DOI: 10.1148/rg.272065083
RadioGraphics 2007;27:325-339
© RSNA, 2007
CT and MR Imaging after Imaging-guided Thermal Ablation of Renal Neoplasms1
Geoffrey E. Wile, MD,
John R. Leyendecker, MD,
Kyle A. Krehbiel, MD,
Raymond B. Dyer, MD and
Ronald J. Zagoria, MD
1 From the Department of Radiology, Wake Forest University Baptist Medical Center, Medical Center Blvd, Winston-Salem, NC 27103. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received May 1, 2006; revision requested July 10 and received August 24; accepted August 28. R.J.Z. is a consultant with and received a research grant from Tyco Healthcare (Valleylab); all remaining authors have no financial relationships to disclose.
Address correspondence to J.R.L. (e-mail: jleyende{at}wfubmc.edu).
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Abstract
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In recent years, thermal tumor ablation techniques such as percutaneous radiofrequency (RF) ablation and cryoablation have assumed an important role in the management of renal tumors, particularly in patients who may be suboptimal candidates for more invasive surgical techniques. Postablation computed tomography (CT) and magnetic resonance (MR) imaging play an important part in evaluation of the ablation zone, surveillance for residual or recurrent tumor, and identification of procedure-related complications. The appearance of the ablation zone may vary depending on the ablation technique used, initial tumor size, and tumor location and composition. Most ablated tumors demonstrate a gradual decrease in size over time once the acute changes have resolved, although tumor involution is more evident after cryoablation than after RF ablation. Exophytic tumor ablation zones typically have a "bull’s-eye" appearance on CT scans and MR images obtained after RF ablation, with a visible mass often persisting in the absence of viable tumor. Residual or recurrent tumor often manifests as a focus of nodular or crescentic enhancement on postablation contrast material–enhanced CT scans and MR images, although a thin peripheral rim of enhancement often persists for several months following cryoablation. Complications following renal tumor ablation are usually minor but may include hemorrhage, ureteral stricture, urine leak, colonic perforation and colonephric fistula, and pneumothorax. As more patients undergo renal ablation procedures, it will become increasingly important that radiologists be able to recognize typical postablation CT and MR imaging findings to prevent confusing them with other pathologic processes.
© RSNA, 2007
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Introduction
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The number of new cancer cases in the United States involving the kidney and renal pelvis was estimated at 36,160 for 2005 and has been increasing (1). This increase may be due in part to an increase in the discovery of small incidental renal cell carcinomas (RCCs) at cross-sectional imaging (2). Before the advent of thermal ablation technologies, most patients with RCC of any size were treated with complete or partial nephrectomy (3). Recently, percutaneous radiofrequency (RF) ablation and cryoablation have become treatment options for patients with small RCCs. Although long-term survival data are limited, studies to date show promising results for small tumors, with a low rate of major complications (4–6).
Percutaneous renal RF ablation is performed by placing an electrode within the renal tumor under imaging guidance. A high-frequency alternating current is passed through the tissues surrounding the active portion of the electrode, causing frictional heating and subsequent cell death (7). The use of internally cooled electrodes allows coagulative necrosis without tissue charring (8). Histologic evaluation of the ablation zone reveals tissue devitalization and loss of normal tissue architecture concentrically centered on the electrode (9). Over time, the central area of coagulative necrosis is replaced by fibrosis and scar tissue rather than resorbed, likely accounting for the presence of a persistent mass at follow-up imaging (7).
Cryoablation relies on tissue freezing rather than heating to destroy tumor and may also be successfully performed percutaneously (10). Cryoablation destroys tissue directly by means of intracellular ice formation and osmotic imbalance resulting from extracellular ice formation. Indirect cellular death results from microvascular damage–induced ischemia (7). With both RF ablation and cryoablation, an attempt is made to create an ablation zone that includes a 5–10-mm tumor-free margin.
Computed tomography (CT) and magnetic resonance (MR) imaging play an important role after imaging-guided renal tumor ablation has been performed. As more patients undergo renal ablation procedures, it is increasingly important to be able to recognize the typical postablation CT and MR imaging findings to prevent confusing them with other pathologic processes. The histopathologic changes that accompany renal ablation result in characteristic imaging findings that can be distinguished from residual or recurrent tumor. Procedure-related complications should be recognized to expedite appropriate management. In this article, we review the techniques of RF ablation and cryoablation, the two thermal ablation technologies currently used at our institution. In addition, we discuss and illustrate the use of CT and MR imaging during and after ablation in terms of (a) surveillance imaging protocols; and (b) the typical appearances of uncomplicated renal thermal ablation sites, residual or recurrent disease, and procedure-related complications. We also describe unusual appearances after RF ablation.
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Surveillance Imaging Protocol after Renal Tumor Ablation
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Long-term surveillance imaging is essential for identifying tumor recurrence, metastatic disease, new tumors, and delayed complications after renal tumor ablation. Patients in whom iodinated contrast material is not contraindicated often undergo contrast material–enhanced CT. We use a collimation of 2.5 mm to allow detection of small renal lesions and facilitate multiplanar reformation. We routinely obtain both unenhanced and contrast-enhanced CT scans through the abdomen to allow assessment of contrast enhancement within the ablation zone. Contrast-enhanced images are obtained through the abdomen at 60 and 120 seconds after the intravenous administration of 125 mL of nonionic contrast material. Excretory phase images may be helpful in detecting collecting system complications such as urine leak. In general, the advantages of CT include widespread availability of suitable equipment and expertise. CT requires less patient cooperation than does MR imaging and is not as prone to artifact. We currently perform ablation under CT guidance and obtain an immediate postablation contrast-enhanced scan in many of our patients, with which subsequent CT scans can then be directly compared.
We have traditionally used MR imaging in patients in whom the use of contrast-enhanced CT is inadvisable. MR imaging does not involve ionizing radiation and has superior tissue contrast compared with CT. Subtle degrees of contrast enhancement may also be more apparent on MR images. Once touted as a significant advantage, the multiplanar capability of MR imaging is less relevant with the widespread availability of multi–detector row CT scanners and three-dimensional (3D) workstations. MR imaging requires significantly greater patient cooperation than CT, and severely claustrophobic patients as well as those with certain implanted devices are not good candidates for MR imaging. Traditionally, we have routinely performed gadolinium-enhanced MR imaging rather than contrast-enhanced CT in patients with renal insufficiency. However, the benefit of this approach in terms of preserving renal function has not been definitively established, and recently there has been renewed interest in the effect of gadolinium-based contrast agents on renal function in patients with preexisting renal disease (11,12).
Our protocol for surveillance MR imaging includes T1- and T2-weighted images as well as gradient-echo (GRE) T1-weighted images obtained before and after the administration of intravenous gadolinium-based contrast material (Table). The use of parallel imaging permits coverage of the entire liver and kidneys during a single breath hold with most sequences. We perform a single-shot fast SE sequence in two planes in every patient because this type of sequence is quick and relatively resistant to motion artifact and other artifacts. For axial T1-weighted images, we use a breath-hold dual-echo spoiled GRE sequence that provides images at both in-phase and opposed-phase echo times. We obtain fat-suppressed T2-weighted images using a respiratory triggered fast SE sequence, although many scanners are capable of producing adequate images of the kidneys during one or two breath holds. The use of fat suppression improves the conspicuity of fluid collections on T2-weighted images. For gadolinium-enhanced images, we use a 3D breath-hold fat-suppressed GRE sequence with parallel imaging and zero-fill interpolation to reduce acquisition time. Unenhanced, arterial phase, venous phase, nephrographic phase, and excretory phase images are acquired. The data set obtained prior to contrast material administration is subtracted from those obtained after its administration on a 3D workstation.
Initial surveillance imaging is usually performed no earlier than 1 month and no later than 3 months following RF ablation. Our surveillance schedule includes scanning at 1 month, 3 months, and then every 6 months after ablation (13,14). Given the lack of ionizing radiation with MR imaging and the need for multiple follow-up imaging examinations, it is likely that MR imaging will assume an increasing role in surveillance imaging after tumor ablation. It is important to note that institutions may differ with respect to follow-up imaging protocols depending on availability of equipment, imaging expertise, and clinical experience. It is also likely that surveillance protocols will continue to undergo modification as long-term follow-up and survival data accumulate and imaging technology evolves.
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Typical Imaging Findings during and Immediately after Ablation
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During CT-guided RF ablation, changes in attenuation within the tumor at unenhanced CT are too subtle for monitoring the adequacy of ablation. However, radiographically evident changes do occur. Stranding develops within the perinephric fat, and the pararenal fascia may become thickened. These changes are also visible on immediate postablation CT scans (Fig 1). A variable amount of perinephric or subcapsular hemorrhage may develop, and small locules of gas often form within the surrounding tissue. Occasionally, sterile water may be infused percutaneously between the bowel and kidney to displace the bowel away from the ablation site, thereby reducing the risk of inadvertent injury (15). This fluid persists on immediate postablation images and should not be confused with hemorrhage, which has higher intrinsic attenuation. Any change in tumor size during ablation is subtle.
On an immediate post-ablation contrast-enhanced CT scan, a completely treated tumor shows no evidence of contrast enhancement. A rim of nonenhancing devitalized renal parenchyma may also be present. Any enhancing tissue at the previous site of the ablated tumor suggests residual viable neoplasm (Fig 2).
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Figure 1. Immediate postablation contrast-enhanced CT scan obtained in an 82-year-old man with RCC shows stranding and fascial thickening (arrows). Arrowhead indicates the ablation zone.
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Figure 2a. Residual tumor in an 83-year-old woman with RCC. (a) Preablation contrast-enhanced CT scan shows a small, enhancing exophytic tumor (arrow). (b) Immediate post-ablation contrast-enhanced CT scan shows a residual crescent of enhancing tumor (arrow), which was re-treated during the same session.
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Figure 2b. Residual tumor in an 83-year-old woman with RCC. (a) Preablation contrast-enhanced CT scan shows a small, enhancing exophytic tumor (arrow). (b) Immediate post-ablation contrast-enhanced CT scan shows a residual crescent of enhancing tumor (arrow), which was re-treated during the same session.
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The appearance of the ablation zone during and immediately following cryoablation differs dramatically from that of an RF ablation site. During cryoablation, a clearly defined region of decreased attenuation develops as the tumor is frozen, which allows monitoring of treatment efficacy (Fig 3).
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Figure 3a. (a) Unenhanced CT scan obtained in a 74-year-old woman with RCC during cryoablation shows a low-attenuation ice ball (arrow) surrounding the cryoprobe (arrowhead). (b) Immediate post-cryoablation unenhanced CT scan obtained following thawing and cryoprobe removal shows residual low attenuation in the tumor ablation zone (arrow). (c) Unenhanced CT scan obtained 5 months later during retreatment of the same kidney with RF ablation for recurrent tumor shows fat stranding (arrow) but no appreciable change in attenuation of the ablation zone. Arrowhead indicates the electrode. (d) On an unenhanced CT scan obtained immediately after RF ablation, the ablation zone (arrow) is mildly hyperattenuating relative to the renal parenchyma.
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Figure 3b. (a) Unenhanced CT scan obtained in a 74-year-old woman with RCC during cryoablation shows a low-attenuation ice ball (arrow) surrounding the cryoprobe (arrowhead). (b) Immediate post-cryoablation unenhanced CT scan obtained following thawing and cryoprobe removal shows residual low attenuation in the tumor ablation zone (arrow). (c) Unenhanced CT scan obtained 5 months later during retreatment of the same kidney with RF ablation for recurrent tumor shows fat stranding (arrow) but no appreciable change in attenuation of the ablation zone. Arrowhead indicates the electrode. (d) On an unenhanced CT scan obtained immediately after RF ablation, the ablation zone (arrow) is mildly hyperattenuating relative to the renal parenchyma.
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Figure 3c. (a) Unenhanced CT scan obtained in a 74-year-old woman with RCC during cryoablation shows a low-attenuation ice ball (arrow) surrounding the cryoprobe (arrowhead). (b) Immediate post-cryoablation unenhanced CT scan obtained following thawing and cryoprobe removal shows residual low attenuation in the tumor ablation zone (arrow). (c) Unenhanced CT scan obtained 5 months later during retreatment of the same kidney with RF ablation for recurrent tumor shows fat stranding (arrow) but no appreciable change in attenuation of the ablation zone. Arrowhead indicates the electrode. (d) On an unenhanced CT scan obtained immediately after RF ablation, the ablation zone (arrow) is mildly hyperattenuating relative to the renal parenchyma.
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Figure 3d. (a) Unenhanced CT scan obtained in a 74-year-old woman with RCC during cryoablation shows a low-attenuation ice ball (arrow) surrounding the cryoprobe (arrowhead). (b) Immediate post-cryoablation unenhanced CT scan obtained following thawing and cryoprobe removal shows residual low attenuation in the tumor ablation zone (arrow). (c) Unenhanced CT scan obtained 5 months later during retreatment of the same kidney with RF ablation for recurrent tumor shows fat stranding (arrow) but no appreciable change in attenuation of the ablation zone. Arrowhead indicates the electrode. (d) On an unenhanced CT scan obtained immediately after RF ablation, the ablation zone (arrow) is mildly hyperattenuating relative to the renal parenchyma.
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At MR imaging, the tumor loses signal intensity on T2-weighted images obtained during RF ablation, becoming significantly hypointense relative to its pretreatment appearance (16). This finding has been reported to be useful for assessing the adequacy of ablation when MR imaging guidance is used (17). The signal intensity of the treated tumor on T1-weighted images obtained at low field strength has been reported as more variable immediately following ablation, ranging from hyperintense to hypointense relative to intact renal cortex (16). During cryoablation, the ice ball appears as a well-demarcated signal void on T1-weighted MR images (18).
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Appearance of Thermal Ablation Sites at Surveillance Imaging
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The acute changes of gas, perinephric stranding, and hemorrhage present immediately after RF ablation decrease over time, and a typical "bull’seye" appearance is often evident on the first follow-up scan, particularly when the initial lesion was peripherally located or exophytic (Fig 4). Nonenhancing soft tissue persists within the central ablation zone, consisting of the ablated tumor and possibly some devitalized renal parenchyma. This central zone should be clearly demarcated from the surrounding normally enhancing renal parenchyma at contrast-enhanced CT. The portions of the central ablation zone that do not abut normal renal parenchyma are surrounded by relatively normal-appearing fat, which is in turn surrounded by a thin, soft-tissue-attenuation rim or halo (Fig 4). This peritumoral rim or halo is more typical of exophytic lesions and percutaneously treated lesions than of laparoscopically or surgically ablated lesions (19). More centrally located (endophytic) tumors may eventually develop fat interposition between the ablated tissue and normal renal parenchyma (19).
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Figure 4. Contrast-enhanced CT scan obtained in a 71-year-old woman 5 weeks after RF ablation demonstrates an exophytic tumor with the typical bull’s-eye appearance. An area of nonenhancing soft-tissue attenuation (ablation zone) persists centrally (arrow) and is surrounded by fat and a thin rim of peritu-moral soft-tissue attenuation (arrowhead).
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Typically, an RF ablation site also resembles a bull’s-eye on follow-up MR images (Fig 5). The ablation zone often appears mildly heterogeneous but predominantly bright with T1-weighted sequences at 1.5 T. With T2-weighted sequences, the central mass tends to be hypointense relative to the normal renal parenchyma, although some mild heterogeneity may be present. With both T1- and T2-weighted sequences, a peritumoral rim of variable signal intensity is usually present (20). The signal intensity characteristics of the fat immediately adjacent to an ablation zone are not altered by the ablation process. In addition to maintaining the expected signal intensity of fat with all sequences, this fat suppresses normally with fat-suppression techniques. Therefore, the ablation zone may become difficult to visualize on fat-suppressed T2-weighted images, since the ablation zone is isointense relative to the surrounding suppressed fat (Fig 5c). For this reason, a T2-weighted sequence without fat suppression is often helpful in delineating the ablation zone.
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Figure 5a. Typical MR imaging appearance of an RF ablation site. The patient was an 83-year-old man with RCC who had undergone RF ablation 14 months earlier. (a) Coronal single-shot fast SE T2-weighted MR image shows the ablation zone (arrow) with low signal intensity. A peritumoral rim (arrowhead) creates a bull’s-eye appearance. (b) Axial GRE T1-weighted MR image demonstrates the ablation zone (arrow) with mild heterogeneity but predominantly high signal intensity. (c) On an axial fat-suppressed fast SE T2-weighted MR image, the low-signal-intensity ablation zone (arrow) is isointense relative to the surrounding suppressed fat.
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Figure 5b. Typical MR imaging appearance of an RF ablation site. The patient was an 83-year-old man with RCC who had undergone RF ablation 14 months earlier. (a) Coronal single-shot fast SE T2-weighted MR image shows the ablation zone (arrow) with low signal intensity. A peritumoral rim (arrowhead) creates a bull’s-eye appearance. (b) Axial GRE T1-weighted MR image demonstrates the ablation zone (arrow) with mild heterogeneity but predominantly high signal intensity. (c) On an axial fat-suppressed fast SE T2-weighted MR image, the low-signal-intensity ablation zone (arrow) is isointense relative to the surrounding suppressed fat.
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Figure 5c. Typical MR imaging appearance of an RF ablation site. The patient was an 83-year-old man with RCC who had undergone RF ablation 14 months earlier. (a) Coronal single-shot fast SE T2-weighted MR image shows the ablation zone (arrow) with low signal intensity. A peritumoral rim (arrowhead) creates a bull’s-eye appearance. (b) Axial GRE T1-weighted MR image demonstrates the ablation zone (arrow) with mild heterogeneity but predominantly high signal intensity. (c) On an axial fat-suppressed fast SE T2-weighted MR image, the low-signal-intensity ablation zone (arrow) is isointense relative to the surrounding suppressed fat.
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Because the nonacute RF ablation zone typically shows increased signal intensity on T1-weighted MR images in the absence of viable tumor at 1.5 T, abnormal gadolinium enhancement may be difficult to see on postcontrast fat-suppressed images. Subtracting a precontrast data set from postcontrast data sets may be helpful in detecting subtle areas of enhancement at follow-up, particularly in heterogeneous ablation zones.Care must be taken to ensure identical scanning parameters and respiratory phase between pre-and postcontrast images. A completely treated tumor will appear totally dark on well-registered subtraction images (Fig 6).
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Figure 6a. Usefulness of image subtraction. (a) Axial precontrast fat-suppressed T1-weighted MR image obtained in a 65-year-old man with RCC shows a high-signal-intensity ablation zone (arrow). (b) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image shows persistent high signal intensity that may be difficult to distinguish from subtle enhancement within the ablation zone (arrow). (c) Subtraction image created by subtracting the precontrast data set from the postcontrast data set shows no enhancement in the region of the ablation zone (arrow).
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Figure 6b. Usefulness of image subtraction. (a) Axial precontrast fat-suppressed T1-weighted MR image obtained in a 65-year-old man with RCC shows a high-signal-intensity ablation zone (arrow). (b) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image shows persistent high signal intensity that may be difficult to distinguish from subtle enhancement within the ablation zone (arrow). (c) Subtraction image created by subtracting the precontrast data set from the postcontrast data set shows no enhancement in the region of the ablation zone (arrow).
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Figure 6c. Usefulness of image subtraction. (a) Axial precontrast fat-suppressed T1-weighted MR image obtained in a 65-year-old man with RCC shows a high-signal-intensity ablation zone (arrow). (b) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image shows persistent high signal intensity that may be difficult to distinguish from subtle enhancement within the ablation zone (arrow). (c) Subtraction image created by subtracting the precontrast data set from the postcontrast data set shows no enhancement in the region of the ablation zone (arrow).
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Although the size of the ablation zone may appear to acutely increase in size after renal RF ablation due in part to hemorrhage, most lesions appear to gradually involute at subsequent imaging examinations (Fig 7) (16). However, the regression in size of a renal tumor treated with RF ablation is less than the degree of tumor involution after cryoablation, and a persistent mass typically remains even years after successful RF ablation despite atrophy of the adjacent renal parenchyma (19,21). The presence of a residual mass alone does not imply residual disease.
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Figure 7a. Expected changes in an RF ablation zone over time. (a) Preablation contrast-enhanced CT scan obtained in a 78-year-old-woman shows a small, intraparenchymal RCC (arrow). (b) Immediate postablation CT scan demonstrates a wedge-shaped unenhanced region (arrows) that encompasses the renal tumor as well as some of the surrounding renal parenchyma. (c) On a CT scan obtained 3 years later, the ablation zone (arrow) shows only mild involution, a finding that demonstrates that a substantial (albeit nonviable) mass may persist years after RF ablation.
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Figure 7b. Expected changes in an RF ablation zone over time. (a) Preablation contrast-enhanced CT scan obtained in a 78-year-old-woman shows a small, intraparenchymal RCC (arrow). (b) Immediate postablation CT scan demonstrates a wedge-shaped unenhanced region (arrows) that encompasses the renal tumor as well as some of the surrounding renal parenchyma. (c) On a CT scan obtained 3 years later, the ablation zone (arrow) shows only mild involution, a finding that demonstrates that a substantial (albeit nonviable) mass may persist years after RF ablation.
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Figure 7c. Expected changes in an RF ablation zone over time. (a) Preablation contrast-enhanced CT scan obtained in a 78-year-old-woman shows a small, intraparenchymal RCC (arrow). (b) Immediate postablation CT scan demonstrates a wedge-shaped unenhanced region (arrows) that encompasses the renal tumor as well as some of the surrounding renal parenchyma. (c) On a CT scan obtained 3 years later, the ablation zone (arrow) shows only mild involution, a finding that demonstrates that a substantial (albeit nonviable) mass may persist years after RF ablation.
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As with RF ablation, the ablation zone following cryoablation fails to demonstrate central enhancement at CT and MR imaging performed with intravenous contrast material (Fig 8) (18, 22). However, some authors have reported a peripheral enhancing rim to be a common MR imaging finding in the first few months after laparoscopic cryoablation (23,24). The signal intensity of the ablation zone following cryoablation tends to be variable on T1- and T2-weighted MR images, particularly shortly after ablation. At follow-up, however, most cryoablation zones are isointense relative to renal parenchyma on T1-weighted images and predominantly hypointense on T2-weighted images (23). Although the ablation zone may exceed the initial tumor size immediately after cryoablation, it undergoes more dramatic involution over time than does the typical RF ablation zone (19,21,23). In one series, 32% of 179 tumors were undetectable by 2 years after cryoablation, whereas a detectable mass (ablation zone) was present in 100% of 81 RF ablation patients (21).
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Figure 8a. (a) Coronal single-shot fast SE T2-weighted MR image obtained in a 65-year-old man 5 months prior to cryoablation shows a heterogeneous mass (arrow) within the left kidney. (b) On a coronal single-shot fast SE T2-weighted MR image obtained 2 months after cryoablation, the ablation zone (arrow) demonstrates markedly heterogeneous signal intensity and is significantly larger than the original tumor. (c) Coronal reformatted MR image from an axial gadolinium-enhanced data set created by subtracting the precontrast data set from the postcontrast data set shows no residual enhancement within the ablation zone. Note the presence of a relatively uniform enhancing rim (arrow), a finding that should not be confused with residual tumor.
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Figure 8b. (a) Coronal single-shot fast SE T2-weighted MR image obtained in a 65-year-old man 5 months prior to cryoablation shows a heterogeneous mass (arrow) within the left kidney. (b) On a coronal single-shot fast SE T2-weighted MR image obtained 2 months after cryoablation, the ablation zone (arrow) demonstrates markedly heterogeneous signal intensity and is significantly larger than the original tumor. (c) Coronal reformatted MR image from an axial gadolinium-enhanced data set created by subtracting the precontrast data set from the postcontrast data set shows no residual enhancement within the ablation zone. Note the presence of a relatively uniform enhancing rim (arrow), a finding that should not be confused with residual tumor.
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Figure 8c. (a) Coronal single-shot fast SE T2-weighted MR image obtained in a 65-year-old man 5 months prior to cryoablation shows a heterogeneous mass (arrow) within the left kidney. (b) On a coronal single-shot fast SE T2-weighted MR image obtained 2 months after cryoablation, the ablation zone (arrow) demonstrates markedly heterogeneous signal intensity and is significantly larger than the original tumor. (c) Coronal reformatted MR image from an axial gadolinium-enhanced data set created by subtracting the precontrast data set from the postcontrast data set shows no residual enhancement within the ablation zone. Note the presence of a relatively uniform enhancing rim (arrow), a finding that should not be confused with residual tumor.
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Unusual Appearances after RF Ablation
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Not all tumors treated with RF ablation have a typical appearance at follow-up imaging. Initial tumor location, size, and composition may affect the appearance after treatment. As mentioned previously, it is unusual for an RF ablation zone to demonstrate complete involution at radiography. However, some smaller lesions may eventually become nearly undetectable at subsequent imaging, a finding that is more typical after cryoablation (Fig 9). Cystic lesions may retain a cystic component after treatment, although the lesion tends to involute with time (Fig 10). RF-ablated tumors may demonstrate areas of fat attenuation at CT or signal intensity loss at opposed-phase GRE MR imaging, findings that are presumably related to the preablation lipid content of the tumor (Figs 11, 12).
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Figure 9a. Marked involution of a central RCC in a 78-year-old man with only one kidney who had undergone RF ablation for life-threatening hematuria. (a) Coronal preablation single-shot fast SE T2-weighted MR image shows a small central mass (arrow). (b) Coronal single-shot fast SE T2-weighted MR image obtained 11 months after ablation shows marked involution of the tumor (arrow). This degree of involution is more typical of tumors treated with cryoablation.
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Figure 9b. Marked involution of a central RCC in a 78-year-old man with only one kidney who had undergone RF ablation for life-threatening hematuria. (a) Coronal preablation single-shot fast SE T2-weighted MR image shows a small central mass (arrow). (b) Coronal single-shot fast SE T2-weighted MR image obtained 11 months after ablation shows marked involution of the tumor (arrow). This degree of involution is more typical of tumors treated with cryoablation.
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Figure 10a. Retained cystic component in a 68-year-old woman who underwent RF ablation for cystic RCC. (a) Axial preablation single-shot fast SE T2-weighted MR image shows a large, complex cystic mass (arrow). (b) Axial single-shot fast SE T2-weighted MR image obtained 3 years after ablation shows a significant decrease in lesion size but only incomplete involution (arrow).
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Figure 10b. Retained cystic component in a 68-year-old woman who underwent RF ablation for cystic RCC. (a) Axial preablation single-shot fast SE T2-weighted MR image shows a large, complex cystic mass (arrow). (b) Axial single-shot fast SE T2-weighted MR image obtained 3 years after ablation shows a significant decrease in lesion size but only incomplete involution (arrow).
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Figure 11. Unenhanced CT scan obtained in an 82-year-old man who had undergone RF ablation for clear cell type RCC 19 months earlier shows fat attenuation within the ablation zone (arrow).
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Figure 12a. Residual lipid within the ablation zone in a 69-year-old man who had undergone RF ablation for intraparenchymal clear cell type RCC. (a) Axial in-phase GRE T1-weighted MR image obtained 3 months after RF ablation shows a region of high signal intensity within the ablation zone (arrow). (b) Corresponding axial opposed-phase T1-weighted MR image shows loss of signal intensity in the periphery of the ablation zone (arrow), a finding that indicates the presence of lipid. Gadolinium-enhanced subtraction images showed no evidence of enhancement within the ablation zone.
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Figure 12b. Residual lipid within the ablation zone in a 69-year-old man who had undergone RF ablation for intraparenchymal clear cell type RCC. (a) Axial in-phase GRE T1-weighted MR image obtained 3 months after RF ablation shows a region of high signal intensity within the ablation zone (arrow). (b) Corresponding axial opposed-phase T1-weighted MR image shows loss of signal intensity in the periphery of the ablation zone (arrow), a find-ing that indicates the presence of lipid. Gadolinium-enhanced subtraction images showed no evidence of enhancement within the ablation zone.
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Residual and Recurrent Disease after Thermal Ablation
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Early detection of residual or recurrent disease following initial treatment is one goal of any surveillance protocol. In the interpretation of follow-up images, it is helpful to also review the preablation and ablation images to determine initial tumor size, tumor location, and electrode or cryoprobe placement in an effort to predict areas that are likely to demonstrate recurrence. Eccentric electrode or cryoprobe placement within a mass is likely to result in residual disease at the tumor margin farthest from the ablation device tip.
Residual tumor is suggested when enhancing nodules or crescents are noted in the vicinity of the treated tumor on contrast-enhanced CT scans or MR images (Fig 13) (25).Gadolinium-enhanced fat-suppressed T1-weighted subtraction MR images are helpful in demonstrating subtle areas of enhancement by eliminating the high signal intensity often present within the tumor on unsubtracted images. Because the ablation zone following RF ablation typically has low signal intensity on T2-weighted MR images, a new or enlarging focus of hyperintensity on these images may also be a sign of viable tumor (16). Ablated tumors remain stable in size or involute over time on follow-up images. Therefore, an increase in tumor size after the acute postablation changes have resolved should raise concern for tumor recurrence. Tumor may also recur within the renal vein (Fig 14). Therefore, it is always important to scrutinize the renal vein and inferior vena cava on surveillance images for evidence of interval enlargement or abnormal enhancement. Occasionally, a new tumor focus may develop in either the same kidney or the contralateral kidney; thus, it is important not only to evaluate the treatment site, but also to treat each surveillance scanning session as a primary tumor search (Fig 15). Finally, metastatic disease may manifest at post-treatment imaging that was either not apparent or unappreciated at pretreatment imaging. A case of skin metastasis from RCC at the electrode insertion site after percutaneous renal RF ablation has been reported (14).
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Figure 13a. Residual RCC in a 57-year-old man who had undergone RF ablation followed by cryoablation. (a) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained 1 month after RF ablation shows peripheral nodular enhancement within the ablation zone (arrows), a finding that is consistent with residual disease. (b) Unenhanced CT scan obtained 1 week after cryoablation for residual tumor shows resolving hemorrhage (arrowhead) and perinephric stranding surrounding the ablation zone (arrow). (c) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained through the same region of the kidney 9 weeks after cryoablation shows that the original areas of residual disease no longer enhance. Arrow indicates the ablation zone. (d) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image through the upper pole of the kidney shows a new focus of nodular enhancement (arrow), a finding that indicates residual tumor.
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Figure 13b. Residual RCC in a 57-year-old man who had undergone RF ablation followed by cryoablation. (a) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained 1 month after RF ablation shows peripheral nodular enhancement within the ablation zone (arrows), a finding that is consistent with residual disease. (b) Unenhanced CT scan obtained 1 week after cryoablation for residual tumor shows resolving hemorrhage (arrowhead) and perinephric stranding surrounding the ablation zone (arrow). (c) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained through the same region of the kidney 9 weeks after cryoablation shows that the original areas of residual disease no longer enhance. Arrow indicates the ablation zone. (d) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image through the upper pole of the kidney shows a new focus of nodular enhancement (arrow), a finding that indicates residual tumor.
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Figure 13c. Residual RCC in a 57-year-old man who had undergone RF ablation followed by cryoablation. (a) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained 1 month after RF ablation shows peripheral nodular enhancement within the ablation zone (arrows), a finding that is consistent with residual disease. (b) Unenhanced CT scan obtained 1 week after cryoablation for residual tumor shows resolving hemorrhage (arrowhead) and perinephric stranding surrounding the ablation zone (arrow). (c) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained through the same region of the kidney 9 weeks after cryoablation shows that the original areas of residual disease no longer enhance. Arrow indicates the ablation zone. (d) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image through the upper pole of the kidney shows a new focus of nodular enhancement (arrow), a finding that indicates residual tumor.
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Figure 13d. Residual RCC in a 57-year-old man who had undergone RF ablation followed by cryoablation. (a) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained 1 month after RF ablation shows peripheral nodular enhancement within the ablation zone (arrows), a finding that is consistent with residual disease. (b) Unenhanced CT scan obtained 1 week after cryoablation for residual tumor shows resolving hemorrhage (arrowhead) and perinephric stranding surrounding the ablation zone (arrow). (c) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image obtained through the same region of the kidney 9 weeks after cryoablation shows that the original areas of residual disease no longer enhance. Arrow indicates the ablation zone. (d) Axial gadolinium-enhanced fat-suppressed T1-weighted MR image through the upper pole of the kidney shows a new focus of nodular enhancement (arrow), a finding that indicates residual tumor.
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Figure 14a. Renal vein tumor in a 73-year-old man who underwent RF ablation for RCC. (a) Axial fast SE T2-weighted MR image obtained 4 months after RF ablation shows expansion of and abnormal signal intensity within the left renal vein (arrow), findings that are indicative of tumor thrombus. Arrowhead indicates the ablation zone. (b) Axial fast SE T2-weighted MR image obtained 11 months after RF ablation shows an increase in the size of the tumor within the left renal vein (arrow). This recurrence had not been appreciated previously. Arrowhead indicates the ablation zone.
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Figure 14b. Renal vein tumor in a 73-year-old man who underwent RF ablation for RCC. (a) Axial fast SE T2-weighted MR image obtained 4 months after RF ablation shows expansion of and abnormal signal intensity within the left renal vein (arrow), findings that are indicative of tumor thrombus. Arrowhead indicates the ablation zone. (b) Axial fast SE T2-weighted MR image obtained 11 months after RF ablation shows an increase in the size of the tumor within the left renal vein (arrow). This recurrence had not been appreciated previously. Arrowhead indicates the ablation zone.
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Figure 15a. Development of a new tumor focus in a 74-year-old man who had undergone RF ablation for RCC. (a) Axial single-shot fast SE T2-weighted MR image obtained 7 months after ablation shows decreased signal intensity in the ablation zone (arrowhead). (b) Axial single-shot fast SE T2-weighted MR image obtained 14 months after ablation shows a new tumor (arrow) adjacent to the previous ablation zone (arrowhead).
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Figure 15b. Development of a new tumor focus in a 74-year-old man who had undergone RF ablation for RCC. (a) Axial single-shot fast SE T2-weighted MR image obtained 7 months after ablation shows decreased signal intensity in the ablation zone (arrowhead). (b) Axial single-shot fast SE T2-weighted MR image obtained 14 months after ablation shows a new tumor (arrow) adjacent to the previous ablation zone (arrowhead).
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Complications
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Fortunately, complications following thermal ablation procedures for RCC are usually minor (26). However, CT scans and MR images obtained during and after thermal ablation of renal tumors should always be evaluated for evidence of procedure-related complications. Hemorrhage is one of the most common radiologically evident complications related to RF ablation and cryoablation (Fig 16). Hemorrhage may be subcapsular or perinephric in location or may extend into the pararenal spaces or collecting system. In general, acute hemorrhage will be of higher attenuation than simple fluid at unenhanced CT. Sterile water that was used to displace bowel (hydrodissection) should not be confused with hemorrhage on immediate postprocedure CT scans (Fig 16b). However, because water quickly dissipates, fluid collections present on subsequent follow-up images cannot be attributed to hydrodissection. Hemorrhage into the collecting system may manifest as gross hematuria and may occasionally require placement of a ureteral stent or bladder catheter (27–29).
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Figure 16a. Typical contrast-enhanced CT appearance of postablation hemorrhage. (a) Contrast-enhanced CT scan obtained immediately after RF ablation for RCC shows a large perirenal and pararenal hemorrhage with active extravasation of contrast material (arrow). The hemorrhage was self limiting and did not require intervention. (b) Contrast-enhanced CT scan obtained immediately after RF ablation in a different patient shows a sub-capsular hematoma (arrowhead). Note the lower-attenuation sterile water (arrow) used to displace bowel during ablation. The patient required no intervention for bleeding.
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Figure 16b. Typical contrast-enhanced CT appearance of postablation hemorrhage. (a) Contrast-enhanced CT scan obtained immediately after RF ablation for RCC shows a large perirenal and pararenal hemorrhage with active extravasation of contrast material (arrow). The hemorrhage was self limiting and did not require intervention. (b) Contrast-enhanced CT scan obtained immediately after RF ablation in a different patient shows a sub-capsular hematoma (arrowhead). Note the lower-attenuation sterile water (arrow) used to displace bowel during ablation. The patient required no intervention for bleeding.
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Ureteral strictures related to proximity of the ureter to the ablation site take time to develop and may not be apparent on initial postprocedure images. However, postablation ureteral thickening and periureteral stranding should raise the possibility of subsequent stricture formation (Fig 17). The appearance of the collecting system on follow-up images should be compared with that on earlier images to detect evidence of developing dilatation.Urine leaks may also occur after renal tumor ablation and may manifest as an accumulation of excreted contrast material outside the collecting system within the ablation zone (25,29). There is some evidence to suggest that cryoablation may be associated with a lower risk of collecting system injury than RF ablation (30,31).
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Figure 17a. Ureteral stricture following RF ablation of an RCC in the lower right renal pole in a 49-year-old man. (a) Unenhanced CT scan shows the RF electrode (arrowhead) in place within the lower pole mass. Note the location of the proximal right ureter (arrow). (b) Immediate postablation contrast-enhanced CT scan shows ureteral thickening and periureteral stranding (arrow). (c) Coronal single-shot fast SE T2-weighted MR image obtained 15 months after RF ablation shows hydronephrosis and proximal ureteral obstruction (arrow) due to a ureteral stricture. The patient elected not to undergo intervention for the stricture.
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Figure 17b. Ureteral stricture following RF ablation of an RCC in the lower right renal pole in a 49-year-old man. (a) Unenhanced CT scan shows the RF electrode (arrowhead) in place within the lower pole mass. Note the location of the proximal right ureter (arrow). (b) Immediate postablation contrast-enhanced CT scan shows ureteral thickening and periureteral stranding (arrow). (c) Coronal single-shot fast SE T2-weighted MR image obtained 15 months after RF ablation shows hydronephrosis and proximal ureteral obstruction (arrow) due to a ureteral stricture. The patient elected not to undergo intervention for the stricture.
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Figure 17c. Ureteral stricture following RF ablation of an RCC in the lower right renal pole in a 49-year-old man. (a) Unenhanced CT scan shows the RF electrode (arrowhead) in place within the lower pole mass. Note the location of the proximal right ureter (arrow). (b) Immediate postablation contrast-enhanced CT scan shows ureteral thickening and periureteral stranding (arrow). (c) Coronal single-shot fast SE T2-weighted MR image obtained 15 months after RF ablation shows hydronephrosis and proximal ureteral obstruction (arrow) due to a ureteral stricture. The patient elected not to undergo intervention for the stricture.
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Significant bowel injuries are unusual following renal tumor ablation, provided appropriate care is taken at the time of the procedure. However, colonic perforation and col
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Abstract
Introduction
