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British Journal of Radiology (2006) 79, 562-571
© 2006 British Institute of Radiology
doi: 10.1259/bjr/51844219
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Full paper

Radiofrequency ablation in pig lungs: in vivo comparison of internally cooled, perfusion and multitined expandable electrodes

J M Lee, MD1,2, J K Han, MD1,2, J M Chang, MD1,2, S Y Chung, MD1,2, S H Kim, MD1,2, J Y Lee, MD1,2 and B I Choi, MD1,2

1 Department of Radiology, and Institute of Radiation Medicine, Seoul National University College of Medicine, 28 Yongon-dong, Chongno-gu, Seoul 110-744, 2 Clinical Research Institute, Seoul National University Hospital, Seoul, Korea


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
The purpose of this study was to compare the amounts of in vivo coagulation obtained by radiofrequency (RF) ablation in porcine lung, using three types of electrodes. 15 in vivo ablation procedures were performed in the lungs of five pigs using three kinds of currently available RF devices under CT guidance. After placing an electrode in the lung, three ablation zones were created at each of three different regimens: Group A: RF ablation with an internally cooled electrode; Group B: RF ablation with a perfusion electrode, with instillation of 0.9% NaCl solution at a rate of 1.5 ml min–1; Group C: RF ablation with a multitined expandable electrode. According to the manufacturer's recommendations, RF application times were 12 min in group A and 20 min in group B. In group C, RF energy was delivered for 7 min after a mean temperature of 110°C was reached at 5 cm deployment. 36 min after the procedures, contrast-enhanced CT scans were obtained to evaluate the volume of zone of coagulation, and lungs were harvested for gross measurements. After macroscopic and histopathological analyses of 5 mm-thick lung sections, diameters, volumes and variation coefficients of regions of central coagulation were assessed. During RF ablation, the perfusion electrode allowed a larger energy delivery than the internally cooled or the multitined expandable electrodes, i.e. 33.6±4.7 kJ in group A, 40.0±8.2 kJ in group B and 23.5±6.1 kJ in group C (p<0.05). On gross observation, the cut surface of the gross specimen containing RF-induced coagulation showed that the ablated tissue appeared to be a central, firm, dark-brown area surrounded by an irregular outer margin (approximately 3–10 mm thick) of bright red tissue. In vivo studies showed that RF ablation using the perfusion electrode achieved larger coagulation volume than RF ablation using the other electrodes (p<0.05): 7.2±4.1 cm3 in group A; 16.9±5.5 cm3 in group B; 7.5±3.3 cm3 in group C. The corresponding variation coefficients were 0.55, 0.31, and 0.45, respectively. Our study shows that RF ablation using a perfusion electrode achieves a larger coagulation volume with an irregular margin than RF ablation using internally cooled or multitined expandable electrodes in the porcine lung.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Lung cancer is one of the major leading causes of cancer-related mortality, with more than 1.2 million new cases of lung cancer being diagnosed worldwide each year [1–3]. In addition, the lung is also the second most frequent site of metastatic disease. Until recently, therapeutic options for primary lung cancers, depending on tumour grading and staging and the presence of comorbidities, included a combination of surgical resection, chemotherapy, and/or XRT. Indeed, surgical resection is the treatment of choice for early-stage non-small cell lung cancer (NSCLC), but unfortunately, patients with NSCLC are frequently poor surgical candidates due to coexistent medical diseases such as chronic obstructive bronchopneumopathy or cardiac disease [4]. Several studies have documented survival benefits in patients with pulmonary metastases with favourable histologies who received complete resection as compared with unresectable individuals [5, 6]. In patients with lung cancer who are not surgical candidates, the treatment options are primarily XRT with or without chemotherapy. However, for stage I/II NSCLC, the effectiveness for XRT relative to surgery remains uncertain [7], and a meta-analysis of trials comparing primary treatment with or without chemotherapy showed that chemotherapy provided only a modest benefit [8]. Therefore, less invasive therapies that can accomplish tumour destruction without the use of general anaesthesia may complement, improve, or even replace existing therapies.

Radiofrequency (RF) ablation has received much recent attention as a minimally invasive strategy for the treatment of various neoplasms of the liver, kidney and bone [9–12]. Also, a number of recent experimental and clinical studies have demonstrated the feasibility and safety of RF ablation for the treatment of inoperable lung malignancies [13–18]. Some preliminary studies have shown that RF ablation enables the successful treatment of relatively small lung malignancies with a high rate of complete response and acceptable morbidity [15–18]. However, others have shown limitations in achieving complete necrosis in large tumours measuring 3 cm or more in diameter [19–21]. Lee et al [20] treated 32 lesions by RF ablation and achieved complete necrosis in 100% of tumours smaller than 3 cm in diameter, and in 23% of larger tumours.

To successfully ablate malignant tumours, it is essential to ensure the coagulation of the entire targeted volume with as few complications as possible. Indeed, the acquisition of a large volume of ablation in a safe manner is of paramount importance if RF is to be accepted as routine form of intervention. Previous clinical studies on lung RF ablation have examined internally cooled needle electrodes [18, 20] and multitined expandable electrodes [19, 21]. Currently in Korea, Valleylab, BerchtoldTM, and RITA® RF systems have been used (personal communication with Drs KY Jin, YK Kim, and GS Jung) for the treatment of primary and secondary lung cancers. Recently, Lee et al [22, 23] demonstrated improved RF ablation efficacy using saline infusion to induce a large volume of coagulation both ex vivo and in vivo rabbit lungs. However, no study has compared the in vivo efficacies of different types of electrodes in the lung using large animals. The purpose of our study was to compare in vivo coagulation obtained with currently available RF ablation devices, namely an internally cooled needle electrode, a multitined expandable electrode and a perfusion electrode.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Animals and preparations
The experimental protocol was approved by the Animal Use and Care Administrative Advisory Committee of our institution. All experiments were performed according to a protocol approved by the local institutional committee on animals, in accordance with the general guidelines issued by the National Institute of Health for the care of laboratory animals. Five female farm pigs were used in this study (weight range 30–40 kg). The animals were fasted overnight, but had free access to water before the experiments. Each of the five pigs was anaesthetised using an intramuscular injection of 50 mg kg–1 of ketamine hydrochloride (Ketamine; Yuhan, Seoul, Korea) and 5 mg kg–1 of Xylazine (Rumpun, Bayer Korea), and prepared for RF ablation. Booster injections of up to half of the initial dose were administered as needed. Ringer's lactate solution was continuously infused during the experiment (500 ml h–1). Endotracheal intubation was performed and anaesthesia was maintained with inhaled enfluorane (Gerolan; Choongwae Pharma Corporation, Seoul, Korea). Mechanical ventilation was used throughout the procedure.

Cardiac and respiratory parameters were monitored throughout the procedures. Each animal's lateral hindquarters were shaved bilaterally, and two 8 cm x 12 cm wire-mesh grounding-pads coated with conductive gel were placed on each hind limb. For the RF ablation procedures, animals were placed in the supine position on the CT scanner. Animals were euthanized approximately 1–2 h after the final RF ablation procedure with a pentobarbital overdose of 60 mg kg–1. The lungs were then removed.

Study design
To minimize potential variations in the RF ablation procedures, all procedures were performed by consensus between two radiologists who had extensive routine experience with all three systems. Electrodes were placed under the same experimental conditions with CT guidance (Somatom plus 4 scanner or Sensation 16; Siemens Medical Solutions, Forchheim, Germany). CT enabled an electrode tip to be positioned at least 2 cm from the pleura, from a previous ablation site in the same lung, and from large vessels (> 3 mm in diameter). RF ablation was performed using one of the three RF systems: Group A: a 200 W generator (CC3: Valleylab TM) and an internally cooled electrode; Group B: a 60 W generator (HiTT 106 Berchtold®, Tuttlingen, Germany) and a perfusion electrode; Group C: a 150 W generator (1500 TM model; RITA® medical Systems; Mountain View, CA) and a multitined expandable electrode.

The RF systems, lung lobes, position and the order in which each ablation procedure was performed were randomly assigned. A new electrode was used for each ablation session. All settings were performed according to manufacturer's recommendations. The automated control mechanisms functioned by measuring the total impedance between the electrodes for the Berchtold®, and ValleylabTM systems and on the temperature of the electrode tip for the RITA® system. Applied current, power output and impedance were continuously monitored using a generator system during RF ablation and were recorded. The technical aspects of the RF ablation, including impedance and wattage changes, and the dimensions of the RF-coagulated area for each system were compared.

RF devices, ablation protocols and procedures
Group A (ValleylabTM system)
A 480 kHz generator (CC3; ValleylabTM) capable of a maximum power of 200 W was used with a 1.6 mm-diameter internally cooled electrode (single cool-tip needle), which has a 3 cm-long active distal region (Figure 1aGo). Electrode cooling was ensured by the peristaltic perfusion of chilled saline using a peristaltic pump (PE-PM; ValleylabTM), which allowed the electrode to maintain a tip temperature of below 25°C during RF delivery. This RF system requires the applications of four neural pads. The circuitry incorporated into the generator allowed continuous monitoring of the impedance between the active electrode and the grounding pads. RF current was passed for 12 min at a maximum generator setting for the impedance control method. This method allows the maximum power to be delivered until impedance rises to 10 {Omega} above the baseline value. At this point, the current is switched off automatically to avoid a further local increase in temperature, which would result in tissue charring. 15 s later, the current is automatically switched on again, thus being referred to as the pulsed RF technique, which increases RF ablation area [24].


Figure 1
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  		Figure 1. Different electrodes used for monopolar radiofrequency ablation.(a) ValleylabTM internally cooled electrode with a 30 mm long exposed tip (Cool-tip). (b) Berchtold® perfusion electrode with a 15 mm long exposed tip and side holes. (c) RITA® multitined expandable electrode (Starburst XL) with nine curved tines.

 
Group B (Berchtold® system)
This perfusion RF system is supported by a 375 kHz generator (HiTT 106; Berchtold®) capable of 60 W at maximum through a 1.7 mm-diameter monopolar electrode with an active tip length of 1.5 cm. The electrode used for this saline-enhanced technique was double walled at its distal part, and the inner wall had small perfusion holes (Figure 1bGo). A 0.9% saline solution was used as the perfusion liquid. This flowed through the hollow shaft of the electrode and permeated through the perfusion holes into the space between the inner and outer walls of the needle tip. Continuous interstitial perfusion of saline was started at 30 s at 90 ml h–1 prior to RF application, and was maintained during RF ablation using a digitally controlled syringe pump (Pilot C; Frensenius Vial, Brezins, France). RF current was applied for 20 min at 40 W, in accordance with the manufacturer's recommendations. The associated control mechanism stabilized RF power within moderate impedance changes of between 100 {Omega} and 350 {Omega}. When the impedance exceeded 900 {Omega}, an additional saline bolus was administered [25].

Group C (RITA® system)
Model 1500TM (RITA® Medical systems) utilizes a 150 W generator operating at 460 kHz. In this RF system, the expandable electrode (Starburst XL; Rita Medical Systems) consists of an insulated outer needle with a diameter of 2.2 mm that houses nine deployable curved tines, which have a maximum diameter of 5 cm when fully expanded (Figure 1cGo). The tips of five of these tines contain a thermocouple and allow temperature monitoring during the ablation procedure. Electrodes were progressively extended deeper into the lung parenchyma with temperature monitoring, and power was controlled according to average temperature. Tines were first deployed at 2 cm with a pre-selected target temperature of 80°C, then advanced to 3 cm with a target temperature of 105°C and, finally, extended to 4–5 cm with a target temperature of 110°C. Target temperatures were maintained for 7 min and then post-ablation temperatures were monitored [26].

Imaging follow-up
A multirow detector CT (Sensation 16, Siemens Medical Solutions) was used to monitor ablations at 30–60 min after RF ablation. Axial CT scans were obtained using a 0.75 mm detector collimation, a reconstruction increment of 3 mm and a 1.0 pitch, and included both lungs, before and after injecting 70 ml of contrast medium (Ultravist 370®; Schering Korea, Seoul, Korea). Contrast medium was injected at a rate of 2 ml s–1 through an ear vein; post-contrast CT scans were obtained at 60 s after contrast administration and CT images were reconstructed at an interval of 3 mm in the axial plane and at an interval of 1 mm to obtain a high-quality data set for multiplanar reconstruction images. The thin section data set was forwarded to a PC containing dedicated 3D software (Rapidia, INFINITT, Seoul, Korea), and data was reconstructed into 3 mm-thick coronal and sagittal slices.

Assessment of coagulation zone (imaging and pathological studies)
Pigs were euthanized after obtaining CT images. Once harvested, lungs were serially sectioned at 5 mm intervals along the axial plane. The histopathological study included staining for mitochondrial enzyme activity, which was performed by incubating thin representative tissue sections for 30 min in 2% 2,3,5,-triphenyl tetrazolium chloride (TTC; Sigma, St Louis, MO), at 20–25°C. This test is a vitalline stain for mitochondrial enzyme activity [27] and can be used to determine irreversible cellular injury during the early stages of RF-induced necrosis [28]. As the unstained area of an RF-induced coagulation has been shown to correspond to the zone of necrosis [28], two observers measured axial diameters along the axis of the electrodes (Dax) and transverse diameters (Dtr) of ablation areas in the axial plane before reaching consensus. The number of slices containing an RF ablated region determined the vertical diameter (Dv). In addition, the slices were photographed using a digital camera (Canon EOS 300D; Canon Inc., Tokyo, Japan), and images were saved to image management software (PhotoShop; Adobe, San Jose, CA). Area analysis was performed on a computer equipped with NIH Image J software (National Institutes of Health; http://rsb.info.nih.gov/ij/) [29]. The area of coagulation, on each slice was calculated using this computer program, and volumes were calculated by multiplying areas by slice thickness and summed to obtain total lesion volumes.

The shapes of RF-induced coagulations were characterized using the ratio between the long axis diameter and the short-axis diameter: Dax/Dtr. Thus, a ratio near 1 indicates a near spherical shape. In addition, volume variations in each group were determined using coefficients of variation, calculated as follows: standard deviation of the ablation volume/mean value of the ablation volume. The closer this ratio is to 0, the more reproducible the coagulation [26]. The RF-induced ablated regions of representative cases in each group were fixed in 10% formalin for routine histological processing, and processed by paraffin sectioning and haematoxylin-eosin staining for light microscopic studies.

On contrast-enhanced CT images, the diameters of hyperattenuated ablation areas in treated lungs were measured on axial images and on sagittal reconstructed images using a dedicated 3D program (Rapidia: INFINITT, Seoul, Korea). The Dax and Dtr values of hyperattenuated coagulation were measured in axial slices showing maximum ablation zone dimension, and Dv was measured in sagittal reconstruction images showing maximum ablation zone dimension. Using the Image J software (http://rsb.info.nih.gov/ij/) [29], the area of coagulation, on each CT image was calculated, and volumes were calculated by multiplying areas by slice thickness, summed to obtain total lesion volumes.

Statistical analysis
The dimensions of thermal ablation areas and the technical parameters such as accumulated RF energy of the three groups were averaged for each group and compared using one-way analysis of variance (ANOVA) test. In comparisons between groups, the Bonferroni multiple comparison test was applied. The volume of the hyperattenuated area seen at CT was correlated with the volume of central white area of the RF-induced coagulation at pathological specimen, and then the degree of correlation between them analysed using Pearson's correlation coefficient. Values are expressed as means±SD. For all statistical analyses, a p-value of <0.05 was considered significant. Statistics were performed using the Instat program (GraphPad Software, Inc., San Diego, CA).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Evaluation of RF methods
All pigs tolerated the RF ablation procedures well. In terms of the duration of energy application, the mean duration of RF energy delivery was 12 min in Group A, 20 min in Group B and 23±1.7 min in Group C (p<0.05). Impedance values decreased linearly during the procedure in group B. However, in Group C, impedance values gradually increased over the first 10 min and fluctuated between 150 {Omega} and 200 {Omega} over the final 10 min. In Group A, the impedance intermittently increased and activated the pulsed RF algorithm (Figure 2Go). In Group B, we were able to deliver 40 W and keep the impedance below 150 {Omega}. As a consequence, RF ablation using the perfusion electrode (Group B) allowed larger energy delivery than RF ablation using the internally cooled (Group A) or multitined expandable (Group C) electrodes, i.e. 33.6±4.7 kJ in Group A, 40.0±8.2 kJ in Group B and 23.5±6.1 kJ in Group C (p<0.05).


Figure 2
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  		Figure 2. Graphic depiction of the electrical parameters during radiofrequency ablation.(a) Tissue impedance (lower row), radiofrequency (RF) current (middle row), and power changes (upper row) during radiofrequency ablation using an internally cooled electrode. (b) Power and impedance changes (lower row), and tissue temperature changes (upper row) during radiofrequency ablation using the multitined expandable electrode.

 
CT and gross examination of the RF-induced coagulation area
On gross observation, the cut surface of gross specimens containing RF-induced coagulation showed that ablated tissue had a two layered structure presenting as a central, firm, dark-brown area surrounded by an outer, irregular margin (approximately 3–10 mm thick) of bright red tissue (Figure 3Go). When the specimens were fixed with 2% 2,3,5-TTC to determine if mitochondrial function was irreversibly damaged after RF ablation, the central dark-brown area did not show any staining with TTC (no enzyme activity), but the outer bright red zone showed inhomogeneous staining with TTC in red colour (admixture of stained and not-stained areas). The viable lung tissue outside of this area also showed homogeneous staining in red with TTC (positive enzyme activity). The lung tissue on CT, RF-ablated regions appeared as irregularly shaped hyperattenuated areas, which agreed with gross examination findings. In some cases, RF-induced hyperattenuated areas consisted of heterogeneous central zones containing air and surrounded by a broad hyperattenuated region (Figure 3Go).


Figure 3
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  		Figure 3. Contrast-enhanced CT scans and photographs of lung treated by radiofrequency (RF) ablation using an internally cooled electrode and a multitined expandable electrode in a pig model. (a) Contrast-enhanced axial CT scan obtained immediately after RF ablation showing an RF-induced hyperattenuated region (arrows) in both lungs. Note that the hyperattenuated region in the right lung treated with an internally cooled electrode is similar to that in left lung treated with a multitined expandable electrode. Photographs of gross specimen containing RF-induced coagulation areas created with (b) an internally cooled electrode and (c) a multitined expandable electrode. Ablated tissues appeared as a central, firm, dark-brown area (asterisk) surrounded by an outer, irregular margin (arrows) of bright red tissue. (d) Contrast-enhanced coronal reconstruction image obtained immediately after RF ablation using a perfusion electrode showing RF-induced hyperattenuated regions in the left lung (arrows). Note that the hyperattenuated region in the left lung treated with a perfusion electrode is larger than those produced using an internally cooled electrode (arrowheads) in the right lung. (e) Photographs of a gross specimen containing an RF-induced coagulation area created with a perfusion electrode. Note that the short- and long-axis diameters of the coagulation area were larger in Group B using a perfusion electrode than in the other Groups (a and c).

 
Evaluation of volumes of coagulation using gross specimens and CT images
The mean Dax's of the RF induced central white zone (unstained area with TTC) in axial planes measured in gross specimens of the three groups, were 1.7±0.4 cm in Group A, and 2.7±0.5 cm in Group B, and 1.9±0.5 cm in Group C (p = 0.018) (Table 1)Go. The corresponding mean Dtrs were 2.6±0.6 cm, 3.5±0.8 cm and 2.9±1.0 cm (p>0.05), and the mean Dvs of ablated regions were 3.1±0.5 cm, 3.6±0.5 cm and 2.9±1.0 cm (p>0.05) (Figure 3Go). Furthermore, the volumes of coagulation areas in the three groups were 7.2±4.1 cm3 in Group A, 16.9±5.5 cm3 in Group B and 7.5±3.3 cm3 in Group C. The differences were significant between Groups A and B (p = 0.002), and between Groups B and C (p = 0.018). The coefficients of variability for internally cooled, perfusion and multitined expandable electrodes were 0.55, 0.31, and 0.45, respectively (p>0.05).


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  		Table 1. Measured values of RF-induced coagulation necrosis in gross specimens in the three groups

 
Dax/Dt ratios of coagulated areas produced with each RF system were 0.7±0.1 in Group A, 0.78±0.2 in Group B and 0.74±0.4 in Group C. Although there was no significant difference in Dax/Dt ratios between the groups (p>0.05), the perfusion electrode created more irregularly bordered coagulations than the internally cooled or multitined expandable electrodes. On CT, the volumes of RF-induced hyperattenuated areas in the three groups were 9.2±4.5 cm3 in Group A, 20.4±6.3 cm3 in Group B and 10.2±4.0 cm3 in Group C, respectively. The volume of RF-induced hyperattenuated areas in Group B was larger than those of Groups A or C (p<0.05). The volumes of RF-induced hyperattenuated areas on CT were larger than the volumes of RF-induced coagulation (central unstained area of the gross specimen with TTC staining) measured on the gross specimens, but there was a significant correlation between them (p<0.05).

Histopathological results
Histological specimens showed central tissue loss at the electrode insertion tract and in the inner ablation zone; lung structures seemed to remain, but parenchymal cells showed eosinophilic cytoplasm with pyknotic nuclei and alveolar exudates, which correspond to the known early changes of coagulation necrosis [30]. The outer ablation zone showed haemorrhagic congestion, which was accompanied by neutrophil infiltration, and pulmonary alveoli were filled with exudates. In this zone, there was an admixture of abnormal cells with pyknotic nuclei and eosinophilic cytoplasm and normal looking parenchymal cells. Beyond this area, lung tissue showed normal histological findings, except acute inflammatory cell infiltrate.


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
The efficacy and safety of percutaneous RF ablation have been firmly established for the treatment of hepatic malignancies [9, 10]. However, this is not true in the lung [18, 19]. The most promising application for lung RF ablation is in the treatment and local control of primary NSCLC. Published studies [16–21] indicate that, although RF ablation has been offered to a heterogeneous cohort of lung cancer patients, initial results are encouraging. According to previous studies, significant differences are apparent in terms of the complete tumour necrosis rates of tumours smaller and larger than 3 cm in diameter [20, 21]. This limitation of lung RF ablation is primarily due to the fact that, when using a single electrode device in monopolar mode, it is limited by the precipitous drop in current density that occurs with distance from the energy source, which makes the periphery of the RF lesion particularly prone to vascular cooling [10, 30]. To circumvent these problems, strategies to increase the dimension of RF-induced ablation zones are needed, and several researchers have suggested that saline-enhanced RF ablation using a perfusion electrode can expand RF-induced coagulation [30–32].

In our study, RF ablation according to manufacturer's recommendations leads to larger volumes of coagulation using the perfusion electrode than using multitined or internally cooled electrodes. These results confirm those of previous studies [22, 23], i.e. that RF ablation using perfusion electrodes produces significantly larger coagulation areas than internally cooled or multitined electrodes, when RF ablation is performed according to manufacturer's recommendations. In our study, the high efficacy of the perfusion electrode in creating a large volume coagulation could be attributed to the low impedance kept during the ablation procedure, and high energy delivery during the RF ablation. This phenomenon could be explained by the presence of highly ionic saline around the electrode with continuous saline infusion during the RF procedure, which improves tissue conductivity [30, 31]. In addition, the larger volumes of coagulation obtained with this device are probably the result of the effects of heated fluid and its higher thermal conductivity [22].

The shape of a coagulated area is at least as important as the coagulation volume because the ablation of a tumour requires that the entire tumour, and a safety margin of grossly normal tissue, are encompassed by the ablation. Thus, the creation of large but complex asymmetric coagulation shapes does not reflect the effectiveness of a device. In clinical practice, coagulation shape is determined by the configuration of the RF electrode, the location of the tumour, tumour consistency, perfusion-mediated cooling effects and, in the case of open-perfusion devices, the saline distribution [26]. One potential disadvantage of RF ablation with saline infusion is the possibility of irregular zones of coagulation, due to uneven distribution of saline and a higher complication rate [32–35]. According to a previous study of RF ablation using a perfusion electrode for treating human pulmonary tumour by Kim et al [34], hypertonic saline-enhanced RF ablation was powerful and efficient in local ablation, but it was difficult to predict the exact extent of ablation. In our study, although there was no significant difference between the Dax/Dtr ratio of RF-induced coagulation areas for the three kinds of electrodes, the perfusion electrode created rather irregular bordered coagulations compared with the internally cooled or multitined expandable electrodes.

Given that prospective surgical data demonstrate 3 and 2.4 fold increases in local-regional recurrence rates for local wedge resection and segmental resection, respectively, compared with lobectomy, RF ablation alone for the treatment of primary lung cancer may not be validated [36]. However, for tumours under 2 cm in diameter (stage IA NSCLC), a recent study that compared limited resection (segmentectomy) with lymph node assessment versus lobectomy showed equivalent 5-year survival and local recurrence rates [37]. Recent studies on lung RF ablation [18–20] have led to the opinion that, for small tumours (<3 cm), RF ablation might provide a viable alternative to surgical resection for local disease control, especially in the non-surgical patient cohort. RF ablation can also be used in conjunction with other treatment modalities. In addition to the previously mentioned combination with XRT, there are ongoing studies on combined RF ablation and brachytherapy in patients with either metastatic lung malignancies or a history of prior treatment that precludes additional external beam radiotherapy [17]. The rationale involves the enhancement of local control by magnifying the cytoreductive and radiation effect by destroying the central hypoxic area of the target tumour. As demonstrated in this study, the larger volume of coagulation created by RF ablation using a perfusion electrode may increase the clinical utility of RF ablation therapy by allowing the successful treatment of larger lung tumours, or by reducing the number of sessions needed to treat a given tumour.

In our study, the measured volume of RF-induced hyperattenuated region on CT was greater than that of the RF-induced coagulation at gross specimen. Differences between the measured volumes of RF-induced hyperattenuation on CT and on gross specimens were attributed to the fact that the hyperattenuated area on CT corresponded to a region including both central and peripheral discoloured zones of ablation area on gross specimen; but on the gross specimens with TTC staining, the only central whitish area showed no enzyme activity (no staining).

Several limitations of this study must be addressed. First, because of the small sample size, interpretations are limited. Second, differences between the results obtained for the three RF devices are valid in healthy lungs, but not for lung tumours. Therefore, the extent to which our findings reflect the clinical situation is limited. Third, the duration of RF energy application was different between the groups. It would be fair to compare the same duration of energy application, because the volume of coagulation using internally cooled electrodes could be increased if RF energy is applied for more than 12 min. However, in our study, all RF ablation procedures were performed according to manufacturer's recommendations as in previous studies of porcine liver [25, 26]. Fourth, although there are modifications of expandable electrodes and internally cooled electrodes (cluster) available, we did not include those electrodes in the present study. Given that the developmental speed of RF technology is rapid, this study represents a snapshot in time because further refinements and improvements of current techniques will undoubtedly increase the effectiveness and further expand the role of RF ablation. In addition, we evaluated the geometry of RF-induced coagulation by the two-dimensional measurements. Three-dimensional virtual modelling of RF-induced coagulation in the gross specimens would be beneficial for the evaluation of coagulation geometry. However, given the explanted lung collapse, its value may not be as good as in the liver. Finally, large volumes of coagulation may not always be beneficial or desirable. In certain circumstances, coagulation extending beyond the tumour boundaries could be detrimental if surrounding structures are damaged or if insufficient tissue is preserved to permit normal organ function.

In summary, RF ablation according the manufacturer's recommendations leads to larger volumes of coagulation using perfusion electrodes. Based on our study results, we believe that large lung tumours could be treated by RF ablation using a perfusion electrode, with longer energy application, more effectively than with other electrode types.

Received for publication September 12, 2005. Revision received November 24, 2005. Accepted for publication January 19, 2006.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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