DOI: 10.1148/rg.24si045512
RadioGraphics 2004;24:S59-S71
© RSNA, 2004
Imaging-guided Radiofrequency Ablation of Renal Masses1
Ronald J. Zagoria, MD
1 From the Department of Radiology, Wake Forest University Health Sciences, Medical Center Blvd, Winston-Salem, NC 27157-1088. Received February 25, 2004; revision requested April 2 and received May 11; accepted May 19. The author has no financial relationships to disclose. Address correspondence to the author (e-mail: rzagoria@wfubmc.edu).
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Abstract
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Substantial and growing evidence indicates that imaging-guided percutaneous radiofrequency ablation of small renal cell carcinomas (RCCs) is effective for complete tumor eradication. The rate of successful radiofrequency treatment of small RCCs ranges from 79% to 97%, with a 1% rate of serious complications. For patients who are considered high-risk candidates for nephrectomy, percutaneous radiofrequency ablation represents another treatment option. The article summarizes the published results for this technique and also describes the indications, techniques, procedural risks, and applications for percutaneous radiofrequency ablation of RCCs. The successful use of radiofrequency ablation for treatment of recurrent and metastatic RCCs is also described.
© RSNA, 2004
Index Terms: Kidney neoplasms, CT, 81.1211 • Kidney neoplasms, MR, 81.1214 • Kidney neoplasms, therapeutic radiology • Kidney neoplasms, US, 81.1298 • Radiofrequency (RF) ablation
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Introduction
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For many years, the only curative treatment of early-stage renal cell carcinoma (RCC) has been surgical resection (1). Surgical resection, however, is associated with a small risk of mortality and substantial morbidity. Some patients with RCCs are poor surgical candidates because of comorbidities, which until recently essentially excluded them from undergoing curative treatment. There is increasing evidence that percutaneous radiofrequency ablation can be a curative treatment with minimal morbidity for some patients with RCC. Herein, I review the principles of radiofrequency ablation of renal tumors and the published data supporting its use in the treatment of RCC. I also describe patient selection, the techniques used for the ablation procedure, complications, and follow-up imaging after ablation of RCCs. Other applications for radiofrequency ablation in the treatment of RCC are also discussed.
The treatment of RCC can be a frustrating problem because of several facts. Fortunately, patients with RCCs that have not spread outside the renal parenchyma have an excellent prognosis for cure if the RCC can be resected (1). Some patients, however, are not ideal candidates for surgical resection of their renal tumors because of comorbidities, limited functional renal reserve, or other complicating factors. Although observational treatment for the high-risk surgical candidate has been advocated by some authors (2), the natural history of RCCs can be unpredictable and systemic spread may develop during periods of surveillance. Unfortunately, advanced-stage RCC does not respond well to therapy and the prognosis for these patients remains extremely poor (1).
The number of RCCs detected has increased substantially. In 2002, more than 30,000 new cases of RCC were diagnosed in the United States (3). This number equates to a greater than 100% increase in the incidence of RCC diag-nosed in the United States since 1950. Most of this increase has occurred because small, localized tumors have been detected incidentally in asymptomatic patients who underwent imaging for other reasons (4). Tumors in these patients might not progress to clinically significant lesions. For some of these patients, an effective nonsurgical approach to treatment would be appealing.
Radical nephrectomy has long been considered the standard treatment for even small, localized RCCs. Meanwhile, renal-sparing surgery has grown in popularity, and the techniques have been refined. Studies in which surgical techniques have been compared (5,6) have shown that open partial nephrectomy is as effective as radical nephrectomy for curing small, localized RCCs. This success indicates that complete eradication of an RCC can result in cure rates comparable to those obtained with complete removal of a tumor-containing kidney. Advances in imaging and thermal ablation techniques, combined with the theory that in situ tumor destruction will yield results comparable to those achieved with tumor resection, have led to increased interest in imaging-guided, minimally invasive percutaneous thermal ablation techniques for treating RCC. There is substantial experience with radiofrequency ablation—which causes tumor destruction by heating—in the treatment of RCCs (7–15). Although radiofrequency devices can be introduced intraoperatively, during an open procedure, the most experience with and the maximum benefit from this technique have been with percutaneous imaging-guided procedures.
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Ablation Process
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In radiofrequency ablation, a high-frequency, alternating current with a wavelength of 460–500 kHz is emitted through an electrode placed within the targeted tissue (11). Grounding pads applied to the patient’s thighs complete the electrical circuit. Deposition of radiofrequency energy results in frictional heating from flowing electrons in cells near the site of energy emission. When living hu-man tissues are heated to more than 49°C, cell death occurs within minutes (16). Temperatures in excess of 60°C cause immediate cell death (11). The cell death is induced by the denaturation of proteins, which results in the loss of enzymatic function, melting of cell membranes, and destruction of cytoplasm (16). These events result in direct cytodestruction of the affected cells. Although some cells are destroyed at temperatures less than 49°C, other cells can survive temperatures approaching 49°C. Alternatively, when temperatures exceed 105°C, cells boil, releasing gas vapor and causing tissue charring (11). Gas and charred tissue inhibit dispersion of radiofrequency energy, which decreases the effective depth of penetration of lethal energy concentrations (11). Hence, radiofrequency ablation devices should ideally induce prolonged heating of target tissue with temperatures sustained between 50° and 105°C.
For percutaneous imaging-guided radiofrequency ablation, the energy is delivered into the target tissue by means of needlelike electrodes. Currently available radiofrequency ablation electrodes range in diameter from 15 to 17 gauge. Three U.S. Food and Drug Administration–approved radiofrequency devices are available in the United States (Fig 1). Each of these devices uses a different strategy to maximize the size of thermal ablation. With the Radionics system, each electrode is shaped like a standard 17-gauge needle and delivered as a single electrode or as a unit of three electrodes arranged in a triangular cluster. The Radionics system increases ablation lesion size by using two enhancements: electrode cooling and pulsed energy delivery (11). The RITA device consists of a generator and a 14- or 15-gauge electrode with numerous retractable tines, which are used to increase the area of ablation. The tines are advanced into the area of treatment. The LeVeen system uses a 14-gauge electrode with 12 retractable tines that are advanced into the area of treatment. Each device also uses a slightly different approach to energy delivery and monitoring for thermal destruction.
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Figure 1a. Radiofrequency electrodes currently available. (a) A, Starburst configuration multi-tine electrode (RITA [radiofrequency interstitial tissue ablation] system; Rita Medical Systems, Mountain View, Calif). B and C are the single and cluster electrodes, respectively, used in the Cool-tip radiofrequency ablation system (Radionics, Burlington, Mass). (b) The LeVeen electrode (Radiotherapeutics, Mountain View, Calif) is another multi-tine electrode design.
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Figure 1b. Radiofrequency electrodes currently available. (a) A, Starburst configuration multi-tine electrode (RITA [radiofrequency interstitial tissue ablation] system; Rita Medical Systems, Mountain View, Calif). B and C are the single and cluster electrodes, respectively, used in the Cool-tip radiofrequency ablation system (Radionics, Burlington, Mass). (b) The LeVeen electrode (Radiotherapeutics, Mountain View, Calif) is another multi-tine electrode design.
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The theoretical maximum size of the treatment zone has been calculated in vitro for radiofrequency ablation (16). In vitro, the theoretical maximum size of the ablated area is two times the length of the energy-emitting segment of the electrode for the long axis of the treatment zone (16). The transverse axis maximum can be up to two-thirds of the length of the long axis of the treatment zone (16). In vivo, the treatment zone varies and is usually smaller than the theoretical maximum (8). The maximum size of the treatment zone can be increased by inducing ischemia or by treating devascularized tissue (16). Alternatively, flowing blood, large fluid-containing spaces, or circulating air can decrease the effective size of the treatment zone (16). The available radiofrequency devices use generators that deliver 150–200 W of energy.
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Efficacy of Radiofrequency Ablation for Treating Renal Tumors
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The first published report about radiofrequency ablation of tumor tissue was released in 1990 (17). In this study, radiofrequency ablation was used in the treatment of hepatic tissue. In 1997, the use of radiofrequency ablation to produce extensive necrosis of kidney tumors in humans was reported (18). In 1998, a case report was pub-lished that described the use of percutaneous radiofrequency ablation under ultrasonographic (US) guidance as the sole treatment of RCC in a human (19).
Since then, Matlaga et al (8) performed a clinical trial to systematically evaluate the in vivo application of radiofrequency ablation in renal parenchymal tumors. Ablations were performed intraoperatively and with US guidance immediately before surgical nephrectomy. A Radionics cool-tip radiofrequency system was used. After tumor biopsy, a single ablation of 12 minutes duration was performed. Then, nephrectomy was performed and the tumor was evaluated at histologic examination by using both standard stains and vital stains, which indicate whether cells were viable at resection. All tumors treated in this study were RCCs and ranged in diameter from 1.4 to 8 cm (average diameter, 3.2 cm). The diameter of the ablated tumor tissue was 1–6 cm per single 12-minute ablation. In no cases were "skip" areas of viable tumor identified within the necrotic regions. Evaluation of the surrounding kidney after nephrectomy demonstrated that a margin of treated, nonviable kidney tissue was identified in all eight of the completely devitalized tumors. The treatment margin was 2–13 mm thick. In no case was the rim of treated kidney greater than 13 mm in diameter. The results of this study indicated that in vivo eradication of small RCCs was feasible, although some tumors necessitate more than one 12-minute treatment for complete devitalization of the tumor. In addition, the authors determined that only a small amount of adjacent normal kidney was destroyed, a finding that suggests that little diminution in renal function will result when this technique is used.
Results from clinical series with relatively short-term follow-up have supported the efficacy of this procedure in carefully selected patients (7,9,11–15). The largest series reported to date included radiofrequency ablation of 42 RCCs in 34 patients (12). In this series, there was no detectable disease after ablation in the 29 RCCs that were exophytic, with the treated tumors in this category ranging in diameter from 1.1 to 5.0 cm. Of 11 RCCs greater than 3 cm in diameter that contacted the renal sinus, six contained detectable residual tumor—even after repeated ablations. There was no detectable disease in patients with small (
3 cm in diameter) tumors treated with radiofrequency ablation, regardless of tumor position.
Another group of investigators (13) documented successful radiofrequency ablation of 31 of 32 (97%) RCCs after one or two ablation sessions. In this series, 26 smaller (mean diameter, 2.4 cm) masses were successfully treated with a single session, whereas five of six larger (mean diameter, 3.5 cm) RCCs necessitated a second RFA session for complete eradication.
A more recent study reviewed treatment with radiofrequency ablation of 24 RCCs in 22 patients (20). Twenty patients with 22 RCCs (92%) were left with no detectable disease, whereas the remaining two patients deferred further treatments because of severe comorbidities. The major determinant of successful complete ablation in this study was tumor size. Tumor location, tumor histologic characteristics, and presence of renal insufficiency did not correlate with the success rate of radiofrequency ablation.
In another series (9), 24 RCCs in 21 patients with von Hippel–Lindau disease or familial papillary RCCs were treated with percutaneous radiofrequency ablation. At 2-month follow-up, 19 of the 24 (79%) tumors demonstrated no evidence of contrast enhancement at computed tomography (CT). The remaining five tumors demonstrated some persistent enhancement. There were no serious complications, and all patients were treated on an outpatient basis.
Other investigators have published similar results for the treatment of RCC with radiofrequency ablation (7,14,15). All series have resulted in high success rates for achieving complete eradication of smaller RCCs with radiofrequency ablation.
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Indications for Radiofrequency Ablation of Renal Masses
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Exact indications for radiofrequency ablation in the treatment of renal malignancies have yet to be validated with prospective scientific studies. Results of clinical studies, however, indicate that radiofrequency ablation is effective and has a very low risk of substantial complications, which makes it a desirable option in some clinical situations. In most cases, ablation is performed in patients with a renal tumor suggestive of RCC on the basis of imaging findings, and who have marginal renal function, serious comorbidities that make them high-risk surgical candidates, or a high risk of developing additional RCCs (eg, patients with von Hippel–Lindau disease).
Contraindications for radiofrequency ablation include uncorrected coagulopathy and acute illness such as sepsis (11). Although many patients referred for radiofrequency ablation have serious comorbidities (eg, chronic congestive heart failure), this should not be considered a contraindication to ablation of the RCC.
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Radiofrequency Ablation Technique
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In preparation for radiofrequency ablation, patients should undergo contrast material–enhanced CT or magnetic resonance (MR) imaging of their abdomen (Fig 2). To achieve curative results, patients should have no tumor spread beyond Gerota fascia, into the renal vein, or to lymph nodes (stage I or II in the Robson classification system or stage T1–T3a in the TNM classification system [21]). Extension of tumor into veins, adjacent organs, or lymph nodes or distant metastases are contraindications to the procedure unless it is being performed for reasons other than cure. There is no absolute size limit of a renal mass that would exclude a patient from radiofrequency ablation; however, the likelihood of complete ablation is higher for smaller tumors (20). It is unlikely that complete ablation will be achieved with tumors larger than 5 cm, and treatment of larger tumors should be considered only for cytoreduction rather than cure. Patients should have a recent serum creatinine value, platelet count, and coagulation profile available before ablation. Patients should discontinue the use of aspirin, other antiplatelet agents, and warfarin sodium far enough in advance of the procedure for coagulation parameters to return to normal.
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Figure 2a. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2b. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2c. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2d. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2e. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2f. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2g. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Figure 2h. Steps in radiofrequency ablation of a renal tumor. The patient, an 83-year-old man with a history of bladder cancer and myocardial infarction, was referred for radiofrequency ablation because of his comorbidities. (a) Contrast-enhanced CT scan shows an avidly enhancing 3.8-cm-diameter mass (arrow) arising from the left kidney. (b) Unenhanced CT scan obtained with the patient in a prone position shows placement of a 19-gauge needle at the edge of the tumor. Fine-needle aspiration and core biopsy were performed at this position immediately before ablation. Biopsy results confirmed that the tumor was a clear cell renal adenocarcinoma. (c) CT scan obtained immediately after biopsy shows a cluster electrode being advanced into the tumor. The electrode was positioned near the interface of the kidney and the tumor in an attempt to ablate tumor at this margin. (d, e) CT scans obtained immediately after the first ablation show the electrode being repositioned into different areas of the tumor to overlap ablation zones. Additional ablations were performed after repositioning. (f) Contrast-enhanced CT scan obtained immediately after the three ablations shows no evidence of tumor enhancement. A thin rim of kidney adjacent to the tumor also does not enhance. This finding indicates ablation of the adjacent renal margin. A small amount of blood and gas is visible in the perinephric space. These findings are expected immediately after ablation. (g, h) Unenhanced (g) and contrast-enhanced (h) CT scans obtained 2 months after ablation show no evidence of residual viable tumor. On the unenhanced scan (g), the tumor appears slightly hyperattenuating, measuring 64 HU. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The tumor does not enhance on the contrast-enhanced scan (h), where it measures 61 HU. The treated tumor is smaller than it was before treatment, which is also a typical feature following ablation.
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Because RCC is accurately diagnosed with CT or MR imaging and no adjuvant therapy has proved advantageous in its treatment, results of renal mass biopsy are not likely to change the treatment plan (1). Biopsy may be performed before the procedure if it is believed that the results will have an effect on patient treatment. At my institution, except in unusual circumstances (eg, coexisting second malignancy), the decision about whether to treat a renal tumor is based on the imaging features alone. However, because ablation does not yield a resection specimen and patients prefer to have a definitive diagnosis, and as an aid to follow-up imaging planning, tumor biopsy is usually performed at the ablation procedure, with results available after completion of the ablation. For instance, a biopsy specimen diagnostic of an angiomyolipoma, in which no fat was detected on pretreatment images, would negate the need for long-term imaging follow-up. My colleagues and I routinely perform biopsy immediately before ablation in sedated patients by using a coaxial technique (Fig 2). Both cytologic and core specimens are obtained.
Use of anesthesia for renal radiofrequency ablation varies from institution to institution. Most authors use conscious sedation, although general anesthesia is required for some patients, particularly those with marked respiratory compromise (11–15,20).
The technique of placing the electrode is analogous to that of performing imaging-guided biopsy of a renal mass (Fig 2). Unlike in a typical biopsy, the electrode tip should be advanced to the deep margin of the tumor for the first treatment because little ablation occurs beyond the electrode tip in some cases. Patients are usually placed in the prone or decubitus position for the procedure. Imaging guidance can be achieved with CT, US, or MR imaging. Although US has the advantages of real-time imaging, absence of ionizing radiation, and portability, it has some limitations for guiding ablation. In some patients, it may be difficult or impossible to visualize very small renal tumors with US. Air-filled bowel adjacent to a renal tumor may not be well seen with US. In addition, the process of radiofrequency ablation produces a hyperechoic zone in the area of treatment (Fig 3). This effect is believed to arise from cell boiling and the release of microbubbles in the treated tissues. With US, this echogenicity severely obscures the area around the treatment zone, which makes electrode repositioning difficult to image. MR imaging has been used rarely for radiofrequency ablation guidance (22). In most cases, renal radiofrequency ablation is performed with CT guidance (Figs 4, 5). CT has the following advantages: (a) it can demonstrate very small RCCs, (b) it lacks the obscuring artifacts after ablation, and (c) in patients without renal insufficiency, intravenous contrast material can be administered to determine the adequacy of ablation before the procedure is terminated (Fig 2).
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Figure 3a. Hyperechoic zone caused by radiofrequency ablation in a patient who was enrolled in a "treat and resect" study (20) of renal tumor ablation. (a) Contrast-enhanced CT scan shows a 1.5-cm-diameter mass (arrow) in the right kidney. Biopsy findings confirmed that the tumor was an RCC. (b) Intraoperative US scan shows the isoechoic renal tumor (cursors) located anteriorly in the right kidney. (c) With US guidance, the electrode (arrow) was advanced into the tumor immediately before ablation. (d) US scan obtained during ablation of the tumor shows the hyperechoic zone (arrow) caused by radiofrequency ablation. This hyperechoic zone obscures the electrode and US detail of the treatment area. After nephrectomy, histologic examination of the specimen showed complete ablation of the tumor.
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Figure 3b. Hyperechoic zone caused by radiofrequency ablation in a patient who was enrolled in a "treat and resect" study (20) of renal tumor ablation. (a) Contrast-enhanced CT scan shows a 1.5-cm-diameter mass (arrow) in the right kidney. Biopsy findings confirmed that the tumor was an RCC. (b) Intraoperative US scan shows the isoechoic renal tumor (cursors) located anteriorly in the right kidney. (c) With US guidance, the electrode (arrow) was advanced into the tumor immediately before ablation. (d) US scan obtained during ablation of the tumor shows the hyperechoic zone (arrow) caused by radiofrequency ablation. This hyperechoic zone obscures the electrode and US detail of the treatment area. After nephrectomy, histologic examination of the specimen showed complete ablation of the tumor.
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Figure 3c. Hyperechoic zone caused by radiofrequency ablation in a patient who was enrolled in a "treat and resect" study (20) of renal tumor ablation. (a) Contrast-enhanced CT scan shows a 1.5-cm-diameter mass (arrow) in the right kidney. Biopsy findings confirmed that the tumor was an RCC. (b) Intraoperative US scan shows the isoechoic renal tumor (cursors) located anteriorly in the right kidney. (c) With US guidance, the electrode (arrow) was advanced into the tumor immediately before ablation. (d) US scan obtained during ablation of the tumor shows the hyperechoic zone (arrow) caused by radiofrequency ablation. This hyperechoic zone obscures the electrode and US detail of the treatment area. After nephrectomy, histologic examination of the specimen showed complete ablation of the tumor.
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Figure 3d. Hyperechoic zone caused by radiofrequency ablation in a patient who was enrolled in a "treat and resect" study (20) of renal tumor ablation. (a) Contrast-enhanced CT scan shows a 1.5-cm-diameter mass (arrow) in the right kidney. Biopsy findings confirmed that the tumor was an RCC. (b) Intraoperative US scan shows the isoechoic renal tumor (cursors) located anteriorly in the right kidney. (c) With US guidance, the electrode (arrow) was advanced into the tumor immediately before ablation. (d) US scan obtained during ablation of the tumor shows the hyperechoic zone (arrow) caused by radiofrequency ablation. This hyperechoic zone obscures the electrode and US detail of the treatment area. After nephrectomy, histologic examination of the specimen showed complete ablation of the tumor.
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Figure 4a. Treatment of a biopsy-proved 3-cm-diameter RCC in a patient with heart failure. (a) Unenhanced CT scan shows a solid, exophytic tumor arising from the right kidney. (b) Contrast-enhanced CT scan obtained at the same level as a shows an enhancing solid RCC (arrow). (c) Unenhanced CT scan obtained 1 month later shows the electrode advanced into the tumor for treatment. (d) Contrast-enhanced CT scan obtained 7 months after ablation shows that the tumor has decreased in size. There is no longer any detectable enhancement. These findings indicate successful tumor ablation.
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Figure 4b. Treatment of a biopsy-proved 3-cm-diameter RCC in a patient with heart failure. (a) Unenhanced CT scan shows a solid, exophytic tumor arising from the right kidney. (b) Contrast-enhanced CT scan obtained at the same level as a shows an enhancing solid RCC (arrow). (c) Unenhanced CT scan obtained 1 month later shows the electrode advanced into the tumor for treatment. (d) Contrast-enhanced CT scan obtained 7 months after ablation shows that the tumor has decreased in size. There is no longer any detectable enhancement. These findings indicate successful tumor ablation.
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Figure 4c. Treatment of a biopsy-proved 3-cm-diameter RCC in a patient with heart failure. (a) Unenhanced CT scan shows a solid, exophytic tumor arising from the right kidney. (b) Contrast-enhanced CT scan obtained at the same level as a shows an enhancing solid RCC (arrow). (c) Unenhanced CT scan obtained 1 month later shows the electrode advanced into the tumor for treatment. (d) Contrast-enhanced CT scan obtained 7 months after ablation shows that the tumor has decreased in size. There is no longer any detectable enhancement. These findings indicate successful tumor ablation.
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Figure 4d. Treatment of a biopsy-proved 3-cm-diameter RCC in a patient with heart failure. (a) Unenhanced CT scan shows a solid, exophytic tumor arising from the right kidney. (b) Contrast-enhanced CT scan obtained at the same level as a shows an enhancing solid RCC (arrow). (c) Unenhanced CT scan obtained 1 month later shows the electrode advanced into the tumor for treatment. (d) Contrast-enhanced CT scan obtained 7 months after ablation shows that the tumor has decreased in size. There is no longer any detectable enhancement. These findings indicate successful tumor ablation.
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Figure 5a. Radiofrequency ablation of a biopsy-proved RCC in an elderly patient with coronary artery disease. (a) Unenhanced CT scan obtained immediately before ablation, with the patient in prone position, shows an exophytic 4.5-cm-diameter tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained during ablation shows the electrode tip positioned within the RCC. Multiple tines can be seen deployed from the electrode. There is some hemorrhage from the tumor puncture surrounding the mass. (c) Gadolinium-enhanced MR image (three-dimensional dynamic image, repetition time msec/echo time msec = 4.6/0.956) obtained 5 weeks after ablation shows residual enhancing tumor (arrows) in the periphery of the treated RCC. (d) Unenhanced CT scan obtained 3 months after a second ablation session shows that the tumor is slightly heterogeneous and has higher attenuation than expected for an uncomplicated tumor. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The peritumor stranding is an expected finding after ablation. (e) Contrast-enhanced CT scan at the same level as d shows no enhancement in the retreated tumor. This finding indicates successful tumor ablation.
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Figure 5b. Radiofrequency ablation of a biopsy-proved RCC in an elderly patient with coronary artery disease. (a) Unenhanced CT scan obtained immediately before ablation, with the patient in prone position, shows an exophytic 4.5-cm-diameter tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained during ablation shows the electrode tip positioned within the RCC. Multiple tines can be seen deployed from the electrode. There is some hemorrhage from the tumor puncture surrounding the mass. (c) Gadolinium-enhanced MR image (three-dimensional dynamic image, repetition time msec/echo time msec = 4.6/0.956) obtained 5 weeks after ablation shows residual enhancing tumor (arrows) in the periphery of the treated RCC. (d) Unenhanced CT scan obtained 3 months after a second ablation session shows that the tumor is slightly heterogeneous and has higher attenuation than expected for an uncomplicated tumor. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The peritumor stranding is an expected finding after ablation. (e) Contrast-enhanced CT scan at the same level as d shows no enhancement in the retreated tumor. This finding indicates successful tumor ablation.
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Figure 5c. Radiofrequency ablation of a biopsy-proved RCC in an elderly patient with coronary artery disease. (a) Unenhanced CT scan obtained immediately before ablation, with the patient in prone position, shows an exophytic 4.5-cm-diameter tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained during ablation shows the electrode tip positioned within the RCC. Multiple tines can be seen deployed from the electrode. There is some hemorrhage from the tumor puncture surrounding the mass. (c) Gadolinium-enhanced MR image (three-dimensional dynamic image, repetition time msec/echo time msec = 4.6/0.956) obtained 5 weeks after ablation shows residual enhancing tumor (arrows) in the periphery of the treated RCC. (d) Unenhanced CT scan obtained 3 months after a second ablation session shows that the tumor is slightly heterogeneous and has higher attenuation than expected for an uncomplicated tumor. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The peritumor stranding is an expected finding after ablation. (e) Contrast-enhanced CT scan at the same level as d shows no enhancement in the retreated tumor. This finding indicates successful tumor ablation.
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Figure 5d. Radiofrequency ablation of a biopsy-proved RCC in an elderly patient with coronary artery disease. (a) Unenhanced CT scan obtained immediately before ablation, with the patient in prone position, shows an exophytic 4.5-cm-diameter tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained during ablation shows the electrode tip positioned within the RCC. Multiple tines can be seen deployed from the electrode. There is some hemorrhage from the tumor puncture surrounding the mass. (c) Gadolinium-enhanced MR image (three-dimensional dynamic image, repetition time msec/echo time msec = 4.6/0.956) obtained 5 weeks after ablation shows residual enhancing tumor (arrows) in the periphery of the treated RCC. (d) Unenhanced CT scan obtained 3 months after a second ablation session shows that the tumor is slightly heterogeneous and has higher attenuation than expected for an uncomplicated tumor. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The peritumor stranding is an expected finding after ablation. (e) Contrast-enhanced CT scan at the same level as d shows no enhancement in the retreated tumor. This finding indicates successful tumor ablation.
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Figure 5e. Radiofrequency ablation of a biopsy-proved RCC in an elderly patient with coronary artery disease. (a) Unenhanced CT scan obtained immediately before ablation, with the patient in prone position, shows an exophytic 4.5-cm-diameter tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained during ablation shows the electrode tip positioned within the RCC. Multiple tines can be seen deployed from the electrode. There is some hemorrhage from the tumor puncture surrounding the mass. (c) Gadolinium-enhanced MR image (three-dimensional dynamic image, repetition time msec/echo time msec = 4.6/0.956) obtained 5 weeks after ablation shows residual enhancing tumor (arrows) in the periphery of the treated RCC. (d) Unenhanced CT scan obtained 3 months after a second ablation session shows that the tumor is slightly heterogeneous and has higher attenuation than expected for an uncomplicated tumor. This finding is believed to reflect coagulation necrosis in the tumor induced by ablation. The peritumor stranding is an expected finding after ablation. (e) Contrast-enhanced CT scan at the same level as d shows no enhancement in the retreated tumor. This finding indicates successful tumor ablation.
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Radiofrequency electrode selection is dependent on tumor size. Each of the three available devices has been used for the ablation of RCCs. An electrode size that will ablate the tumor and a surrounding margin of normal renal parenchyma should be selected. For tumors larger than 3 cm in diameter, multiple overlapping ablations will likely be necessary for complete tumor destruction. Adequacy of tumor treatment is difficult to determine at ablation. Results of one study (20) indicated that performance of contrast-enhanced CT at the end of a presumed-adequate ablation session, but before sedation was ended or the patient was moved, was very useful for determining whether viable tumor remained. Enhancing tumor could then be treated with additional ablations before terminating the session. Most radiologists recommend obtaining unenhanced and contrast-enhanced CT or MR images 1–3 months after ablation (11). Although ablated tumors often have internal areas of increased attenuation or increased signal intensity at CT and MR imaging (Fig 2), respectively, there should be no contrast enhancement of nonviable tumor after the intravenous administration of contrast material (Figs 4, 5). Areas of contrast enhancement (>10 HU or >15% with CT and MR imaging, respectively) are indicative of residual viable RCC (Fig 6). Residual viable tumor can be treated with additional ablation sessions (Fig 5). Two in vivo studies in which radiofrequency ablation of RCCs was evaluated showed different results in terms of the importance of absent contrast enhancement in treated RCCs (8,23). One study (8) showed complete, uniform cellular necrosis in ablated areas. There were no "skip" areas in the treated tumors. The second study, in which a different electrode device was used (23), however, found nonuniform ablation of RCCs; 5%–10% viable tumor remained in most treated tumors. This study did not use vital stains for evaluating the histologic specimens. The interpretation of standard ablation specimens with standard hematoxylin and eosin staining may be less accurate than when vital stains are used. This may have limited their findings and conclusions. This group also found that contrast enhancement could not be detected with CT in areas where viable tumor was histologically demonstrated after nephrectomy (23). This result suggests that lack of contrast enhancement may cause overestimation of the amount of tumor destruction caused by radiofrequency ablation. Further studies and longer follow-up of treated patients should help clarify this issue.
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Figure 6a. Local tumor recurrence after radiofrequency ablation of RCC in a patient with renal insufficiency. (a) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows a 5.2-cm-diameter, enhancing exophytic tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained with the patient prone shows placement of the electrode in the tumor. An MR image obtained 2 months later (not shown) demonstrated no evidence of tumor enhancement. (c) T2-weighted MR image (45/4.652) obtained 1 year after ablation shows enlargement of the left renal vein with solid tissue within its lumen (arrow). (d) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows enhancement of the tumor thrombus and of tissue in the tumor bed (arrows), findings that indicate recurrence of RCC. Examination of the resected specimen showed clear cell adenocarcinoma in the kidney and left renal vein.
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Figure 6b. Local tumor recurrence after radiofrequency ablation of RCC in a patient with renal insufficiency. (a) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows a 5.2-cm-diameter, enhancing exophytic tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained with the patient prone shows placement of the electrode in the tumor. An MR image obtained 2 months later (not shown) demonstrated no evidence of tumor enhancement. (c) T2-weighted MR image (45/4.652) obtained 1 year after ablation shows enlargement of the left renal vein with solid tissue within its lumen (arrow). (d) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows enhancement of the tumor thrombus and of tissue in the tumor bed (arrows), findings that indicate recurrence of RCC. Examination of the resected specimen showed clear cell adenocarcinoma in the kidney and left renal vein.
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Figure 6c. Local tumor recurrence after radiofrequency ablation of RCC in a patient with renal insufficiency. (a) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows a 5.2-cm-diameter, enhancing exophytic tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained with the patient prone shows placement of the electrode in the tumor. An MR image obtained 2 months later (not shown) demonstrated no evidence of tumor enhancement. (c) T2-weighted MR image (45/4.652) obtained 1 year after ablation shows enlargement of the left renal vein with solid tissue within its lumen (arrow). (d) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows enhancement of the tumor thrombus and of tissue in the tumor bed (arrows), findings that indicate recurrence of RCC. Examination of the resected specimen showed clear cell adenocarcinoma in the kidney and left renal vein.
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Figure 6d. Local tumor recurrence after radiofrequency ablation of RCC in a patient with renal insufficiency. (a) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows a 5.2-cm-diameter, enhancing exophytic tumor (arrow) in the left kidney. (b) Unenhanced CT scan obtained with the patient prone shows placement of the electrode in the tumor. An MR image obtained 2 months later (not shown) demonstrated no evidence of tumor enhancement. (c) T2-weighted MR image (45/4.652) obtained 1 year after ablation shows enlargement of the left renal vein with solid tissue within its lumen (arrow). (d) Gadolinium-enhanced T1-weighted MR image (4.368/0.972) shows enhancement of the tumor thrombus and of tissue in the tumor bed (arrows), findings that indicate recurrence of RCC. Examination of the resected specimen showed clear cell adenocarcinoma in the kidney and left renal vein.
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Complications
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Few complications caused by radiofrequency ablation have been reported, and most patients can be treated on an outpatient basis. Hematuria occurs uncommonly (24), is self-limited, and resolves within 24 hours of treatment. Because of the kidney’s location, care must be taken to avoid thermal injury to adjacent bowel. At a minimum, 5 mm of intervening fat should be present between bowel and the target tumor to avoid causing bowel necrosis (24). Fat is an effective insulator, and 5 mm or more is thought to be adequate protection for adjacent bowel (24). If bowel abuts the tumor to be treated, sterile water can be injected to displace the bowel and allow for safe ablation of the RCC (25) (Fig 7). Ablation of renal tumors adjacent to the adrenal gland can cause sudden release of vasoactive catecholamines. For ablation of these tumors, the operator should be prepared to administer 
-adrenergic blocking medications. The risk of clinically significant thermal injury to liver or spleen, when ablating an RCC, is thought to be insignificant (24). Although perirenal hemorrhage (Fig 8) is common after RCC ablation, there have been no reports of clinically significant bleeding caused by this treatment (24). To my knowledge, one case of needle track seeding (13) and one case of ureteral stricture (24) have been reported. It appears that there is little renal damage associated with radiofrequency ablation (8). Even in the treatment of central tumors, the development of clinically important pelvicaliceal damage has been rarely reported (24). In vivo studies have demonstrated only a small amount of kidney destruction in the area surrounding the tumor (8,10); hence, renal function should remain nearly intact after this procedure.
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Figure 7a. Technique for displacing adjacent organs before radiofrequency ablation of a renal tumor. (a) Contrast-enhanced CT scan shows a 1-cm-diameter enhancing, exophytic tumor (arrow) arising anteriorly from the left kidney. The small intestine is immediately adjacent to the tumor. The patient was referred for radiofrequency ablation because of his severe emphysema. (b) CT scan obtained with the patient in the prone position for the ablation procedure shows that the tumor (arrow) has a calcified rim and is adjacent to a loop of small intestine, which increases the risk of thermal damage to the intestine. (c) With CT guidance, a 22-gauge needle was advanced into the perinephric space. Fifty milliliters of sterile water was injected at this point to displace the intestine away from the tumor and thus avoid thermal damage to the small intestine during ablation. (d) Repeat CT scan obtained after the injection of sterile water shows that there was sufficient space between the tumor and small intestine to advance the injection needle farther. This scan illustrates the needle position after advancing it farther. An additional 100 mL of sterile water was injected at this point to increase the space between the tumor and the intestine. (e) CT scan obtained after sterile water injection shows the electrode positioned to bisect the exophytic tumor. Ablation was performed with the electrode in this position. (f) Contrast-enhanced CT scan obtained immediately after ablation shows no enhancement of the tumor or of a thin rim of adjacent kidney (arrows). This finding indicates successful ablation of the tumor and the adjacent margin of kidney.
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Figure 7b. Technique for displacing adjacent organs before radiofrequency ablation of a renal tumor. (a) Contrast-enhanced CT scan shows a 1-cm-diameter enhancing, exophytic tumor (arrow) arising anteriorly from the left kidney. The small intestine is immediately adjacent to the tumor. The patient was referred for radiofrequency ablation because of his severe emphysema. (b) CT scan obtained with the patient in the prone position for the ablation procedure shows that the tumor (arrow) has a calcified rim and is adjacent to a loop of small intestine, which increases the risk of thermal damage to the intestine. (c) With CT guidance, a 22-gauge needle was advanced into the perinephric space. Fifty milliliters of sterile water was injected at this point to displace the intestine away from the tumor and thus avoid thermal damage to the small intestine during ablation. (d) Repeat CT scan obtained after the injection of sterile water shows that there was sufficient space between the tumor and small intestine to advance the injection needle farther. This scan illustrates the needle position after advancing it farther. An additional 100 mL of sterile water was injected at this point to increase the space between the tumor and the intestine. (e) CT scan obtained after sterile water injection shows the electrode positioned to bisect the exophytic tumor. Ablation was performed with the electrode in this position. (f) Contrast-enhanced CT scan obtained immediately after ablation shows no enhancement of the tumor or of a thin rim of adjacent kidney (arrows). This finding indicates successful ablation of the tumor and the adjacent margin of kidney.
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Abstract
Introduction
