European Urology
European Urology 48 (2005) 584–592
Renal Cancer
Minimally InvasiveTreatment of Renal Cell Carcinoma: Comparison of 4 Different Monopolar Radiofrequency Devices Axel Ha¨ckera,*, Stefan Valloa, Christel Weissb, Rainer Grobholzc, Peter Alkena, Thomas Knolla, Maurice Stephan Michela a
Department of Urology, University Hospital Mannheim, Faculty of Clinical Medicine Mannheim, Ruprecht-Karls University of Heidelberg, Germany Department of Biomathematics, University Hospital Mannheim, Faculty of Clinical Medicine Mannheim, Ruprecht-Karls University of Heidelberg, Germany c Institute of Pathology, University Hospital Mannheim, Faculty of Clinical Medicine Mannheim, Ruprecht-Karls University of Heidelberg, Germany b
Accepted 14 June 2005 Available online 1 July 2005
Abstract Objectives: Radiofrequency Ablation is an investigational treatment option for RCC. The aim of our study was to test the ablation algorithms of four different RF systems in a standardized ex vivo perfused porcine kidney model. Materials and methods: A multitine monopolar dry electrode (impedance-based system), a multitine monopolar dry electrode (temperature-based system), a single monopolar wet electrode (impedance-based system) and a single monopolar dry, internally-cooled electrode (impedance-based system) were selected. RF energy was applied at different treatment parameters (power with and without control, tissue temperature, saline enhancement) for 1, 3, 5 and 9 minutes in healthy perfused ex vivo porcine tissue. Each treatment parameter was repeated 5 times. Maximum vertical, long-axis and short-axis diameters of the macroscopic lesion were measured and lesion volumes/ shapes were calculated. Results: Lesion volumes increased significantly with the pre-selected tissue temperature and saline enhancement. Saline enhancement created larger, but irregular shaped lesions. The impedance-based system created lesion volumes that were predictable by treatment time and generator power. Lesions were unpredictable when uncontrolled generator power was applied. The created lesion shape was dependent on the selected electrode configuration. Conclusions: The currently available monopolar RFA systems offer different specific technical features to control tissue ablation. Detailed knowledge of the specific characteristics of each RF system is necessary to provide a higher chance of successful clinical outcome by complete and reliable ablation. # 2005 Elsevier B.V. All rights reserved. Keywords: Radiofrequency (RF) ablation; Kidney; Renal cell carcinoma; Experimental study
1. Introduction Small renal masses are discovered incidentally at increasing rates through the widespread use of radiographic imaging modalities [1]. The natural history of * Corresponding author. Present address: Department of Urology, University Hospital Mannheim, Theodor-Kutzer-Ufer 1-3, 68135 Mannheim, Germany. Tel. +49 621 383 2629; Fax: +49 621 383 1923. E-mail address:
[email protected] (A. Ha¨cker).
these masses that are discovered at an early stage is often of slow growth and low metastatic risk [2,3]. Depending on the individual clinical situation, currently available treatment options include surgical excision by radical or partial open/laparoscopic nephrectomy, watchful waiting and investigational energy-based minimally-invasive ablation techniques. These techniques have the potential of avoiding open or laparoscopic surgical morbidity and better preser-
0302-2838/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.eururo.2005.06.010
A. Ha¨cker et al. / European Urology 48 (2005) 584–592
ving renal function. Examples of these techniques are Cryotherapy, Microwave Thermal Therapy, Interstitial Laser, High Intensity Focused Ultrasound (HIFU) and Radiofrequency Ablation (RFA). The mechanism of RFA is heat-based tissue destruction using alternating electrical current at very high frequency (>400.000 Hz) delivered through an electrode inserted into the tumor. Different electrode systems have been developed, including single probes such as elongated, internally cooled and salineenhanced electrodes, and multi probes such as array, bipolar, clustered cooled and expandable electrodes. Energy is applied under temperature or impedancebased monitoring. Although RFA has already been used in clinical practise, variables affecting the coagulative effect of RFA are incompletely evaluated. Animal studies and small clinical series have mainly demonstrated feasibility and safety [4–6]. Systematical investigations are rare with a large variability in the ablation protocols. The aim of our study was to investigate the ablation algorithms of four different commercially available monopolar RF systems under similar conditions in a standardized perfused kidney tissue model. 2. Materials and methods 2.1. Ex-vivo tissue model of perfused porcine kidneys The standardized model of the isolated perfused ex vivo porcine kidney was used and previously described in detail [7,8]. Kidneys were removed from pigs within 5 minutes after slaughtering and were immediately perfused with cold (4 8C) sodium chloride
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(0.9%) solution through the intubated renal artery (10 F catheter). Organs were stored at 4 8C when the effluent from the renal vein ran clear. During the trials the kidneys were continuously perfused with sodium chloride (0.9%) solution at 37 8C by a roller pump. The perfusion pressure was set to 110–130 cm H2O (measured by a water column) resulting in a perfusion rate of 60–100 ml/min. The kidney and the grounding pad were placed in a plastic basin filled with saline solution (0.9%) controlled at 37 8C with the groundling pad approximately 30 cm distant to the electrode [9]. 2.2. Radiofrequency devices, settings and protocols of delivery The RF electrodes were inserted and (in case of expandable electrodes) deployed in the center of the renal parenchyma perpendicular to the surface of the kidney. Each parameter setting was repeated 5 times. Treatment times were 1, 3, 5 and 9 minutes each in all cases. The deployment diameter of 2.0 cm for expandable electrodes and the electrode sizes for non expandable electrodes were selected on the basis of the anatomic characteristics of porcine kidneys and previous protocols developed for RFA of these kidneys [6,10]. The technical data of the four devices are presented in Table 1. 2.2.1. Multitine monopolar dry electrode - impedance-based system (Fig. 1a) Two different treatment regimes were performed: first, without impedance control, power output was set constant at 20, 40 or 60 W. Secondly, an impedance-based treatment algorithm was performed, which is described in details elsewhere [11]. Briefly, power output was initially set at 20 W and was increased automatically in 10 W increments until an uncontrolled impedance rise occurred (called ‘‘roll-off’’). Power was reapplied after 30 s with 50% of the power at which the impedance rose. The procedure was determined until the next uncontrolled impedance rise occurred. 2.2.2. Multitine monopolar dry electrode - temperature-based system (Fig. 1b) Energy was delivered until the average of four temperature thermocouples at the tips of the prongs of 708, 908, 1108 and 1208
Table 1 Technical characteristics of the RF systems Technical data
RF 3000a
RITA 1500b
HiTT 106c
COOL TIPd
Max. power output (W) Frequency (kHz) Monitoring of ablation Electrode type
200 480 Impedance Monopolar dry expandable 8-tine needle 2.5
150 460 Temperature Monopolar dry expandable 3-tine needle 2.2
60 375 Impedance Monopolar wet non-expandable single needle perfusion 1.6
250 480 Impedance Monopolar dry non-expandable single needle internally cooled
LeVeen Umbrella
Starburst Christmas tree
EZ 708-15 Straight
Cool-Tip Straight
NA 2.0
NA 2.0
1.5 NA
1.0 NA
Needle diameter (mm) Active electrode (Model) Configuration Tip Length (cm) Diameter (cm)
Note: NA = not applicable. a Boston Scientific, Natic, Mass., USA. b RITA Medical System, Mountain View, Calif., USA. c Integra, formerly Berchtold, Tuttlingen, Germany. d Radionics, Burlington, Mass., USA.
1.6
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Fig. 1. Electrode configuration of 4 different systems; (a): Expandable 8-tine monopolar dry electrode; (b) Expandable 3-tine monopolar dry electrode; (c) Single monopolar wet electrode; and (d) Single monopolar dry, internally cooled electrode.
was reached. This temperature level was maintained automatically for 1, 3, 5 and 9 minutes. 2.2.3. Single monopolar wet electrode - impedance-based system (Fig. 1c) At the distal part of the electrode, three groups of two sides holes were placed with an 1208 angle from each other. A digitally controlled syringe pump was connected to the needle to provide continuous basis flow of 20 ml/h of saline solution (0.9%) at room temperature through the needle sites holes into surrounding tissue during ablation. Additionally, the flow rate was controlled automatically (range 32 ml/h to 120 ml/h) on the basis of the measured tissue impedance during treatment. RF current was applied at power levels of 5, 15, and 25 W. Additionally, ablation was performed without saline infusion at identical treatment times and generator power settings. 2.2.4. Single monopolar dry, internally cooled electrode impedance-based system (Fig. 1d) A peristaltic pump cooled the electrode internally by providing cold (4 8C) saline solution (0.9%) in its cannula sheet. Power was delivered either continuously at power levels of 30, 50 and 70 W (‘manual-control mode’-without impedance control) or in the pulsed current mode (‘auto-control mode’, with impedance control). In the latter, the maximum generator power is delivered until the impedance rose 10 V above the baseline value. At this point, an algorithm interrupted generator output automatically in order to
avoid further impedance increase. It was switched on automatically, when the tissue was again able to receive a new impulse of energy. 2.3. Lesion size, volume, shape and histology Lesions were cut along the longitudinal plane (L-plane), passing through the axis of the electrode and then cut transversely (T-plane) into slices. Maximum lesion diameters (mm) were measured: the vertical diameter (DV) along the needle axis, the long-axis diameter (DL) perpendicular to it and the short-axis diameter (DS) in the Tplane. The lesion volumes (mm3) were calculated by assuming the volume (V) of a prolate ellipsoid (V = (4p/3) (DV DL DS)/ 8). The shape of the lesion (S) was assessed by the ratio of DV and the average of DL and DS (according to Pereira et al. [11]). A value of S = 1 corresponds to a spherical shape, a value of S > 1 to an oval shape with the longer diameter parallel to the needle axis (elliptical shape), and a value of S < 1 to an oval shape with the longer diameter perpendicular to the needle axis (flattened sphere). Representative tissue sections were stained with HE. 2.4. Statistical analysis Lesion volumes are presented as mean standard deviation. Differences in lesion volume were analysed using two-way analysis of variance (ANOVA) and Student t-test; one sample t-test was performed to compare if lesion shape is different to 1 (spherical shape). A value p < 0.05 was considered to be statistically significant.
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Fig. 3. Multitine monopolar dry electrode without impedance control.
Fig. 2. Histology of RF lesion (HE staining): Border between normal (bottom) and RF-treated (top) renal tissue.
3. Results 3.1. Lesion morphology (Fig. 2) The macroscopic appearance of the ablation area was yellow-white. Histology of representative HEstained specimens showed consistent findings without morphological differences between the different RF systems. Untreated tissue had a normal anatomical morphology. The border between treated and untreated renal parenchyma was macroscopically abrupt and no distinct transition zone was observed with either RF system. The treated area showed zones of cauterisation with small substantial defects, tissue tears and cellular necrosis. The macroscopic visible RF zone corresponded to the microscopic ablation zone, as also shown by others [12,13]. 3.2. Lesion sizes and volume Maximum lesion diameters (vertical, long-axis and short-axis diameter) are presented in detail in Table 2. The lesion sizes produced with the perfused single
Fig. 4. Multitine monopolar dry electrode - temperature-based system.
monopolar wet electrode could only be approximated due to irregular tongue-shaped extensions of the coagulation zone. Lesion volumes are presented in Figs. 3–6. For comparison, a renal mass with a maximum measured diameter of 0.5 cm on CT or MR scans has a spherical volume of 65.47 mm3, a 10 mm mass of 523.75 mm3, a 15 mm mass of 1767.65 mm3 and a 20 mm mass has a spherical volume of 4190 mm3. 3.2.1. Multitine monopolar dry electrode without impedance control (Fig. 3) At the lowest energy setting (power 20 W, 1 min. treatment time), coagulated tissue was only seen at the tip of the electrodes. It was not possible to induce
Fig. 5. Single monopolar wet electrode - impedance-based system; (a) Perfused electrode; (b) Unperfused electrode.
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Table 2 Lesion sizes with maximum vertical, long-axis and short-axis diameter Device
Vertical diameter (mm) 3
5
9
1
3
Short-axis diameter (mm)
5
9
1
3
5
9
Multitine monopolar dry electrode
20 W 0 11.2 3.0 14.3 2.2 13.2 2.9 40 W 11.4 3.7 15.6 2.8 16.0 1.0 14.7 1.7 60 W 13.7 2.4 14.5 1.87 16.1 1.8 14.7 1.7 Impedance mode 19.2 2.3
Multitine monopolar dry electrode temperature-based
70 8C 90 8C 110 8C 120 8C
8.5 2.9 8.8 1.4 10.6 2.2 13.8 1.1 14.8 2.5 15.1 2.5 17.5 2.0 18.3 1.6
10.6 4.0 14.4 2.5 18.7 0.8 19.4 2.8
Single monopolar wet electrode-unperfused
5W 15 W 25 W
11.2 0.7 11.5 1.6 10.8 2.0 11.2 2.5 10.8 0.6 11.0 2.4
9.5 2.5 12.8 3.3 10.5 0.5 12.6 2.2 10.6 1.4 11.6 2.6
Single monopolar wet electrode-perfuseda
5W 15 W 25 W
9.7 2.2 12.1 2.6 13.4 2.0 14.4 2.1 13.1 1.7 16.7 2.9
14.0 2.0 11.3 2.0 15.7 3.0 13.8 2.7 15.6 2.3 14.1 3.6
7.0 1.4 9.3 1.6 10.3 0.7 10.3 2.0 8.00 1.6 7.7 1.7 10.8 1.1 10.3 2.5 9.3 1.8 13.5 3.1 15.6 4.1 13.5 3.3 8.5 1.9 10.6 2.5 12.7 2.4 12.6 5.3 10.0 2.1 13.7 2.1 13.6 3.3 15.0 3.3 9.6 1.4 13.2 2.0 12.7 3.1 13.0 4.5
13.2 0.8 12.2 1.6 13.4 1.5 11.0 1.9
14.0 2.5 14.0 1.2 14.6 2.6 12.8 1.2
11.3 1.1 10.3 1.0 9.8 2.7 9.5 1.5
Single monopolar dry internally cooled electrode
a
Lesion sizes approximated.
30 W 50 W 70 W Impedance mode
10.8 0.9 12.0 1.1 11.8 1.3 12.8 2.0
0 23.0 1.5 23.6 3.5 23.8 3.1 20.0 1.4 20.0 1.4 20.6 1.3 22.4 3.4 20.6 3.2 23.3 2.7 24.8 3.4 23.5 1.7 18.6 1.8 20.1 1.4 21.2 1.5 20.7 1.4 20.5 1.2 21.7 2.2 23.2 1.7 23.5 1.7 18.8 2.1 19.3 1.8 20.2 2.7 20.7 1.4 25.8 2.5 21.6 1.7
11.8 2.0 9.0 3.0 15.0 2.74 14.1 1.4 17.0 2.3 18.8 2.7 19.0 2.3 19.0 1.4
13.6 2.3 13.4 2.0 11.2 0.7 13.9 1.8
5.2 0.7 5.2 0.9 5.4 0.9
10.2 2.8 15.5 2.6 20.1 2.2 21.5 1.7
13.8 2.3 17.5 3.2 22.0 1.6 23.0 1.7
6.0 1.0 5.7 1.1 5.1 1.3
5.0 1.4 5.3 0.8 6.5 1.0
11.2 0.7 11.4 1.3 12.2 1.4 12.0 1.8
14.4 1.6 12.6 1.8 12.0 1.6 13.7 1.7
12.2 2.0 8.3 3.4 8.7 1.9 16.0 1.8 12.1 2.6 14.8 2.0 20.4 2.0 17.8 2.7 17.8 3.8 22.4 1.1 18.0 2.1 19.1 2.6 8.3 1.8 8.0 1.8 7.0 0.6
4.7 0.9 4.5 1.1 4.5 0.9
5.0 1.7 5.2 1.3 5.2 1.3
13.6 1.8 12.2 1.3 10.2 0.9 12.4 1.5 10.1 2.1 10.4 0.8 12.6 1.3 9.5 2.4 10.0 0.8 16.2 3.2 8.8 1.6 10.9 2.5
10.8 1.3 17.3 3.9 18.2 3.1 20.5 2.4
13.4 2.7 17.6 0.5 19.0 1.4 20.0 1.4
4.6 1.3 5.1 1.3 5.9 1.4
7.3 1.3 7.6 1.3 6.3 1.6
12.4 2.8 10.8 1.1 10.4 1.3 12.8 1.9
13.6 2.3 11.2 1.4 10.8 1.3 15.0 2.3
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Long-axis diameter (mm) Treatment time (min)
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consistent and homogeneous lesions, indicating a threshold dosis. Longer treatment times created larger lesions: lesion volumes increased significantly with treatment time from 1 to 9 minutes at power levels of 20, 40 and 60 W (p < 0.0001). Generator power significantly (p = 0.0193) influenced lesion volume at identical treatment times (1–9 minutes): A power level of 40 W turned out to be optimal for lesion control. Lesions produced at 20 W were significantly smaller compared to 40 W (p = 0.0058). Lesions produced at 60 W power level showed effects of carbonization near the needle electrodes indicating a too high power level. They were smaller than those produced at 40 W. Using the automatical treatment algorithm, mean treatment time was 13.44 4.99 minutes (range 6– 21 minutes) and the mean treatment volume was 5648.47 1101.91 mm3. 3.2.2. Multitine monopolar dry electrode temperature-based system (Fig. 4) Lesion volume increased significantly (p < 0.001) with treatment time from 1 to 5 minutes at all selected tissue temperatures (708–1208C). After 5 minutes treatment time, no further statistical significant (p = 0.449) increase of lesion volume was observed, independent of the selected tissue temperature, indicating a plateau. Higher tissue temperatures created larger lesions: At identical treatment times (1–9 minutes), lesion volumes increased significantly (p < 0.001) with higher tissue temperatures (708–1208 C).
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utes; an increase was seen at 9 minutes, with statistical significance to 1, 3 and 5 minutes (p < 0.0001). After 9 minutes treatment time, lesion produced with lower power (5/15 W) were larger than those produced with a higher power level (25 W) and had a large range of the volume. 3.2.3.3. Comparative studies. Using saline enhanced perfused electrodes, lesion volumes were at least 2 times larger than those produced with unperfused electrodes at all identical treatment times and power levels. This difference was statistically significant (p < 0.0001; 3-way ANOVA). 3.2.4. Single monopolar dry, internally cooled electrode - impedance based system 3.2.4.1. Manual mode – without impedance control (Fig.6a). Lesions produced with the lowest power level of 30 W were statistically significant (p = 0.0174) larger (exception: 3 minutes) than those produced with higher power levels of 50 W and 70 W. These lesions had similar volumes without statistical differences. Lesion volumes were also influenced by treatment time: At longer treatment times, larger lesion volumes were created at all selected power levels (30, 50 and 70 W; p = 0.0313 for 1 versus 9 minutes). This increase varied during treatment time from 1 to 9 minutes. 3.2.4.2. Auto mode – with impedance control (Fig. 6b). Lesion volumes ranged from 488.29 161.39 (1 minutes) to 1802.33 617.21 mm3 (9 minutes). Longer treatment times created larger lesions: lesion volume increased significantly with treatment time from 1 to 3, 5 and 9 minutes (p < 0.0001).
3.2.3. Single monopolar wet electrode - impedancebased system 3.2.3.1. Perfused electrode (Fig. 5a). Higher power levels (15 W, 25 W), created statistically significant (p < 0.0001) larger lesions than a lower power level of 5 W at identical treatment times (1–9 minutes). Power levels above 15 W did not further increase lesion volume (no statistically significant differences between 15 and 25 W at all treatment times; p = 0.5219). At identical power levels (5/15/25 W), lesion volumes increased significantly with treatment time (p = 0.005 for 1 versus 3 min); longer treatment times (5 and 9 minutes) did not further increase lesion volume. A large range of lesion volume was observed at longer treatment times (5 and 9 minutes).
3.2.4.3. Comparative studies. Lesion volumes at 1 minutes were statistically significant (p = 0.0014) smaller in the impedance-based mode than without impedance-based ablation (30/50/70 W). After 3 and 5 minutes, no differences in lesion volume between the modes were seen. After 9 minutes, lesion volume was significantly larger in the impedance-based mode (p = 0.001). Using the impedance-based mode, lesion volume increased significantly and linear with longer treatment times. This is in contrast to the variable increase without impedance control.
3.2.3.2. Unperfused electrode (Fig. 5b). Generator power (5/15/25 W) did not influence lesion volume: no statistically significant (p = 0.8087) differences were seen at identical treatment times (1–9 minutes). At identical power levels (5/15/25 W) lesion volume was not influenced by treatment time from 1 to 5 min-
3.3. Lesion shape All lesions of the 8-tine monopolar dry electrode (with and without impedance control) were shaped like a flattened sphere (p < 0.01). All lesions were spherical when created by the 3-tine monopolar dry electrode and the single monopolar dry internally cooled
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Fig. 6. Single monopolar dry, internally cooled electrode - impedance-based system; (a) Manual mode - without impedance control; and (b) Auto mode – with impedance control.
electrode (manual and auto mode). An elliptic shape was produced for all lesions by the unperfused single monopolar wet electrode (p < 0.008) and by the perfused single monopolar wet electrode (for all lesion produced at 1 to 5 min; p < 0.007). Using this electrode, a switch of shape to spherical was observed when treatment time was exceeded to 9 minutes (5, 15 and 25 W). However, the calculated lesion shape produced with the perfused electrode could only be approximated due to irregular tongue-shaped extensions of the coagulation zone (see also lesion size above).
4. Discussion The majority of clinical experience with RFA is derived from treatment of liver tumors. Treatment protocols for renal tumors have been mainly extrapolated from these experiences [14,15]. However, the kidney is a unique and complex organ with different hemodynamic, morphological and electrical characteristics. The impact of these and other yet undefined features must be evaluated before direct extrapolation from liver to renal tumors is made. During RFA, high-frequency alternating current is transferred into the tissue. The current creates molecular friction resulting in local production of heat, denaturation of proteins and cell membrane disintegration. Our histological findings are in accordance with reports from in vivo porcine kidney studies [10,16,17]. Heat distribution around the probe is a function of tissue impedance, native tissue temperature, thermal conductivity and heat loss through the blood flow. Different technical systems have been developed to monitor the ablation process. Our results show that lesion volume can be unpredictable
without monitoring the ablation processing. Uncontrolled application of power can slow down lesion development resulting in smaller lesions at higher power levels. Temperature-based systems determine completion of treatment when the tissue temperature surrounding the probe have been held for a set amount of time. Our results with this system demonstrate, that lesion volume increases with the pre-selected tissue temperature. A disadvantage of the system this that the temperature recorded at the tip of the RF tine may not accurately reflect the tissue temperature distant from the tines [10]. Impedance-based systems determine completion of treatment when the tissues surrounding the probe create essentially infinite impedance and when electric current is unable to further flow through the tissue. Optimum power levels are selected automatically to create the maximum possible lesions. Precise (treatment time dependent) lesion development is possible (Radionics system). However, the impedance control can increase treatment time (Boston Scientific system). A limitation of the impedance-based systems is, that treated areas may not reach the required temperatures for coagulative necrosis, as the temperature is not directly measured. Another drawback is that impedance can increase prematurely due to bubble formation and tissue charring. Carbonization of tissue around the needle tip causes a significant increase of impedance, the lesion volume cannot further increase at longer treatment times. A strategy to minimize carbonization is to perfuse the needle internally. Goldberg et al. [18] have shown that tissue cooling throughout perfused electrodes decreases tissue impedance and improves tolerance of increased generator output with subsequent higher deposit of energy and larger lesions [19].
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Another strategy to increase lesion size is to infuse liquid into the tissue. Our results show, that saline enhanced RF ablation increases lesion volumes. The infused electrolyte fluid improves electric and thermal conductivity. Restrictive heating/impedance rise near the metal electrode can be reduced allowing greater energy input [20]. Uncontrolled diffusion and spread of heat through the boiling fluid may cause market distortions and irregularities in the lesion shape. Along these lines, in vivo studies have shown, that leakage of saline can result in spread of current to surrounding structures with thermal injury of adjacent organs [11,21,22]. In accordance with these findings, we also observed coagulated tissue running away from the RF lesion along vessels or the renal surface into untreated tissue. However, this process is mainly influenced by the amount of saline and the injection velocity [23]. Further investigations in this area are necessary to find out the optimum velocity and amount of injection to ensure reproducible lesion shapes. Additional to the lesion volume, the likely shape of the thermal lesion is of importance when planning the ablation procedure. The lesion shape of a tumor can be calculated on CT/MR imaging. Successful treatment requires that the entire tumor and a margin of normal tissue is ablated. Our results confirm, that lesion shape is highly dependent on the electrode configuration. These findings may help to decide, which electrode should be selected to ablate a tumor of a known shape. Some limitations of this study have to be mentioned. This study evaluated RFA of healthy renal parenchyma and not of human renal tumors. It is at present unclear, whether ex vivo measurements predict exactly coagulation in human tumors [21]. Unfortunately, a reliable in vivo renal tumor model is missing.
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Although being performed by others [11], direct comparison of different systems and electrodes with concluding the superiority of one system over another (e.g. in terms of lesion size/volume) is in our opinion not helpful. Only comparison of identical electrode configurations and varying energy setting and/or treatment time can introduce study bias. To date, only few data are available on basic performance of commercial electrodes for RFA on renal masses, although used increasingly in clinical application. In our view, it is worrying to expose patients to treatments with new RF devices and electrodes in the absence of valid experimental data on the volume and geometry of the lesions. Detailed knowledge of the advantages and disadvantages of different RF systems should be available before starting routine application.
5. Conclusions The currently available monopolar RFA systems offer different specific technical features to control tissue ablation. Lesion size, volume and shape is dependent on the selected RF system, procedure time and electrode configuration. Detailed knowledge of the specific characteristics of each RF system should provide a higher chance of successful clinical outcome by complete and reliable ablation.
Acknowledgment This project was supported by a research grant of the Tumorzentrum Heidelberg/Mannheim. The authors thank the companies Boston Scientific, RITA Medical Systems, Integra and Radionics for providing the RF devices as well as their technical support.
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