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Clinical Observations |
1 Department of Urology and Comprehensive Cancer Center of Wake Forest
University, Winston-Salem, NC.
2 Present address: Department of Urologic Surgery, Vanderbilt University Medical
Center, Nashville, TN.
3 Department of Pathology, Wake Forest University Health Sciences,
Winston-Salem, NC.
4 Department of Radiology, Wake Forest University Health Sciences, Medical
Center Blvd., Winston-Salem, NC 27157.
5 Present address: Piedmont Urological Associates, High Point, NC.
Received December 20, 2005;
accepted after revision March 30, 2006.
Address correspondence to R. J. Zagoria
(rzagoria{at}wfubmc.edu).
Abstract
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CONCLUSION. For a single 10-minute ablation, ablated volumes averaged 27 cm3 and 105 cm3 with a single-probe and a three-probe ablation array, respectively. There were no skip areas within the ablated zone. Microwave ablation can safely and quickly generate large ablation lesions and renal neoplasms.
Keywords: interventional radiology • kidney • microwave coagulation therapy • oncology • percutaneous ablation
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Although it has had considerable initial success in the management of renal cell carcinoma, radiofrequency ablation has a number of potential limitations. These include restrictions on the size of the tumor that can be managed, the number of repeated treatments needed for management of larger tumors, the time needed to perform the procedure on larger tumors, the need for grounding pads and the risk of cutaneous burns, and inability to perform real-time monitoring of the extent of ablation. Microwave energy has the potential to overcome some of these limitations [7]. We initiated a phase 1 study to assess tumor destruction and to further develop a minimally invasive method of in situ tumor ablation with a microwave device. Microwave energy has been used extensively for ablation of hepatic lesions [8-15]. This study was designed to assess through histologic examination the utility of microwave energy for in vivo ablation of renal neoplasms.
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Patients underwent standard radical nephrectomy through either an open or a hand-assisted laparoscopic approach. Before ligation of the renal blood vessels, needle biopsy of the tumor was performed. After biopsy, one or an array of three microwave ablation probes (VivaWave, Vivant Medical), depending on the size of the lesion and surgeon preference, were inserted into the tumor under direct visual guidance. The 13-gauge probe shafts were cooled by continuous flow of saline solution according to the manufacturer's specifications. Microwave energy was delivered through the probes at 60 W for a total of 10 minutes. The probes were removed, the renal blood vessels were promptly ligated, and the surgical specimen, consisting of the kidney and tumor within the Gerota fascia, was removed en bloc.
The specimen was immediately subjected to pathologic processing. The main end point of the study was the size of the ablative tissue produced in one ablation cycle with one of three configurations of probes. For the purposes of this trial, it was not relevant whether the entire tumor was ablated, because no specific effort was made to ensure complete coverage of the tumor. The size of the ablated tissue was evaluated. In two cases, the ablated area was in an already necrotic tumor, making thermal changes difficult to detect at microscopic examination, so visual inspection (color change) and palpation (dense texture) in addition to standard histologic examination were used in combination to determine the size of the ablation.
In eight of the 10 cases, ablation area and cell death were confirmed by
staining for reduced nicotinamide adenine dinucleotide (NADH) diaphorase with
our previously described technique
[16]. In the other two cases,
NADH staining could not be done for technical reasons. All histopathologic and
NADH-stained specimens were reviewed by one pathologist. Size of the ablative
lesion was expressed as both 3D rectilinear volume and volume of a prolate
spheroid according to the formula V= 4/3
ab2, where
a is the major semiaxis and b is the minor semiaxis. If the
volume had two different minor axes, an average of the two was used. Toxicity
was recorded according to the revised National Cancer Institute Common
Terminology Criteria for Adverse Events version 3.0.
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In no case did thermal ablation interfere with determining the stage and grade of the tumor. Most of the tumors were conventional renal cell carcinoma (Table 1). Lesion size at visual inspection and palpation correlated well with size at histopathologic examination, and the results of NADH staining confirmed the presence of uniform cell death in the apparent ablation zone (Fig. 1A, 1B, 1C, 1D). Histopathologic examination and staining for NADH diaphorase revealed no cell death beyond the visually observed ablation zone. In four cases, the ablation zone included tumor tissue and benign renal parenchyma next to the tumor. There were no bleeding complications.
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Microwave thermal ablation has been used clinically in Japan for several years. Like radiofrequency ablation, this technology entails the use of heat to destroy tumor cells. Microwave thermal ablation, however, does not consist of electrical current and therefore does not require grounding the patient. Instead, microwave energy is delivered through an antenna probe placed in the center of the tumor, causing local heating. Microwave energy techniques of thermal ablation have not been widely adopted in the United States because the ablative lesions generated with the original technology were relatively small. Advances in engineering have led to development of an advanced microwave energy-based thermal ablation device that has overcome much of this size limitation. The manufacturer of the device used in this study has developed an antenna with improved material and structural properties that allows optimized electromagnetic power deposition into a tumor and a resultant larger, hotter thermal lesion. This device has been approved by the U.S. Food and Drug Administration for thermocoagulation of human tissue.
In our study, the mean size of ablative lesion achieved with a single probe was 4.1 x 2.7 x 2.2 cm compared with a lesion size of 1.4-2.4 cm achieved with a single radiofrequency probe (Cool-tip radiofrequency ablation system, Radionics) and 2.2-6.3 cm with a three-probe array in a previous treat-and-resect protocol at our institution [16]. Unlike radiofrequency ablation, microwave ablation can be readily performed with multiple probes simultaneously. Multiple probes can be used for radiofrequency ablation, but the procedure is cumbersome and time consuming.
With the microwave system, large tumors can be easily managed with a single ablation or with concurrent ablation of several anatomically separate lesions [17]. In our study, the mean ablative lesion size with a three-probe array was 5.7 x 4.7 x 3.8 cm. Achievement of this large lesion size may decrease the need for repeated treatments, avoid inadequate management of large tumors, and increase the speed of therapy, thereby decreasing the complication rate, although we did not directly assess this factor. Total treatment time may be substantially reduced with the new microwave thermal ablation device. Testing these potential advantages will require direct comparison of radiofrequency and microwave technologies.
Microwave thermal ablation has been shown safe and efficacious in the management of liver tumors in both porcine models and a phase 1 trial with human subjects [8-15]. To our knowledge, this study is the first to examine the performance characteristics of this technology in the setting of renal tumors. The results of our study confirm that the technology can reliably and reproducibly produce a large ablative lesion with uniform tissue necrosis, that is, no skip areas, of solid renal neoplasms. One case of grade 1 toxicity occurred but was caused by human error rather than a failure of the technology. Additional studies are needed to determine the safety and efficacy of this technology in the setting of a treat-and-leave protocol, as in the setting of percutaneous, image-directed therapy.
This novel microwave ablation system can safely and quickly generate large ablative lesions in renal neoplasms. The test cases showed excellent uniform cell kill. Further studies of this system are warranted in a setting of minimally invasive management of renal neoplasms.
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