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10.1245/s10434-006-9033-9
Annals of Surgical Oncology 14:1381-1396 (2007)
© 2007 Society of Surgical Oncology
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Original Article

Experimental and Clinical Radiofrequency Ablation: Proposal for Standardized Description of Coagulation Size and Geometry

Stefaan Mulier, MD1, Yicheng Ni, MD, PhD1, Lars Frich, MD2, Fernando Burdio, MD3, Alban L. Denys, MD4, Jean-François De Wispelaere, MD5, Benoît Dupas, MD, PhD6, Nagy Habib, MD, FRCS7, Michael Hoey, PhD8, Maarten C. Jansen, MD9, Marc Lacrosse, MD5, Raymond Leveillee, MD10, Yi Miao, MD, PhD11, Peter Mulier12, Didier Mutter, MD, PhD13, Kelvin K. Ng, MBBS, PhD, FRCSEd (Gen)14, Roberto Santambrogio, MD15, Dirk Stippel, MD16, Katsuyoshi Tamaki, MD17, Thomas M. van Gulik, MD9, Guy Marchal, MD, PhD1 and Luc Michel, MD, FACS18

1 Department of Radiology, Gasthuisberg University Hospital, Herestraat 49, 3000 Leuven, Belgium
2 Department of Surgery, Rikshospitalet University Hospital, Sognsvannsveien 20, 0027 Oslo, Norway
3 Unity of Liver Surgery and Liver Transplantation, Bellvitge University Hospital, Feixa Llarga, sn, 08907 Hospitalet de Llobregat, Barcelona, Spain
4 Department of Diagnostic and Interventional Radiology, Centre Hospitalier Universitaire Vaudois, Rue du Bugnon 46, 1011 Lausanne, Switzerland
5 Department of Radiology, Mont-Godinne University Hospital, Avenue du Dr. Thérasse 1, 5530 Yvoir, Belgium
6 Central Service of Radiology and Medical Imaging, Hôtel Dieu, University Hospital of Nantes, Place Alexis Ricordeau 1, 44093 Nantes Cedex 01, France
7 Department of Liver and Pancreas Surgery, Imperial College London, Hammersmith Hospital, Du Cane Road, W12 0NN London, UK
8 Biomedical Engineer, 5733 Pond Drive, Shoreview, Minnesota 55126, USA
9 Department of Surgery, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands
10 Division of Endourology, Laparoscopy and Minimally Invasive Surgery, University of Miami School of Medicine, Professional Arts Center, 1150 NW 14th Street, Suite 309, Miami, Florida 33101, USA
11 Department of General Surgery, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China
12 Biomedical Engineer, 8140 Lake Elmo Avenue, Stillwater, Minnesota 55082, USA
13 Department of General Surgery, University Hospital of Strasbourg, Place de l’Hôpital, 67091 Strasbourg Cedex, France
14 Department of Surgery, University of Hong Kong, Queen Mary Hospital, 102 Pofulam Road, Hong Kong 852, China
15 Unit of Bilio-pancreatic Surgery, Ospedale San Paolo, University of Milan, via A. di Rudinì 8, 20142 Milan, Italy
16 Department of Visceral and Vascular Surgery, University of Cologne, Joseph Stelzmann Strasse 9, 50924 Cologne, Germany
17 Department of Digestive and Cardiovascular Medicine, Tokushima University School of Medicine, (3-18-15) Kuramoto-cho, Tokushima 770-8503, Japan
18 Department of Surgery, Mont-Godinne University Hospital, Avenue du Dr. Thérasse 1, 5530 Yvoir, Belgium

Correspondence: Address correspondence and reprint requests to: Luc Michel, MD, FACS; E-mail: michel{at}chir.ucl.ac.be


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Background: Radiofrequency (RF) ablation is used to obtain local control of unresectable tumors in liver, kidney, prostate, and other organs. Accurate data on expected size and geometry of coagulation zones are essential for physicians to prevent collateral damage and local tumor recurrence. The aim of this study was to develop a standardized terminology to describe the size and geometry of these zones for experimental and clinical RF.

Methods: In a first step, the essential geometric parameters to accurately describe the coagulation zones and the spatial relationship between the coagulation zones and the electrodes were defined. In a second step, standard terms were assigned to each parameter.

Results: The proposed terms for single-electrode RF ablation include axial diameter, front margin, coagulation center, maximal and minimal radius, maximal and minimal transverse diameter, ellipticity index, and regularity index. In addition a subjective description of the general shape and regularity is recommended.

Conclusions: Adoption of the proposed standardized description method may help to fill in the many gaps in our current knowledge of the size and geometry of RF coagulation zones.

Key Words: Radiofrequency ablation • Tumor ablation • Size • Geometry • Liver • Kidney


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Radiofrequency ablation (RF ablation) is a valuable technique to obtain local control of unresectable tumors in liver,1–7 kidney,8 prostate,9 and other organs. An important clinical limitation is the inability to monitor the growing coagulation zone accurately during the procedure. Real-time monitoring of the area of coagulation with ultrasound is unreliable.10–13 A too small coagulation zone will inevitably lead to local recurrence.2 Although local recurrence is lower than 10% in the best series,1,2,4–7 in other series it can be as high as 60%.14,15 On the other hand, a too large coagulation zone may lead to collateral damage.3,16–18 Therefore, exact knowledge of the expected size and geometry of coagulation zones is essential to correctly prepare and perform the intervention. A recent review showed that much of this crucial information is lacking for the current commercial RF ablation electrodes.19 The aim of this study was to develop and propose an optimal set of descriptive parameters for different RF ablation electrodes and protocols. The second aim was to standardize description terminology.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
For experimental RF ablation, the optimal set of descriptive parameters for coagulation zones created by single and dual electrodes20 was developed. This set was defined as the minimal information needed to accurately document the coagulation zone size and geometry. We adopted the principle that not only the coagulation zone but also the spatial relationship of the coagulation zone with the electrode(s) should be described.

In a second step, a standard term was assigned to each parameter. This term had to match predifined quality criteria and was either chosen from the literature, if available, or newly created. Therefore, we carried out a PubMed search for the period from January 1, 1990, to May 1, 2005, using the key words radiofrequency (or radio-frequency or radio frequency) and liver (or hepatic or hepatocellular) on articles written in English, French, German, Italian, Spanish, Danish, or Dutch. Relevant papers were also identified from the reference lists of the papers previously obtained through the search. Only papers with a main aim to describ the in vivo and ex vivo coagulation zones after a single RF ablation session in animal liver with commerciel or experimental electrodes were retained. Publications using both single electrodes and multiple-electrode systems20 were included. For each parameter, numerous synonyms were found in the literature. In order to distill the most unequivocal term for each parameter, we first rejected synonyms that suggested a ranking of size, such as "long(est) axis" and "short(est) axis", because the longest axis does not necessarily correspond to the axial diameter; neither does the shortest axis always correspond to the transverse diameter.19,21 We then rejected synonyms that suggested a position in space, such as "vertical diameter", because an RF ablation electrode can be inserted in any direction in the laboratory as well as in a patient. Thirdly, from the remaining terms, the most "expressive" and "intuitively suggestive" term was selected. Finally, some lacking terms had to be newly created.

For clinical RF ablation, applicability of these experiment-derived definitions to the clinical setting was studied.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Current Unstandardized Descriptive Parameters in the RF Ablation Literature
Up to 12 synonyms were identified for describing the same parameter of the coagulation zone. For other valuable parameters, no terms at all were available (Tables 1Go–5GoGoGoGo).


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  		TABLE 1. Description of experimental coagulation zones made by a single electrode
 

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  		TABLE 2. Description of experimental coagulation zones made by a dual-electrode system
 

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  		TABLE 3. Description of clinical coagulation zones made by a single electrode
 

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  		TABLE 4. Frequency of use of proposed descriptive parameters in experimental RF ablation literature, single electrode
 

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  		TABLE 5. Frequency of use of proposed descriptive parameters in experimental RF ablation literature, dual-electrode system
 
Proposal for Standardized Description
Experimental RF Ablation: Minimal Descriptive Parameters
In order to obtain accurate measurements that can be related to the position of the electrode, the electrode should be left in place until the liver is sectioned (Table 1Go). The tines of expandable electrodes are withdrawn into the shaft. With the knife shaving the electrode, the liver is first cut along an axial plane, which is defined as a plane along the electrode axis (Fig. 1Go). Measurements are performed and pictures are taken. All measurements include the central tan-white zone, which corresponds to irreversibly damaged tissue, and exclude the surrounding hypeaemic red rim (for in vivo experiments), which corresponds to viable tissue on acute histochemical staining.22 Coagulation zones that extend to the liver surface should not be taken into account for measurement since the obtained measurements represent an underestimation of the size the coagulation zone would reach in the middle of the parenchyma. Measurements can be performed macroscopically using calipers or on digital photographs using public domain software packages such as ImageJ (National Institutes of Health, Bethesda, MD; http://www.rsb.info.nih.gov/ij/ [accessed May 16, 2006]) or commercially available image analysis software. Measurements should be performed before histological tissue fixation with associated tissue shrinkage.


Figure 1
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  		FIG. 1. Preparation technique and definition of planes for single-electrode RF ablation.

 
Second, both halves of the coagulation zone are cut in the transverse plane, which is defined as the plane perpendicular to the electrode (and to the axial plane) at the site of the largest transverse diameter of the coagulation zone. The two quarters nearest to the electrode tip are then reassembled; measurements are performed and pictures are taken.

Measurements in the Axial Plane (Fig. 2AGo)
The axial diameter (AD) is defined as the distance in mm between the proximal and the distal edges of the coagulation zone, in the axis of the electrode.


Figure 2
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  		FIG. 2. Description of coagulation zones made by a single electrode.

 
The front margin (FM) is defined as the distance in mm between the distal edge of the coagulation zone and the electrode tip. For expandable electrodes, the tines are not taken into account.

The coagulation center (CC) is defined as the section point of the transverse plane and the electrode (axis). Its position is expressed as the distance in mm of the CC to the electrode tip. In other words, its position is measured as the distance between the projection of the site of the maximal transverse diameter on the electrode (axis) and the electrode tip. For expandable electrodes, the tines are not taken into account. The value is positive when the CC is distal to the electrode tip, i.e., into the tissue, and negative when it is proximal to the electrode tip, i.e., on the electrode itself.

Measurements in the Transverse Plane (Fig. 2BGo)
The maximal radius (Rmax) is defined as the maximal distance in mm between the electrode shaft and the edge of the coagulation zone in the transverse plane.

The minimal radius (Rmin) is defined as the minimal distance in mm between the electrode shaft and the edge of the coagulation zone in the transverse plane. Maximal and minimal radiuses are not necessarily perpendicular to each other.

The maximal transverse diameter (TDmax) is defined as the maximal distance in mm between two opposite edges of the coagulation zone in the transverse plane.

The minimal transverse diameter (TDmin) is defined as the minimal distance in mm between two opposite edges of the coagulation zone in the transverse plane, measured on a line crossing halfway the line of the maximal transverse diameter.

Both transverse diameters cross at the center of the coagulation zone. This center does not necessarily correspond to the site of the electrode shaft. The maximal and minimal transverse diameters are not necessarily perpendicular to each other.

General Shape in the Axial Plane (Fig. 2CGo)
The ellipticity index (EI) quantitatively describes the general coagulation zone shape in the axial plane and is calculated as the ratio of axial diameter (AD) and mean transverse diameter [(TDmin + TDmax)/2]:


Formula

Provided that TDmin is close to TDmax, a ratio of 1.0 roughly corresponds to a spherical coagulation zone; a ratio > 1.0, to an elliptical coagulation zone; and a ratio < 1.0, to a flattened sphere.

Regularity of Shape in the Transverse Plane (Fig. 2DGo)
The regularity index (RI) quantitatively describes the regularity of the coagulation zone shape in the transverse plane and is calculated as the ratio of Rmin and Rmax:


Formula

This way, a ratio close to 1.0 corresponds to a nearly spherical coagulation zone. The lower the ratio, the more irregular the coagulation zone (asymmetrical, with indentations, or with extensions). For a ratio inferior to 0.80, the (most frequently found) type(s) of irregularity should be specified in the subjective description (see further).

Dual-Electrode System (Table 2Go)
A dual-electrode system is defined as the combined use of two single electrodes inserted into the target tissue, either monopolar or bipolar.20 Only electrodes inserted in parallel are considered here.

Preparation Technique and Definition of Planes (Fig. 3Go)
The preparation technique is similar as described above for single electrodes, except for the following steps. The coagulation zone is first cut along the axial plane, which is defined as the plane along both electrode axes. Measurements are performed and pictures are taken. Then, both halves of the coagulation zone are cut in the transverse plane, which is defined as the plane perpendicular to the electrode axes at the site of the largest transverse diameter of the coagulation zone in the axial plane. The two quarters nearest to the electrode tip are then reassembled; measurements are performed and pictures are taken.


Figure 3
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  		FIG. 3. Preparation technique and definition of planes for dual-electrode RF ablation.

 
Measurements in the Axial Plane (Fig. 4AGo)
The axial diameter (AD1, AD2) is defined as the distance in mm between the proximal and the distal edges of the coagulation zone, and in the axis of one electrode. It is measured for both electrodes.


Figure 4
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  		FIG. 4. Description of coagulation zones made by a dual-electrode system.

 
The front margin (FM1 and FM2) is defined as the distance in mm between the distal edge of the coagulation zone and the electrode tips. For expandable electrodes, the tines are not taken into account. It is measured for both electrodes.

The mid-axial diameter (MAD) is defined as the distance in mm between the proximal and the distal edges of the coagulation zone, in the axis halfway and parallel to both electrodes.

Measurements in the Transverse Plane (Fig. 4BGo)
The inline transverse diameter (ITD) is defined as the distance in mm between the edges of the coagulation zone in the interelectrode axis in the transverse plane.

The lateral margin (LM1 and LM2) is defined as the distance in mm between the edges of the coagulation zone and the electrode in the interelectrode axis in the transverse plane. It is measured for both electrodes.

The perpendicular transverse diameter (PTD1 and PTD2) is defined as the diameter of the coagulation zone perpendicular to the interelectrode axis where it crosses an electrode in the transverse plane. It is measured for both electrodes.

The mid-transverse diameter (MTD) is defined as the diameter of the coagulation zone perpendicular to the interelectrode axis halfway between both electrodes in the transverse plane.

General Shape in the Axial Plane (Fig. 4CGo)
The axial fusion index (AFI) quantitatively describes the completeness of fusion of the coagulation zones around each electrode in the axial plane and is calculated as the ratio of the mid-axial diameter (MAD) and the mean AD [(AD1 + AD2)/2]:


Formula

This way, a ratio ≥ 1.0 corresponds to complete fusion and a ratio < 1.0 to incomplete fusion.

General Shape in the Transverse Plane (Fig. 4DGo)
The transverse fusion index (TFI) quantitatively describes the completeness of fusion of the coagulation zones around each electrode in the transverse plane and is calculated as the ratio of the mid-transverse diameter (MTD) and the mean: PTD [(PTD1 + PTD2)/2]


Formula

This way, a ratio ≥ 1.0 corresponds to complete fusion and a ratio < 1.0, to incomplete fusion.

Multiple (>2) Electrode System
A multiple (>2) electrode system is defined as the combined use of more than two single electrodes inserted into the target tissue.20 As these systems can be used in a very versatile way, the description should be individualized to each system, bearing in mind the goal to offer to the clinician accurate and clinically useful descriptive parameters, which should always be related to the position of the electrodes. For example, a coagulation zone created by three symmetrically inserted electrodes close to each other can be described as if it was created by a single electrode. A coagulation zone created by three symmetrically inserted electrodes further from each other can be described using elements of the proposed description for a dual-electrode system. However, a coagulation zone created by four or more electrodes, or asymmetrically placed electrodes, requires an individualized description.

Variability
Each parameter should be presented as mean value ± standard deviation (SD). In papers that compare different electrodes or protocols, the Coefficient of Variation (CV) can be added: CV = SD/mean (expressed in %).66

Subjective Description
In addition to the objective descriptive parameters, a subjective description of shape (e.g., spherical, conical, mushroom-shaped, teardrop-shaped, etc.) and regularity (e.g., regular, with irregular spiky extensions, etc.) is recommended (Fig. 5Go).


Figure 5
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  		FIG. 5. Subjective description of coagulation zones: some examples.

 
Pictures
The inclusion of pictures of the coagulation zone is recommended, both in the axial and in the transverse plane. Pictures shown should be representative of the various sizes and shapes that were obtained with the same electrode and the same protocol. All pictures should be at the same scale and should include a centimeter rule. Three-dimensional imaging reconstructions35,67 of the coagulation zone are optional but highly illustrative.

Clinical RF Ablation
Measurement Technique
In order to obtain accurate measurements that can be related to the position of the electrode in the clinical setting, a prospective registration of the position of the electrode axis and the electrode tip during the procedure is necessary. The inclination of the electrode axis versus the three planes (transverse, sagittal, and coronal) of the patient should be recorded. The position of the electrode tip should be described as accurately as possible in relation to the tumor border and other anatomical landmarks. This information can be completed with imaging registration during the procedure, with the electrode in place.

After the RF ablation, the coagulation zone is defined as the area without contrast uptake on contrast-enhanced computed tomography (CT) or magnetic resonance imaging (MRI). The hyperaemic rim is not taken into account.

In a first step, measurements are performed in the axial plane, which is defined as a plane along the electrode axis. For CT, this, often oblique, plane has to be reconstructed postimaging using multiplanar reconstruction; a technique which requires a dedicated acquisition protocol with overlapping thin (millimetric) slices, as commonly achievable with recent multidetector helical CTs. The prerecorded inclination values of the electrode axis versus the three patient planes are used for this reconstruction. Measurements of the coagulation zone are performed digitally, and imaging in the axial plane is recorded. With MRI, the plane along the electrode axis can be obtained either through direct oblique acquisition or through secondary multiplanar reformations from transverse thin slices.

Second, measurements are performed in the transverse plane, which is defined as the plane perpendicular to the electrode (and to the axial plane) at the site of the largest transverse diameter of the coagulation zone. Measurements of the coagulation zone are performed digitally, and imaging in the axial plane is recorded.

Measurements (Table 3Go)
Definitions of geometric dimensions are the same for the experimental and the clinical settings. However, as imaging without the electrode in place is inevitably less accurate than sectioning with the electrode in place, the number of reliable measurements in the clinical setting is smaller. For the same reason, practical application of the geometric definitions in the clinical setting is limited to single RF ablation sessions with a single electrode. In the Material and Methods section of clinical reports, it should be clearly stated that the definitions of "axial" and "transverse" describe the spatial relation with the axis of the electrode and that they should not be confounded with the axial and transverse planes of classical CT or MRI imaging, which describe the spatial relation with the axis of the patient. Further, in clinical reports, the exposant "el" is added to the abbreviations for clarity.

Frequency of Use of Proposed Descriptive Parameters in Experimental RF Ablation Literature
Sixty papers with the main aim describe single-session coagulation zones in animal liver with commercial or experimental electrodes were identified: 50 for a single electrode (Table 4Go).12,21–41,48–55,68–86 Curley (unpublished data) and 10 for a dual-electrode system (Table 5Go).59–65,87,88 For single-electrode experiments, the most essential descriptive parameters, i.e., the axial diameter and the minimal transverse diameter, were available in only 18 and 5 of the 50 papers, respectively. For dual-electrode experiments, the axial diameter, the inline transverse diameter and the lateral margin were available in only 6, 7, and 0 of the 10 papers, respectively. Forty of 60 papers described in vivo experiments. Only 15 of these 40 papers specified whether the red rim was excluded from the measurements: in 13 papers it was excluded, while in 2 papers the red rim was included.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
In RF ablation of liver tumors, precise tailoring of the size and shape of the coagulation zone is important. The coagulation zone should be large enough to encompass both the tumor and a safety margin of 1 cm at all sides.15,16,89 On the other hand, it should be small enough to avoid collateral damage.17 Real-time ultrasound monitoring of the coagulation zone is unreliable.10–13 Diameters measured by ultrasound show a poor correlation with the pathological diameter,10–12 with overestimation of the minimal transverse diameter and underestimation of the maximal transverse diameter of the coagulation zone.13

MRI and elastography thermometry are promising but still experimental methods for real-time monitoring of tissue temperature and, indirectly, the development of the coagulation zone.90–92

Therefore, knowledge of the expected size and shape of a single-session coagulation zone and its relation to the electrode tip is essential.

A recent review showed that much of this crucial information is lacking for current commercial RF ablation electrodes.19 This paucity of data was incriminated explicitly as cause of several local recurrences in a recent study.93 The situation is even worse when experimental electrodes are included in the analysis, as shown in the present study (Tables 4Go and 5Go). The development of a minimal set of descriptive parameters and the corresponding expected variation for each RF ablation electrode and protocol may improve the results of RF ablation and will help to avoid the loss of some of its credibility by poor results due to brochure-based overoptimistic expectations of a perfectly spherical coagulation zone with a constant diameter.42,94

This report should be seen as complementary to several recent efforts to standardize reporting on RF ablation. A paper from the International Working Group on Image-Guided Tumor Ablation (IWGIG-TA) proposes standardized terms for general aspects in the broad field of image-guided tumor ablation.95 Other papers specifically focused on standardization of one particular aspect of RF ablation, such as a scoring system for complications96 and a logical terminology for RF electrodes and RF electrode systems.20 All these efforts are crucial to improve scientific communication in the field of RF ablation.

The proposed terminology has been developed to describe RF coagulation zones but can also be applied to coagulation zones created by other interstitial techniques that use an applicator, such as microwave antennas, laser fibers, and cryoprobes.

Limitations
The current proposal deals only with a standardized macroscopic description of coagulation zones. The exact generator type, electrode type, and treatment algorithm should be documented as well. In order to reliably compare coagulation zones obtained by different electrodes and protocols, a standardized experimental setup is necessary, e.g., regarding the time interval between RF ablation and the measurements; but this falls out of the scope of this report.

The proposed standardized description method may give the impression that size and geometry of coagulation zones are very predictable. In fact, as several authors have recently shown,19,67,94,97 variability of coagulation size and geometry remains an important problem, even when using standardized treatment protocols. Description of this variability is an integral part of the current proposal and should be considered as important as the description of the "mean" size and geometry.

The feasibility of the proposed algorithm for reconstructing the original axes of the electrode on follow-up CT scans needs to be assessed.

Predictive Value of Experimental Measurements
The predictive value of data on size and geometry obtained in animal experiments using the proposed description method is high only for the experimental setting in which the data were measured. Size and geometry obtained in ex vivo experiments tend to overestimate the coagulation diameter and volume and underestimate variations in geometry compared to in vivo experiments.34 Results from ex vivo experiments are useful only as a first step to optimize RF technology in the laboratory. Before clinical application of a new electrode or protocol, coagulation size and geometry should always be assessed in in vivo experimental studies. Even in vivo, the results obtained in the ideal situation, i.e., in the middle of the parenchyma, are not necessarily predictive for the result near a large vessel or near the capsule. Furthermore, findings in animal models should be compared to those observed in patients. The in vivo results of RF ablation in healthy animal liver may not be the same as when applied to a tumor and a surrounding rim of nontumorous liver in a patient. The tumor may be hypo- or hypervascular. It may also have different electrical and thermal characteristics as well as a different sensibility to the generated heat. The liver itself may be altered by fibrosis, cirrhosis, or chemotherapy-induced fatty changes. Hydration may differ between patients. The results of RF ablation obtained in one organ are very different from results in other organs, due to differences in vascularity and composition (water, salts, lipids, and proteins). There are well-known differences for kidney (higher salt content and radial blood flow) versus liver (greater uniformity) versus breast or prostate (higher lipid content, i.e., more insulating capacity).98

Preparation Technique for Experimental Measurements
The proposed preparation technique carries the advantage that the exact relation of the edges of the coagulation zone to the electrode shaft and the electrode tip can be recorded. This is difficult to impossible if the electrode is retracted from the tissue prior to cutting the tissue into slices. Those slices will only by chance be perfectly perpendicular to the electrode track or parallel to the axial plane.

The border of the central tan-white zone should be taken as the border of the coagulation zone. Within this area, all tissue has been shown to be irreversibly damaged.22 The tannish color is due to differential light absorption and reflection by denatured proteins. The surrounding hyperaemic red rim should be excluded from the measurements. This rim still contains viable cells, as proven by histochemical staining and intracellular adenosine triphosphate measurement techniques in the acute phase,20 though some of them may die later (ATP).

Measurements should be performed before histological tissue fixation with associated tissue shrinkage.

Standardization and Interpretation of Terms
Numerous and often contradictory synonyms have been used in the literature for the terms that have been defined in this proposal (Tables 1Go and 2Go). In order to distill the most unequivocal and universally acceptable terms, we first rejected synonyms that suggested a ranking of size, such as "length" and "width", "long(est) axis" and "short(est) axis", "minimum diameter" and "maximum diameter", because the longest axis does not necessarily correspond to the axial diameter; neither does the shortest axis always correspond to the transverse diameter.19,21 We then rejected synonyms that suggested a position in space, such as "vertical diameter", "height", and "depth", because an RF ablation electrode can be inserted in any direction in the laboratory as well as in a patient. Next, from the remaining terms, the most "expressive" term was selected, as was the case for the "ellipticity index (EI)" which is more "intuitively suggestive" than "aspect ratio"56 or "shape value".35 Finally, some lacking terms had to be newly created.

For the interpretation of these terms, we adopted the principle that all terms describe the spatial relationship of the coagulation zone with the electrode (and not with the patient nor with the treated organ). Therefore, "axial" should be interpreted as "in the axis of the electrode" and "transverse" as "perpendicular to the axis of the electrode."

Volume Measurements
In many experimental papers, the description of a three-dimensional coagulation zone is reduced to a single value for its volume. This may be useful in the laboratory to optimize energy deposition but is not helpful to plan RF ablation as it leaves the physician ignorant about the size and shape of the coagulation zone. On the other hand, calculating expected volumes of coagulation zones based on available data on expected axial and transverse diameter can be useful to prevent liver failure when planning RF ablation in patients with limited liver reserve and/or extensive tumor burden. For a perfect ellipsoid, volume is calculated as ({pi}/6+ · AD · TDmax · TDmin.

Random Diameters
In many papers, two or three dimensions of a coagulation zone are measured and reported, e.g., the "longest diameter" and the "shortest diameter," without reference to the position of the electrode. Especially in the clinical setting, we acknowledge that measuring random diameters is far easier than measuring dimensions in relation to the electrode as described in the above proposal. However, while random diameters may be useful for volume calculation, they are of little value to accurately describe coagulation zone geometry, and they may even be misleading. Although for many electrodes the longest diameter of the coagulation zone corresponds to the axial diameter, for other electrodes it corresponds to the transverse diameter.19,21

Front Margin
There are no data available to the physician on the front margin, except very recently for the cooled electrode.22 This information, however, is essential to calculate whether the tip of the electrode should be inserted up to the center of a tumor or rather up to the posterior edge or even behind this edge, to ascertain the safety margin.

Awaiting reliable data, the "best guess" of the front margin for straight electrodes may correspond to half the difference between the axial diameter and the length of the noninsulated tip of the electrode, assuming that the coagulation zone is distributed symmetrically at the front and at the back of the active tip.

For expandable electrodes, no "best guess" is possible. For some expandable electrodes, the center of the coagulation zone may correspond to the end of the electrode shaft, while for other expandable electrodes, the center of the coagulation zone is probably deeper into the tissue than the end of the electrode shaft.99

Coagulation Center (Fig. 6Go)
Many coagulation zones are not spherical but rather mushroom- or cone-shaped.35 Moreover, an elongation of the coagulation zone along the electrode track is often observed. In other words, the transverse plane, defined by the maximal transverse diameter in the axial plane, is not always situated halfway between the proximal and distal edges of the coagulation zone in the axial plane. The expected location of the maximal transverse diameter in relation to the tip of the electrode may prove to be a very useful parameter when planning RF ablations in patients.


Figure 6
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  		FIG. 6. Importance of description of the position of the coagulation center. These three coagulation zones are quite different despite having the same axial diameter, transverse diameter, and frontal margin. Addition of the position of the coagulation center (arrow) increases accuracy of description.

 
Maximal and Minimal Transverse Diameter
In some papers, only the maximal transverse diameter of a coagulation zone is measured and reported. The maximal diameter is important to prevent complications by too large coagulation zones17 but is oncologically irrelevant. In other papers, only the mean of the maximal and minimal transverse diameters or the mean of two perpendicular transverse diameters is reported, which again represents a loss of essential information. Knowledge of the minimal diameter is essential to be sure to cover the tumor and a margin of 1 cm at all sides.

Maximal and Minimal Radius
Knowledge of the radius is more accurate than knowledge of diameter as the radius equals half of the transverse diameter only in symmetrical coagulation zones. Up to 60% of coagulation zones may be asymmetrical.12 A minimal diameter that has been obtained by measuring asymmetrical coagulation zones represents an overestimation of minimal radius and misleads the clinician. Up to now, no data on maximal and minimal radiuses of coagulation zones have been available for any electrode. Values for maximal and minimal radiuses allow calculation of the useful RI (see below).

Ellipticity Index
The EI quantitatively describes the general coagulation zone shape in the axial plane. The higher the index, the more elliptical the coagulation zone, as suggested by the name.19

Regularity of Shape in the Transverse Plane
The regularity index (RI) quantitatively describes the regularity of the coagulation zone shape in the transverse plane. It is very easy to calculate and clinically relevant. An increasing number of reports stress the fact that many coagulation zones are less regular than previously assumed.19,21,34,35,75,94,97 The numerator of the regularity index is the minimal radius, which is related to the risk of local recurrence,15,21,75,97 while the denominator is the maximal radius, which is related to the risk of collateral damage.17

A ratio close to 1.0 corresponds to a nearly spherical coagulation zone. The lower the ratio, the more irregular the coagulation zone. Because the risk of local recurrence and collateral damage is different for asymmetrical coagulation zones, zones with indentations, or zones with extensions, the (most frequently found) type(s) of irregularity should be specified for RI inferior to 0.80 in the subjective description (see further).

The heat sink effect is common to all electrodes when used with normal blood flow and causes local indentations of the coagulation zone near blood vessels.19 For coagulation zones created by expandable electrodes, superficial to deep clefts can be present between tines.19,75 For coagulation zones made by wet electrodes or its combinations (cooled-wet, expandable-wet, bipolar-wet)20 or by saline-enhanced RF ablation,20,100 spiky extensions of the coagulation zone due to irregular spread of saline have been observed.34,35,100 Alternative scoring systems that have been used in the literature to quantify (ir)regularity of coagulation zones include:

Fusion Index for a Dual-Electrode System
Apart from the objective measurements of the coagulation zone, the single most important information needed by the clinician is whether coagulation between the two electrodes is complete or not, i.e., whether the coagulation zones around each electrode are completely fused.

Variability
Some reports give only mean values for certain measurements. The standard deviation is essential to assess the range of coagulation zone sizes that one can expect. In a Gaussian distribution, the mean value ± 1 SD includes 68% of observations and the mean value ± 2 SD includes 95% of observations. In other words, the minimal coagulation zone size that can be expected with 97.5% confidence equals the mean size minus 2 standard deviations and the maximal coagulation zone size that can be expected with 97.5% confidence equals the mean size plus 2 standard deviations.

Some papers confuse the reader by describing the standard error of the mean (SEM) instead of the SD. Formula, with n = number of observations. The SEM is a measure for the reliability of the estimation of the mean value of a population by the observed mean value in a sample. It is not at all a measure of variability of the observed values around the observed mean. The SEM is by definition (much) smaller than the SD. Use of the SEM instead of SD gives a false impression of high reproducibility to the statistically less trained reader.

Subjective Description (Fig. 5Go)
On top of the objective descriptive parameters, a subjective description of shape and regularity is recommended. Examples of shape description include terms such as ellipsoid,35 oval,68 ovoid,12 conical,56,68 inverted conical,35 pear-shaped,25 mushroom-shaped,73 droplet-shaped,73 teardrop-shaped,94 dumbbell-shaped,25,56 and butterfly-shaped 87.

For single-electrode coagulation zones with an RI < 0.8, the (most frequently found) type(s) of irregularity should be specified: asymmetrical with indentations (including cloverleaf-shaped27 or with deep clefts78) or with irregular (flamelike or spiky) extensions.100 Subjective descriptions of shape and regularity may at first glance seem to be imprecise and unscientific. They are, however, a powerful warning to the clinician against a too optimistic expectation of a regular and spherical coagulation zone for the electrodes and protocols that have been tested.

Pictures
"A picture tells more than a thousand words" is true in this context too. It completes the numerical and subjective description. A picture can disclose an irregular shape that was not obvious with objective measurements. Three-dimensional imaging reconstructions35,67 of the coagulation zone are even more illustrative.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Hopefully, the widespread adoption of this proposed minimal set of descriptive parameters will soon fill in the many gaps in our knowledge about the size and geometry of coagulation zones.

Companies are strongly encouraged to provide this set of data for all RF ablation electrodes that they have or intend to bring on the market. In the liver, these data should be produced with and without the Pringle maneuver, both within a predefined short distance to large intrahepatic vessels as well as in hepatic parenchyma without nearby vessels.

A better recognition of shortcomings in size, shape, and regularity of coagulation zones produced by the actual RF ablation electrodes may lead to less local recurrence and collateral damage and may boost research to improve these features.


   ACKNOWLEDGMENTS
 
The authors thank Marie-Bernadette Jacqmain and Christian Deneffe for the illustrations.

Received for publication October 19, 2005. Accepted for publication April 3, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 

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