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1 Medical School, The University of Texas Health Science Center at San Antonio,
San Antonio, TX 78229-3900.
2 Department of Radiology, The University of Texas Health Science Center at San
Antonio, Mail Code 7800, 7703 Floyd Curl Dr., San Antonio, TX
78229-3900.
Received July 3, 2003;
accepted after revision September 9, 2003.
Address correspondence to G. D. Dodd III
(dodd{at}uthscsa.edu).
Abstract
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MATERIALS AND METHODS. A review of the clinical records of 208 patients who underwent radiofrequency ablation of malignant hepatic tumors during a 6-year period revealed 31 patients with small tumors that were treated with a single ablation. Clinical data were recorded using standardized work sheets. Tumor and lesion sizes after ablation were measured from CT scans. The influences of tumor size, tumor type, presence or absence of cirrhosis, and tissue temperature on the ablation size were analyzed.
RESULTS. The size of tumor before treatment ranged from 0.8 to 4.0
cm (mean diameter [± SD] = 1.8 ± 0.9 cm) with corresponding
volumes of 0.27–30.24 mL (mean volume = 27.1 ± 15.9 mL). The
lesion sizes after ablation ranged from 1.7 to 5.3 cm (mean diameter = 3.6
± 0.7 cm) with corresponding volumes of 2.29–75.87 mL (mean
volume = 4.9 ± 7.1 mL). Tumor type (p > 0.25), presence or
absence of cirrhosis (p > 0.45), and tissue temperature
(p = 0.055) had no relationship to ablation size. Tumor size had a
statistically significant influence on ablation lesion size (p <
0.04). Ablation of small tumors (diameter 
2.25 cm, n = 32)
produced random lesion sizes whereas ablation of large tumors (diameter >
2.25 cm, n = 11) produced larger lesions (mean diameter = 4.0
± 0.8 cm).
CONCLUSION. Significant variation occurs in the lesion size produced using the same ablation device and algorithm. These findings must be considered when planning ablation strategies.
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The surgical standard for resection of hepatic tumors includes the resection of the tumor and a 1-cm margin of normal liver between the tumor and the resection edge. A tumor-free margin of less than 1 cm is directly related to an increase in local tumor recurrence [5, 6]. In a similar fashion, successful radiofrequency ablation of liver tumors depends on inducing coagulation necrosis of the entire tumor as well as a 1-cm-thick margin of normal liver around the 360° perimeter of the tumor [7]. Thus, ablation strategies are designed so that the induced thermal injury encompasses the tumor and the tumor-free margin. Predictable ablation volumes are a necessary precursor to an effective treatment strategy. Variations in ablation volume from the expected values may cause an incomplete ablation of a tumor and lead to a higher incidence of local tumor recurrence.
Although the manufacturers of radiofrequency ablation devices claim consistent lesion sizes after ablation with each of their devices, in our clinical practice we have observed considerable variability in the size of the lesion created after ablation in different patients treated with the same ablation device and algorithm. Additionally, we have noted that the actual size of a lesion after ablation is often markedly less than that claimed by the manufacturers. If these observations are correct, modifications in ablation strategies may be necessary to effectively treat patients with hepatic tumors.
On the basis of our clinical observations, we performed a retrospective study to determine the variation in the size of the lesions created using the same radiofrequency ablation device to treat different patients with small malignant hepatic tumors. The sizes of the lesions after ablation were correlated with tumor size, tumor type, the presence or absence of cirrhosis, and the tissue temperature after ablation to identify potential relationships.
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Radiofrequency Ablation System
This study was limited to patients treated with the Cool-tip radiofrequency
ablation system (Radionics, Burlington, MA). The system consists of a 480-kHz
alternating electric current generator (model CC-1) that has a maximum power
output of 200 W, an assortment of 17-gauge internally cooled needle
electrodes, a perfusion pump, and adhesive dispersive electrodes (ground
pads). The generator contains an internal program that automatically runs a
12-min pulsed-energy ablation cycle. The program adjusts the power output
relative to tissue impedance to optimize the diameter of ablated tissue. The
ablation sequence begins with a gradual increase in power over the first
minute to reach a peak power output of up to 200 W. Peak power is maintained
until tissue impedance rises 20 
above the beginning value. Once the
20- threshold is exceeded, power is decreased to 10 W for 15 sec. Power
is then increased back to the maximum value until the tissue impedance rises
again. If the maximum power cannot be maintained for at least 10 sec, a
reduced power setting is used to limit the rise in tissue impedance.
Successive cycles are continued for 12 min to complete the ablation.
Although several different needle electrodes are available for the system, this study was limited to patients whose tumor or tumors were treated with the cluster electrode (model CTC 2025, Radionics). The cluster electrode consists of three parallel 17-gauge needles arranged in a triangular configuration and mounted on a common hub. Each needle has internal channels through which chilled sterile water (20°C) is circulated; none of the perfusate enters the patient's tissues. The unit is operated with four large dispersive electrodes applied to the patient's thighs perpendicular to the long axis of the body.
Ablation Procedure
Each patient was treated as an outpatient. Both a local anesthetic and IV
sedation were administered for patient comfort. IV sedation consisted of
either Diprivan (propofol, AstraZeneca, Wilmington, DE) or Ultiva
(remifentanil hydrochloride, Glaxo Wellcome, Research Triangle Park, NC). All
tumors were treated percutaneously using sonographic guidance. Each tumor
included in the study was treated with a single 12-min ablation using the
cluster electrode. The exposed 2.5-cm tips of the needle electrode were
positioned symmetrically within each tumor before the ablation. One minute
after completion of an ablation, the temperature of the ablated tissue was
recorded, and the electrode was withdrawn.
CT Scans
Each patient underwent CT of the abdomen within 1 month before and 1week
after the ablation procedure. Fifteen of the CT scans before ablation were
obtained at facilities outside our institution; all the remaining CT scans
were obtained at our institution. All CT scans were obtained on helical
scanners with IV contrast enhancement. All the CT scans obtained at other
facilities were judged to be of adequate quality on which to plan appropriate
patient management and for use in this study. All the CT scans obtained at our
institution followed a standardized protocol that consisted of unenhanced and
dual-phase contrast-enhanced CT of the entire liver with images obtained in a
craniocaudal direction. Unenhanced images were obtained as contiguous axial
scans. Arterial and portal venous phase contrast-enhanced CT scans were
obtained in the helical mode 25 and 65 sec, respectively, after the initiation
of infusion of a 3–5 mL/sec injection of 130 mL of nonionic IV contrast
material (Optiray 320 [ioversol] 68%, Mallinckrodt, St. Louis, MO). IV
contrast material was administered via a power injector. All scans were
obtained using 7- to 8-mm collimation, 220 mA, and 120 kVp. The pitch
(1–1.5) was adjusted as necessary to allow a single helical acquisition
through the entire liver in each vascular phase. Hard copies of the studies
were available for all patients.
Image Analysis
The sizes of tumors and lesions after ablation were measured directly from
the CT scans using handheld calipers and the standardized measurement scale
present on each study. The tumors were measured from the vascular phase images
in which they were best visualized; however, in general, hepatocellular
carcinomas were measured from arterial phase images and metastases were
measured from portal venous phase images. All lesions after ablation were
measured from portal venous phase images. The diameters of both the tumors and
the lesions after ablation were recorded in three axes: anterioposterior,
transverse, and craniocaudal. The craniocaudal dimension was measured by
counting the number of slices that showed the lesion and multiplying that
number by the slice thickness. To ensure consistency, one person performed all
the measurements. The measurements were then reviewed independently by a
second person. Any discrepancies were resolved by consensus.
The three measurements from each tumor and lesion after ablation were used
to calculate mean diameters. The volume of each tumor and lesion after
ablation was calculated using the standard equation for the volume of an
ellipsoid:
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Statistical Analysis
The range, mean, and standard deviation (SD) of the mean diameter of the
tumors and postablation lesions were calculated using SPSS version 11.0
(Statistical Package for the Social Sciences, Chicago, IL) for Windows
(Microsoft, Redmond, WA). In the evaluation of correlation between tumor and
postablation lesion size, tumor sizes before treatment were categorized into
two groups: small tumors that were less than or equal to 2.25 cm in diameter
(n = 32) and large tumors that were greater than 2.25 cm in diameter
(n = 11). A division point of 2.25 cm was selected to approximate the
separation of mean diameters at the upper quartile. Thirty-two (74.4%) of the
43 tumors had values below the 2.25-cm division point with a median of 1.3 cm
and a skewness of 0.42, and 11 (25.6%) had values above the 2.25-cm division
point with a median of 3.0 cm and a skewness of 0.5. The unpaired Student's
t test was used to test the influence of tumor size before treatment
and the presence or absence of cirrhosis on the lesion size after ablation.
Types of tumors were separated into three categories: hepatocellular carcinoma
(n = 17), metastatic colorectal carcinoma (n = 14), and
other metastases (n = 12). One-way analysis of variance was used to
test the relationship, the three categories of tumor and the lesion size after
ablation. Pearson's r test and Spearman's rank correlation test were
used to examine the relationship of tissue temperature after ablation to the
lesion size after ablation.
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When the lesions after ablation were classified by tumor type, hepatocellular carcinoma had a mean diameter of 3.7 ± 0.57 cm and a mean volume of 27.3 ± 12.85 mL. Lesions from colon metastases had a mean diameter of 3.8 ± 0.75 cm and a mean volume of 30.9 ± 19.69 mL. Lesions from the other metastases had a mean diameter of 3.3 ± 0.85 cm and a mean volume of 22.3 ± 14.85 mL. Tumors in patients with cirrhosis had a mean diameter after ablation of 3.7 ± 0.56 cm and a mean volume of 27.9 ± 12.74 mL. Patients without cirrhosis had a mean diameter after ablation of 3.6 ± 0.82 cm and a mean volume of 26.5 ± 18.03 mL. The tissue temperature after ablation ranged from 54°C to 83°C with a mean temperature of 71.6 ± 7.3°C.
Our analysis did not reveal a relationship between tumor type (p
> 0.25), the presence or absence of cirrhosis (p > 0.45), or
the tissue temperature after ablation and the size of the lesions after
ablation. However, tumor size did show a statistically significant
relationship to the size of the lesions after ablation (p < 0.04).
Ablation of small tumors ( 2.25 cm in diameter, n = 32) resulted
in random ablation lesion sizes with no pattern of size progression. However,
ablation of large tumors (> 2.25 cm in diameter, n = 11) resulted
in consistently larger ablation lesions with a mean diameter of 4.0 ±
0.78 cm (Figs. 1A,
1B,
2A,
2B,
3A,
3B). In addition, our analysis
did not reveal a difference between the results when examined from the
perspective of mean diameter or volume.
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A study by Shen et al. [9] used the Starburst XL probe (RITA Medical Systems, Mountain View, CA) to create lesions in porcine livers in vivo [9]. The probe used was a 14-gauge needle with nine curved retractable tines. The tines extended to 2 cm for the study. The ablation sequence was controlled by an automatic algorithm that monitored tissue temperature via thermocouples embedded in the tines. The radiofrequency ablation generator (model 1500) had a maximum power output of 150 W. In the study, those researchers performed solitary ablations that produced lesions with a median minimum diameter of 1.5 cm (minimum = 0.7 cm, maximum = 2.3 cm) and a median maximum diameter of 1.9 cm (minimum = 1.8 cm, maximum = 3.0 cm).
Sugimori et al. [10] used the LeVeen needle (RadioTherapeutics, Sunnyvale, CA) to create lesions in porcine livers in vivo. The electrode used in the study had eight tines that were deployed to a depth of 2 cm in the liver. The radiofrequency ablation generator (model RF 2000) operated at a frequency of 460 kHz and produced a maximum of 100 W of power. The ablation algorithm used a stepwise increase in generator power to ultimately achieve "roll off" (a marked increase in tissue impedance indicative of tissue desiccation). Single ablations produced lesions that measured 2.4 ± 0.3 x 2.0 ± 0.4 cm.
Last, a study performed by Dodd et al. (Dodd et al., Radiological Society of North America meeting, 1999) showed that three radiofrequency ablation devices (Radionics, RITA Medical Systems, and RadioTherapeutics) produced highly variable lesions in cirrhotic livers. The investigators performed ablations in five patients who underwent liver transplantation for end-stage cirrhosis. After surgical exposure of the liver and while the liver was normally perfused, three ablations (one per device) were performed in each patient. Examination of the explanted livers showed that each of the three devices from Radionics, RITA Medical Systems, and RadioTherapeutics produced lesions that varied considerably in diameter among patients: 3.8 ± 0.31, 1.9 ± 0.51, and 1.8 ± 0.92 cm, respectively.
Our study differs from the previous studies in that it is the only study, to our knowledge, to specifically investigate the variability in the size of the lesions created by a single ablation device in patients with small hepatic tumors. As documented in the previous studies, we found marked variability in the size of the lesions produced after ablation in different patients. Of the four variables that we evaluated for potential impact on the size of lesions after ablation, the type of tumor, the presence or absence of cirrhosis, and the tissue temperature after ablation showed no statistically significant relationship. The only statistically significant relationship that we discovered was between tumor size and the size of the lesion after ablation. We found that radiofrequency ablation of larger tumors (mean diameter > 2.25 cm) produced significantly larger lesions (mean diameter = 4.0 ± 0.78 cm) than were seen after radiofrequency ablation of smaller tumors. This finding correlates with the finding of Goldberg et al. [8] that solitary ablations in large colorectal hepatic metastases (3.5–6.5 cm in diameter) produced large lesions (diameter: range, 4.2–7.0 cm; mean, 5.3 cm), and ablations performed in porcine liver without tumors produced smaller lesions (3.3 ± 0.2 cm).
The most plausible explanation for the variation in the size of lesions after ablation is differences in hepatic and tumor perfusion. Several studies have shown the effect of hepatic perfusion on the size of lesions after ablation [11, 12]. Goldberg et al. [11] showed that radiofrequency ablation performed in nontumoral in vivo porcine liver during interruption of hepatic blood flow produced larger areas of coagulation necrosis than radiofrequency ablation with unaltered blood flow: 2.9 ± 0.1 cm versus 2.4 ± 0.2 cm, respectively. Washburn et al. [12] showed that the Pringle maneuver (interruption of portal venous and hepatic arterial blood flow) during radiofrequency ablation of nontumoral cirrhotic liver produced significantly larger lesions than were achievable with normal hepatic perfusion (range, 2.7–4.0 cm vs 3.4–5.3 cm, respectively; mean, 3.5 cm vs 4.5 cm, respectively). With these studies as a background, it is reasonable to conclude from our data that variation in hepatic blood flow is the primary factor controlling the size of lesions produced when ablating hepatic tumors equal to 2.5 cm or smaller. It appears that the presence of a small tumor, irrespective of cell type, has little if any impact on the size of the lesion after ablation. In fact, the size of the lesions that we produced ablating small tumors closely approximated the size of lesions reported in multiple studies that used the same radiofrequency ablation device in nontumoral liver models [8, 11, 12].
Hepatic blood flow seems to have less of an impact on the size of the lesion after ablation in large tumors than in small ones. The ability to produce large lesions in large tumors is due to the diminished blood flow relative to nontumoral hepatic parenchyma found in tumors; that is, the "heat sink" effect is less. This phenomenon appears to hold for large tumors that are both hypo- and hypervascular. Although there is a clear difference in the degree of perfusion of hypo- and hypervascular tumors, the lack of correlation in our study between cell type (hepatocellular carcinoma vs hepatic metastases) and the size of lesions after ablation suggests that the magnitude of the difference may be insignificant. A note of caution is appropriate; the ability to create large lesions in large tumors does not necessarily translate into an improved ability to eradicate large tumors. The larger lesions after ablation are almost completely confined to the tumor itself; the tumor-free margin remains as significant a problem as it is with small tumors.
In conclusion, we found a substantial variation in the size of the lesion produced when using the same radiofrequency ablation device and ablation algorithm to treat small malignant hepatic tumors in different patients. This variability is independent of tumor type, the presence or absence of cirrhosis, and the temperature of the tissue after ablation. However, the size of the lesion after ablation is related to the size of the tumor being treated; larger lesions are produced in tumors larger than 2.25 cm. Although we limited our study to a single radiofrequency ablation device, based on our review of other published studies, this variability appears to affect several devices. Furthermore, the average size of the lesions after ablation in our study and in the studies of other devices is substantially smaller than that claimed by the manufacturers.
These findings have implications in regard to designing effective ablation strategies. If ablation strategies are designed using a falsely inflated expectation of the size and reproducibility of the lesion after ablation, the chance of performing an inadequate tumor ablation will be substantially increased. On the basis of our results, we urge physicians to adopt an aggressive ablation strategy to overcome the variability and diminished performance of ablation devices.
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