg/kg xylazine hydrochloride (Xyla-Ject; Phoenix Pharmaceutical, St Joseph, Mo). Atropine (0.05 mg/kg; Phoenix Pharmaceutical) was administered to facilitate intubation. The animals were intubated, and anesthesia was induced and maintained with inhaled 2% isofluorane (Halocarbon Laboratories, River Edge, NJ). The lungs were exposed at thoracotomy (L.A.S., 5 years experience) performed by using a midline incision. An open surgical approach was used to reduce experimental error and verify proper antenna placement.

After the ablations were performed (as described in Experimental Groups section), the animals were euthanized with an intravenous injection of 0.2 mL of pentobarbital sodium plus phenytoin sodium (Beuthaniasia-D; Schering-Plough, Kenilworth, NJ) per kilogram of body weight. Zones of coagulation were excised en bloc without inflation and were manually cut (P.F.L., 3 years experience performing this procedure) into approximately 4-mm transverse slices. The slices were then optically scanned (Perfection 2450 Photograph Model G860A; Epson, Long Beach, Calif) for analysis. One representative slice from the middle of each coagulation zone was chosen for detailed measurement and histopathologic evaluation (described in Experimental Groups section). The slice that best showed complete ablation was selected and immediately stained with triphenyltetrazolium chloride (TTC) for better demarcation of the zone of complete necrosis (29).

Experimental Groups
A total of 24 lung ablations were performed in three experimental groups—of eight swine each—by using (a) single antennas, (b) single antennas with bronchial occlusion performed before ablation, and (c) an array of three antennas spaced 2 cm apart in a triangular configuration and powered simultaneously. To prevent errors caused by interanimal variability, one ablation from each experimental group (single antenna, single antenna with bronchial occlusion, and three antennas) was performed (N.A.D., P.F.L., each with 3 years experience) in each animal (n = 8). All antennas were tuned (C.L.B., 5 years experience) for the dielectric properties of normal lung tissue (described in Antenna Tuning section).

Bronchial occlusion was performed by using a hemostat clamp applied to the right bronchus at peak inspiration (L.A.S., P.F.L., each with 3 years experience). The clamp was removed at the end of the ablation, which lasted approximately 10 minutes. The 2-cm spacing and parallel positioning of the three antennas were maintained by using an acrylic template. No unexpected complications or deaths were encountered throughout the study, and no technique-specific complications were experienced in the bronchial occlusion or multiple-antenna groups.

Triaxial Microwave Ablation System
All ablations were performed by using a previously described 17-gauge triaxial microwave ablation system designed to minimize the detrimental effects of reflected power while maximizing the power applied through a given diameter cable (23). The triaxial design enables one to tune the antenna for a specific tissue type and frequency by adjusting the active length and insertion depth of the antenna (Fig 1). During each ablation, 55 W of power from a single 2.45-GHz generator (Cober-Muegge, Norwalk, Conn) was applied to each antenna for 10 minutes. For multiple-antenna applications, the generator output was divided into four separate channels with a power divider (SM Electronics; Fairview, Tex) such that each channel received 55 W. One of the channels was connected to an external power meter (Agilent E4419A; Agilent Technologies, Palo Alto, Calif). Power was simultaneously applied to all antennas by using a 100% duty cycle.


Figure 1
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  		Figure 1: Prototypic lung- and liver-tuned antennas. The main components (active length and insertion depth) of the triaxial system are labeled. The tuning is dependent on the active length, which is 3.2 mm longer than that of the lung-tuned antenna.

 
Antenna Tuning
Precise matching of the microwave antenna to a given tissue type results in increased power deposition and decreased reflected power. For our study, lung-tuned antennas were created by modifying liver-tuned triaxial antennas for the unique properties of the lungs (Table 1). The antennas were cut to an active length of 15.5 mm on the basis of a theoretic calculation of resonant length in lung tissue (22,24,28). The antenna was inserted into a region of in vivo lung tissue while the power reflection was measured with a vector network analyzer (Agilent 8753D; Agilent Technologies). The active length and insertion depth were then altered until the power reflections were minimized at the operating frequency of 2.45 GHz (C.L.B., 5 years experience). Throughout the study, these lengths were used in antennas tuned for normal lung tissue.

Measurement of Coagulation Zone Size and Shape
For viability testing, an author (P.F.L., 3 years experience) stained one representative slice from each coagulation zone with TTC, which highlights cells with functioning mitochondria by means of dark blue staining. The author chose the slice that most accurately showed the entire zone of coagulation. Viable tissue stained purple, enabling rapid discrimination between viable and nonviable tissue (29). After the TTC reactions were complete and the coagulation zones could be accurately identified, the slices were optically scanned, the images were saved as electronic files, and measurements were obtained by using ImageJ software (National Institutes of Health, Bethesda, Md) (C.L.B., P.F.L., each with 5 years experience).

The minimal and maximal diameters of the coagulation zones were measured through the insertion axis (for single-antenna ablations) or through the isocenter of the insertion axes (for multiple-antenna ablations). The cross-sectional area and isoperimetric ratio of each representative slice were also measured and recorded. The isoperimetric ratio is an estimation of roundness in two dimensions, where values close to 1 indicate more circular coagulation zones (27,30). Values close to 0 indicate cleft, oblate, or irregular shapes. Only those measurements that could be directly obtained from the representative slice were included in our analysis, because the identification of coagulation zones that were not stained was deemed to be unreliable.

Histopathologic Assessment
Select portions of each representative slice were mounted in paraffin blocks, cut into 7-µm-thick sections, and stained with hematoxylin-eosin. The tissue was examined microscopically (T.F.W., 15 years experience as a pathologist) for evaluation of the following postablation features: preservation of alveolar architecture, presence of lung cells (ie, cell membranes, cytoplasm, and nucleus), presence of alveolar capillaries, lysis of erythrocytes, and presence of bronchi and large vessels. The histologic findings were then correlated with the zones of gradation seen on the gross samples.

Statistical Analyses
The shapes and sizes of the coagulation zones were compared among the three ablation groups. Analysis of variance was used to test for overall differences in isoperimetric ratio, mean diameter, minimal diameter, maximal diameter, and area of coagulation. Two-sample t tests were used to perform pairwise comparisons of each metric among the three groups. Bonferroni correction was used because of the large number of experiments performed; this adjustment was made for seven tests per measurement (six pairwise comparisons, one overall comparison). Therefore, statistical significance (with a standard cutoff of P < .05) was noted for P < .007 (.05/7). P values of between .007 and .05 were considered to be suggestive of a significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Tuning Antennas for Specific Tissue
Manual antenna tuning revealed that use of an active antenna with a length of 15.5 mm—which is somewhat longer than the 12.3-mm length that would be used for the liver—minimized the reflection coefficient at 2.45 GHz in normal aerated lung tissue (Fig 2).


Figure 2
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  		Figure 2: Graph illustrates reflection coefficient measurements in aerated lung obtained by using antennas tuned for the lung and liver. At the design frequency of 2.45 GHz, minimal reflections occurred with the lung-tuned antenna but high reflections occurred with the liver-tuned antenna. Thus, if a liver-tuned antenna were to be used in the lungs, it would reflect approximately 25% of the incident power at 2.45 GHz, while the lung-tuned antenna would reflect less than 1% of the incident power. Because of its shorter length, the liver-tuned antenna resonated at a frequency (~3.5 GHz) higher than the operating frequency.

 
Single- versus Multiple-Antenna Ablation
The mean minimal diameter for the coagulation zones created in the multiple-antenna group (3.30 cm ± 0.94 [standard deviation]) was greater than that for the zones created in the single-antenna group (2.74 cm ± 0.73). The mean maximal diameter for the coagulation zones created in the multiple-antenna group (4.75 cm ± 0.65) was significantly greater than that for the zones created in the single-antenna group (3.39 cm ± 1.00, P < .007). It is notable that differences in isoperimetric ratio between the coagulation zones created by using single and multiple antennas were not significantly different (P > .05), despite the fact that the three antennas were separated by a 2-cm space in a triangular configuration (Table 2, Fig 3).


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  		Table 2. Sizes and Shapes of Coagulation Zones

 

Figure 3A
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  		Figure 3a: Gross lung tissue samples stained with TTC, a marker for mitochondrial activity, show representative coagulation zones achieved by using (a) a single lung-tuned antenna (minimal diameter, 2.9 cm; maximal diameter, 3.6 cm), (b) a single lung-tuned antenna with bronchial occlusion (minimal diameter, 3.6 cm; maximal diameter, 4.7 cm), and (c) three lung-tuned antennas spaced 2.0 cm apart and powered simultaneously (minimal diameter, 3.4 cm; maximal diameter, 5.7 cm). Specimens are shown at an identical scale.

 

Figure 3B
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  		Figure 3b: Gross lung tissue samples stained with TTC, a marker for mitochondrial activity, show representative coagulation zones achieved by using (a) a single lung-tuned antenna (minimal diameter, 2.9 cm; maximal diameter, 3.6 cm), (b) a single lung-tuned antenna with bronchial occlusion (minimal diameter, 3.6 cm; maximal diameter, 4.7 cm), and (c) three lung-tuned antennas spaced 2.0 cm apart and powered simultaneously (minimal diameter, 3.4 cm; maximal diameter, 5.7 cm). Specimens are shown at an identical scale.

 

Figure 3C
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  		Figure 3c: Gross lung tissue samples stained with TTC, a marker for mitochondrial activity, show representative coagulation zones achieved by using (a) a single lung-tuned antenna (minimal diameter, 2.9 cm; maximal diameter, 3.6 cm), (b) a single lung-tuned antenna with bronchial occlusion (minimal diameter, 3.6 cm; maximal diameter, 4.7 cm), and (c) three lung-tuned antennas spaced 2.0 cm apart and powered simultaneously (minimal diameter, 3.4 cm; maximal diameter, 5.7 cm). Specimens are shown at an identical scale.

 
Effects of Bronchial Occlusion
The mean minimal diameter for the coagulation zones created with occlusion of the proximal bronchus (3.60 cm ± 1.26) was greater than that for the zones created with single-antenna ablation (2.74 cm ± 0.73, P = .12). In addition, the mean maximal diameter in the bronchial occlusion group (4.58 cm ± 0.99) was significantly greater than that in the single-antenna ablation group (3.39 cm ± 1.00, P = .032). Two important findings were that there was no significant difference in size or shape between the coagulation zones created by using bronchial occlusion and those created by using multiple antennas (P > .05 for all comparisons) and that the mean minimal diameter in the single-antenna bronchial occlusion group (3.60 cm ± 1.26) was slightly greater than that in the multiple-antenna group (3.30 cm ± 0.94).

Histopathologic Assessment
On the histologic sections, the most central coagulation zone (zone 1) consisted of shrunken distorted alveolar walls with precipitates of lysed erythrocyte material (Fig 4). There were also compressed basophilic collagen and distorted tissues with large spaces due to gas bubble formation near the antenna. In zone 2, the alveolar pattern was intact, with intraalveolar edema and pyknotic nuclei. Erythrocyte lysis had also occurred. Zone 3 contained intensely congested alveolar capillaries with pyknotic nuclei. In zone 4, the lung tissue was normal and was stained purple with TTC; this was in contrast to the coagulated tissue, which did not stain. The gross and histologic specimens were compared. Regarding the gross specimens, zone 1 was gray secondary to necrosis, while zones 2 and 3 demonstrated gradations of pink and red, corresponding to the degrees of congestion and lysis.


Figure 4A
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  		Figure 4a: (a) Gross lung tissue sample shows the four histologic zones created with a single microwave antenna. (b) Corresponding histologic specimen (hematoxylin-eosin stain; original magnification, x20) shows the four zones seen in a: Zone 1, the light innermost zone, is completely necrotic. Zone 2 is a smaller light zone characterized by edema in the alveolar spaces, mild congestion, and pyknotic nuclei. Zone 3, the red-pink area, represents a transition zone. Zone 4, the most peripheral zone, is blue and purple and represents uptake of TTC stain, which is consistent with viable tissue. (c) Corresponding histologic section (magnification, x200) from zone 1 shows compressed and disordered alveolar walls with precipitate from lysed erythrocytes. (d) Corresponding histologic section (magnification, x200) from zone 2. (e) Corresponding histologic section (magnification, x200) from zone 3 shows intense congestion of the alveolar capillaries and pyknotic nuclei; these findings correlate with the zone 3 findings seen in b.

 

Figure 4B
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  		Figure 4b: (a) Gross lung tissue sample shows the four histologic zones created with a single microwave antenna. (b) Corresponding histologic specimen (hematoxylin-eosin stain; original magnification, x20) shows the four zones seen in a: Zone 1, the light innermost zone, is completely necrotic. Zone 2 is a smaller light zone characterized by edema in the alveolar spaces, mild congestion, and pyknotic nuclei. Zone 3, the red-pink area, represents a transition zone. Zone 4, the most peripheral zone, is blue and purple and represents uptake of TTC stain, which is consistent with viable tissue. (c) Corresponding histologic section (magnification, x200) from zone 1 shows compressed and disordered alveolar walls with precipitate from lysed erythrocytes. (d) Corresponding histologic section (magnification, x200) from zone 2. (e) Corresponding histologic section (magnification, x200) from zone 3 shows intense congestion of the alveolar capillaries and pyknotic nuclei; these findings correlate with the zone 3 findings seen in b.

 

Figure 4C
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  		Figure 4c: (a) Gross lung tissue sample shows the four histologic zones created with a single microwave antenna. (b) Corresponding histologic specimen (hematoxylin-eosin stain; original magnification, x20) shows the four zones seen in a: Zone 1, the light innermost zone, is completely necrotic. Zone 2 is a smaller light zone characterized by edema in the alveolar spaces, mild congestion, and pyknotic nuclei. Zone 3, the red-pink area, represents a transition zone. Zone 4, the most peripheral zone, is blue and purple and represents uptake of TTC stain, which is consistent with viable tissue. (c) Corresponding histologic section (magnification, x200) from zone 1 shows compressed and disordered alveolar walls with precipitate from lysed erythrocytes. (d) Corresponding histologic section (magnification, x200) from zone 2. (e) Corresponding histologic section (magnification, x200) from zone 3 shows intense congestion of the alveolar capillaries and pyknotic nuclei; these findings correlate with the zone 3 findings seen in b.

 

Figure 4D
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  		Figure 4d: (a) Gross lung tissue sample shows the four histologic zones created with a single microwave antenna. (b) Corresponding histologic specimen (hematoxylin-eosin stain; original magnification, x20) shows the four zones seen in a: Zone 1, the light innermost zone, is completely necrotic. Zone 2 is a smaller light zone characterized by edema in the alveolar spaces, mild congestion, and pyknotic nuclei. Zone 3, the red-pink area, represents a transition zone. Zone 4, the most peripheral zone, is blue and purple and represents uptake of TTC stain, which is consistent with viable tissue. (c) Corresponding histologic section (magnification, x200) from zone 1 shows compressed and disordered alveolar walls with precipitate from lysed erythrocytes. (d) Corresponding histologic section (magnification, x200) from zone 2. (e) Corresponding histologic section (magnification, x200) from zone 3 shows intense congestion of the alveolar capillaries and pyknotic nuclei; these findings correlate with the zone 3 findings seen in b.

 

Figure 4E
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  		Figure 4e: (a) Gross lung tissue sample shows the four histologic zones created with a single microwave antenna. (b) Corresponding histologic specimen (hematoxylin-eosin stain; original magnification, x20) shows the four zones seen in a: Zone 1, the light innermost zone, is completely necrotic. Zone 2 is a smaller light zone characterized by edema in the alveolar spaces, mild congestion, and pyknotic nuclei. Zone 3, the red-pink area, represents a transition zone. Zone 4, the most peripheral zone, is blue and purple and represents uptake of TTC stain, which is consistent with viable tissue. (c) Corresponding histologic section (magnification, x200) from zone 1 shows compressed and disordered alveolar walls with precipitate from lysed erythrocytes. (d) Corresponding histologic section (magnification, x200) from zone 2. (e) Corresponding histologic section (magnification, x200) from zone 3 shows intense congestion of the alveolar capillaries and pyknotic nuclei; these findings correlate with the zone 3 findings seen in b.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
Our study results demonstrate the capability of an experimental small-gauge triaxial microwave ablation system to create large confluent zones of coagulation in normal porcine lung tissue. The 17-gauge antenna (outer diameter, 1.47 mm) is tuned to increase power deposition and reduce the degree of reflected power. Compared with the most clinically used multiple-prong RF ablation systems, which consist of a 13-gauge introducer with 10 25-gauge prongs when they are deployed, the triaxial antenna system produces larger zones of coagulation with considerably less tissue trauma (2,13). We would expect fewer complications with use of such a small antenna. Reports of pneumothorax rates as high as 52% and of difficulty withdrawing the hooks of multiple-prong electrodes indicate that smaller-gauge, higher-efficiency antennas could have a higher safety profile (17,18,31,32). This theory is supported by reports of lower complication rates with use of smaller-gauge needles during lung biopsy (16).

Compared with RF treatment for liver and kidney tumors, RF treatment for lung tumors has been less successful (1,8,11,12,33). The lower success in treating lung tumors may be explained by the interaction between electrical energy and lung tissue. Lung tissue is a poor thermal conductor and does not readily conduct heat away from the energy source into adjacent tissue. Although the electrical impedances of lung tumors may be lower than those of the normal lung parenchyma, it is essential that a margin of normal aerated lung tissue surrounding the tumor be ablated (10,11,13,34). The experimental RF lung ablation procedures performed to date have yielded coagulation zones of only 2–3 cm in maximal diameter with use of relatively large (13–15-gauge) RF electrodes. Volumes of 90–107 cm3 have been reported with use of saline infusion and a large (15-gauge) multiple-tine system (35,36); however, because maximal diameter, minimal diameter, and roundness values were not reported, it is difficult to make contextual interpretations of the data. Also, substantial risks associated with saline infusion and large multiple-tine systems have been demonstrated (14,15,17,18). In addition, investigators in an animal study had difficulty achieving necrosis and effective ablative margins, even in small (1–2-cm) nodules (11).

In contrast, the system evaluated in our study enabled us to create zones of coagulation with a mean minimal diameter greater than 2.5 cm with one 17-gauge antenna. With bronchial occlusion and a single antenna, the mean minimal diameter was greater than 3.5 cm. If these results could be reproduced in humans, it might be possible to treat lung tumors as large as 2.5 cm and produce a 0.5-cm ablative margin with a single antenna. Multiple antennas could be used when treating larger or irregularly shaped tumors, as has been demonstrated in the liver (20,25). Although the three antennas were set in a triangular configuration, the shapes of the resultant coagulation zones were not significantly different from the shapes of zones created with single-antenna ablation.

Compared with RF energy, microwave energy has several theoretic advantages in terms of tumor ablation: Microwave heating is relatively insensitive to tissue desiccation and charring; true 100% duty cycle, multiple-antenna ablation is possible without switching; and the penetration of the microwave field is deeper than the penetration of the RF field (22,37–39). Microwave heating can maintain high temperatures because it is not subject to impedance spikes or "roll-off" (13). Microwave energy should also enable the treatment of tumor margins, regardless of the presence or absence of aerated lung tissue, because electromagnetic waves propagate easily through air. However, these fundamental advantages of microwave ablation have not been fully realized because commercial microwave systems are not widely available and existing systems have relatively large-diameter (typically 12–14-gauge) antennas. There are obvious drawbacks to using these systems in the lungs, particularly in patients with comorbidities such as emphysema.

An advantage of the triaxial antenna is the ability to tune it for maximal energy delivery (Fig 3). Virtually all other ablation systems are designed for use in the liver and are applied to other organs as new indications develop. Whether organ- and tissue-specific tuning is a clinically important advantage remains to be seen. The coagulation and dehydration induced by heating cause potentially large changes in the dielectric properties of tissue (28). These changes may be more marked in tissues that normally have low density, such as lung tissue, and may cause the heated tissue to develop properties more similar to those of denser tissue such as liver parenchyma. More data on how the dielectric properties of tissues change during ablation are needed to better quantify this effect. The best tuning strategy may be to tune the antenna for tissue that has already been treated for several minutes or, ideally, to implement a dynamic tuning process that includes receiving feedback regarding the tissue properties throughout the ablation. Further experimentation is needed to determine how these optimization strategies can be best applied.

Bronchial occlusion has also been used with RF energy to increase zones of coagulation. The mechanism underlying the increased effectiveness of thermal ablation after bronchial occlusion is probably related to decreased heat loss from cooling bronchial air flow and increased stability of the tissue around the antenna. Bronchial occlusion also leads to a 30%–60% decrease in pulmonary blood flow and thus a reduction in perfusion-mediated cooling (9). With stabilization of the antenna due to the absence of respiratory motion, the microwave energy is focused on a single location during the entire ablation period. Because of the increased effectiveness of bronchial occlusion, we were unable to demonstrate a significant difference in size or shape between the coagulation zones created with bronchial occlusion and those created with multiple antennas. The concept that a single antenna with organ-specific maneuver capability can yield zones of coagulation similar to those created with multiple antennas represents another possible option for improving the safety of ablation procedures. Bronchial occlusion is considered safe and is routinely used in thoracic surgery by anesthesiologists who are skilled in the technique (40).

There were limitations in our study. The lack of a widely available inexpensive and reproducible large-animal tumor model has been a long-standing limitation of preclinical tumor ablation studies. The limiting factor in lung ablation procedures is not necessarily inadequate treatment of the targeted tumor mass but rather inadequate treatment of the tumor margins in the surrounding normal aerated lung parenchyma. Owing to substantial tissue shrinking during ablation and some degree of alveolar collapse in the postmortem specimens, it is also likely that our measurements were underestimations of the ablated tissue volume. An additional limitation is the fact that we used open thoracotomy to maximize the number of ablations that could be performed in each animal while minimizing the total number of animals used. Because this was the first study in which we used this system, use of an open technique was essential for proper positioning and control of the experiments. It is possible that a percutaneous approach would yield different results. The described triaxial microwave system has not yet been optimized for percutaneous use, but the results obtained in our study seem to justify further development.

Practical applications: Our study results demonstrate the feasibility of tissue-specific microwave ablation for creating large zones of coagulation in normal porcine lung tissue with use of a small-diameter (17-gauge) triaxial antenna. Coagulation zones were significantly larger with use of simultaneously powered multiple antennas and bronchial occlusion. Small-gauge organ-specific microwave ablation represents a potential method of increasing the success of future clinical lung ablation procedures.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   FOOTNOTES
 

Abbreviations: RF = radiofrequency • TTC = triphenyltetrazolium chloride

See Materials and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, N.A.D., T.F.W., C.L.B.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, N.A.D., P.F.L., F.T.L., C.L.B.; experimental studies, N.A.D., P.F.L., L.S.B., F.T.L., L.A.S., T.M.F., T.F.W., D.W.v.d.W., C.L.B.; statistical analysis, N.A.D., J.P.F., C.L.B.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

  1. Lee JM, Jin GY, Goldberg SN, et al. Percutaneous radiofrequency ablation for inoperable non–small cell lung cancer and metastases: preliminary report. Radiology 2004;230(1):125–134.[Abstract/Free Full Text]
  2. VanSonnenberg E, Shankar S, Morrison PR, et al. Radiofrequency ablation of thoracic lesions. II. Initial clinical experience: technical and multidisciplinary considerations in 30 patients. AJR Am J Roentgenol 2005;184(2):381–390.[Abstract/Free Full Text]
  3. Simon CJ, Dupuy DE. Current role of image-guided ablative therapies in lung cancer. Expert Rev Anticancer Ther 2005;5(4):657–666.[CrossRef][Medline]
  4. Lencioni R, Crocetti L, Cioni D, Della Pina C, Bartolozzi C. Percutaneous radiofrequency ablation of hepatic colorectal metastases: technique, indications, results, and new promises. Invest Radiol 2004;39(11):689–697.[CrossRef][Medline]
  5. Nguyen CL, Scott WJ, Young NA, Rader T, Giles LR, Goldberg M. Radiofrequency ablation of primary lung cancer: results from an ablate and resect pilot study. Chest 2005;128(5):3507–3511.[CrossRef][Medline]
  6. de Baere T, Palussiere J, Auperin A, et al. Midterm local efficacy and survival after radiofrequency ablation of lung tumors with minimum follow-up of 1 year: prospective evaluation. Radiology 2006;240(2):587–596.[Abstract/Free Full Text]
  7. Akeboshi M, Yamakado K, Nakatsuka A, et al. Percutaneous radiofrequency ablation of lung neoplasms: initial therapeutic response. J Vasc Interv Radiol 2004;15(5):463–470.[Medline]
  8. Goldberg SN, Gazelle GS, Compton CC, et al. Radiofrequency ablation of VX2 tumor nodules in the rabbit lung. Acad Radiol 1996;3(11):929–935.[CrossRef][Medline]
  9. Oshima F, Yamakado K, Akeboshi M, et al. Lung radiofrequency ablation with and without bronchial occlusion: experimental study in porcine lungs. J Vasc Interv Radiol 2004;15(12):1451–1456.[Medline]
  10. Ahmed M, Liu Z, Afzal KS, et al. Radiofrequency ablation: effect of surrounding tissue composition on coagulation necrosis in a canine tumor model. Radiology 2004;230(3):761–767.[Abstract/Free Full Text]
  11. Nomori H, Imazu Y, Watanabe K, et al. Radiofrequency ablation of pulmonary tumors and normal lung tissue in swine and rabbits. Chest 2005;127(3):973–977.[CrossRef][Medline]
  12. Steinke K, Glenn D, King J, Morris DL. Percutaneous pulmonary radiofrequency ablation: difficulty achieving complete ablations in big lung lesions. Br J Radiol 2003;76(910):742–745.[Abstract/Free Full Text]
  13. Morrison PR, vanSonnenberg E, Shankar S, et al. Radiofrequency ablation of thoracic lesions. I. Experiments in the normal porcine thorax. AJR Am J Roentgenol 2005;184(2):375–380.[Abstract/Free Full Text]
  14. Kim TS, Lim HK, Kim H. Excessive hyperthermic necrosis of a pulmonary lobe after hypertonic saline-enhanced monopolar radiofrequency ablation. Cardiovasc Intervent Radiol 2006;29(1):160–163.[CrossRef][Medline]
  15. Gillams AR, Lees WR. CT mapping of the distribution of saline during radiofrequency ablation with perfusion electrodes. Cardiovasc Intervent Radiol 2005;28(4):476–480.[CrossRef][Medline]
  16. Geraghty PR, Kee ST, McFarlane G, Razavi MK, Sze DY, Dake MD. CT-guided transthoracic needle aspiration biopsy of pulmonary nodules: needle size and pneumothorax rate. Radiology 2003;229(2):475–481.[Abstract/Free Full Text]
  17. Steinke K, King J, Glenn D, Morris DL. Percutaneous radiofrequency ablation of lung tumors: difficulty withdrawing the hooks resulting in a split needle. Cardiovasc Intervent Radiol 2003;26(6):583–585.[Medline]
  18. Steinke K, King J, Glenn DW, Morris DL. Percutaneous radiofrequency ablation of lung tumors with expandable needle electrodes: tips from preliminary experience. AJR Am J Roentgenol 2004;183(3):605–611.[Free Full Text]
  19. Wright AS, Sampson LA, Warner TF, Mahvi DM, Lee FT Jr. Radiofrequency versus microwave ablation in a hepatic porcine model. Radiology 2005;236(1):132–139.[Abstract/Free Full Text]
  20. Wright AS, Lee FT Jr, Mahvi DM. Hepatic microwave ablation with multiple antennae results in synergistically larger zones of coagulation necrosis. Ann Surg Oncol 2003;10(3):275–283.[Abstract/Free Full Text]
  21. Simon CJ, Dupuy DE, Mayo-Smith WW. Microwave ablation: principles and applications. RadioGraphics 2005;25(suppl 1):S69–S83.[Abstract/Free Full Text]
  22. Brace CL, Laeseke PF, van der Weide DW, Lee FT Jr. Microwave ablation with a triaxial antenna: results in ex vivo bovine liver. IEEE 2005;53:215–220.
  23. Brace CL, Laeseke PF, Sampson LA, Frey TM, van der Weide DW, Lee FT Jr. Microwave ablation with a single small-gauge triaxial antenna: in vivo porcine liver model. Radiology 2007;242(2):435–440.[Abstract/Free Full Text]
  24. Gabriel S, Lau RW, Gabriel C. The dielectric properties of biological tissues. II. Measurements in the frequency range 10 Hz to 20 GHz. Phys Med Biol 1996;41(11):2251–2269.[CrossRef][Medline]
  25. Brace CL, Laeseke PF, Sampson LA, Frey TM, van der Weide DW, Lee FT Jr. Microwave ablation with multiple simultaneously powered small-gauge triaxial antennas: results from an in vivo swine liver model. Radiology 2007;244(1):151–156.[Abstract/Free Full Text]
  26. Institute of Laboratory Animal Resources, Commission on Life Sciences, National Research Council. Guide for the care and use of laboratory animals. Washington, DC: National Academy Press, 1996.
  27. Do Carmo M. Differential geometry of curves and surfaces. 8th ed. Englewood Cliffs, NJ: Prentice-Hall, 1976; 393.
  28. Duck FA, ed. Physical properties of tissue: a comprehensive reference book. San Diego, Calif: Academic Press, 1990.
  29. Goldlust EJ, Paczynski RP, He YY, Hsu CY, Goldberg MP. Automated measurement of infarct size with scanned images of triphenyltetrazolium chloride-stained rat brains. Stroke 1996;27(9):1657–1662.[Abstract/Free Full Text]
  30. Chinn SB, Lee FT Jr, Kennedy GD, et al. Effect of vascular occlusion on radiofrequency ablation of the liver: results in a porcine model. AJR Am J Roentgenol 2001;176(3):789–795.[Abstract/Free Full Text]
  31. Hiraki T, Tajiri N, Mimura H, et al. Pneumothorax, pleural effusion, and chest tube placement after radiofrequency ablation of lung tumors: incidence and risk factors. Radiology 2006;241(1):275–283.[Abstract/Free Full Text]
  32. Steinke K, King J, Glenn D, Morris DL. Radiologic appearance and complications of percutaneous computed tomography-guided radiofrequency-ablated pulmonary metastases from colorectal carcinoma. J Comput Assist Tomogr 2003;27(5):750–757.[CrossRef][Medline]
  33. Herrera LJ, Fernando HC, Perry Y, et al. Radiofrequency ablation of pulmonary malignant tumors in nonsurgical candidates. J Thorac Cardiovasc Surg 2003;125(4):929–937.[Abstract/Free Full Text]
  34. Yamamoto A, Nakamura K, Matsuoka T, et al. Radiofrequency ablation in a porcine lung model: correlation between CT and histopathologic findings. AJR Am J Roentgenol 2005;185(5):1299–1306.[Abstract/Free Full Text]
  35. Gananadha S, Morris DL. Saline infusion markedly reduces impedance and improves efficacy of pulmonary radiofrequency ablation. Cardiovasc Intervent Radiol 2004;27(4):361–365.[Medline]
  36. Goldberg SN, Grassi CJ, Cardella JF, et al. Image-guided tumor ablation: standardization of terminology and reporting criteria. J Vasc Interv Radiol 2005;16(6):765–778.[Medline]
  37. Skinner MG, Iizuka MN, Kolios MC, Sherar MD. A theoretical comparison of energy sources—microwave, ultrasound and laser—for interstitial thermal therapy. Phys Med Biol 1998;43(12):3535–3547.[CrossRef][Medline]
  38. Deardorff DL, Diederich CJ, Nau WH. Control of interstitial thermal coagulation: comparative evaluation of microwave and ultrasound applicators. Med Phys 2001;28(1):104–117.[CrossRef][Medline]
  39. Haemmerich D, Laeseke PF. Thermal tumour ablation: devices, clinical applications and future directions. Int J Hyperthermia 2005;21(8):755–760.[CrossRef][Medline]
  40. Benumof J. Anesthesia for thoracic surgery. 2nd ed. Philadelphia, Pa: Saunders, 1987; 224.



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