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Renal imaging for diagnosis and staging of renal cell carcinoma
Urol Clin N Am 30 (2003) 499–514
Renal imaging for diagnosis and staging of renal cell carcinoma Gary M. Israel, MD*, Morton A. Bosniak, MD Division of Abdominal Imaging, Department of Radiology, New York University Medical Center, 560 First Avenue, New York, NY 10016, USA
Radiologic evaluation of renal masses has rapidly progressed from conventional angiography and intravenous urography to CT and MRI. As more patients are imaged for various reasons, the incidental detection of renal masses has increased. Although most of these lesions are benign cysts, neoplastic disease is also uncovered. Some of these lesions require surgery, whereas others do not. The ability to characterize these lesions accurately, to determine their appropriate management, and to stage renal malignancies has been afforded by continuously improving imaging techniques. This article reviews the use of CT and MRI in the diagnosis and staging of malignant renal disease and its differentiation from nonsurgical renal masses. Technique and quality of the radiologic examination The radiologic diagnosis of a renal mass is dependent on high-quality imaging studies, independent of the imaging modality used. In clinical practice, many lesions are labeled ‘‘indeterminate’’ not secondary to their character but owing to the quality of the examination. Although radiologists are responsible for performing and optimizing each examination, referring urologists must also be cognizant of what constitutes an acceptable examination. In this way, urologists can assume a more informed role in the diagnosis
* Corresponding author. E-mail address: [email protected] (G.M. Israel).
and can realize when the examination is suboptimal. Many renal masses are discovered during routine abdominal CT scanning performed for other reasons. In many of these examinations, the diagnosis of the renal mass is obvious and clear cut. Most of these lesions are simple cysts, but some large neoplasms are found as well. Often, the diagnosis is not certain, and a repeat CT scan dedicated to the kidneys is necessary to diagnose accurately the lesion in question. A repeat scan also may be needed when a complex lesion is detected on sonography and needs further evaluation. A high-quality CT scan dedicated to the kidneys includes images obtained before and after intravenous contrast administration. An adequate amount of contrast material given as a rapid bolus with a power injector is necessary to ensure that a high blood concentration of contrast is present at the time of scanning. The slice collimation can be variable depending on the size of the lesion in question. The authors recommend starting with 5mm sections, followed by thinner sections (3 mm or less) through the lesion or the suspect portion of the lesion if necessary. The scan delay (the time after the contrast is given at which the CT scan is started) is critical. After contrast administration, images are acquired after a minimum of 90 seconds (the nephrographic phase of enhancement). This delay ensures that the images are obtained when renal enhancement and renal neoplasm enhancement are maximal. Examinations obtained too early after the administration of intravenous contrast (during the corticomedullary phase of enhancement) are not adequate for optimal characterization of renal neoplasms [1,2].
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The scan parameters of the precontrast and postcontrast examinations must be constant to ensure that the most accurate Hounsfield unit (HU) measurements are obtained. More recently, multidetector CT has been introduced into clinical practice. This modality allows for the acquisition of large volumetric data sets with nearly isotropic voxels during a single breath hold. The increased speed of multidetector CT allows for dynamic multiphase image acquisition, which enables the radiologist to create meaningful multiplanar reformations and to perform three-dimensional volume rendering. With the advent of nephron-sparing surgery and the recent surgical advances in laparoscopic techniques, multidetector CT provides important preoperative information to the surgeon. In addition, it may be helpful in cases in which percutaneous ablation therapy is possible. One can create images that clearly demonstrate the relationship of a neoplasm to the surrounding renal parenchyma, perirenal fat, and neighboring organs, as well as three-dimensional images with angiographic and urographic renderings (Fig. 1) [3,4]. When the images are analyzed at a workstation in real time, the surgeon can view them in any orientation, depicting invaluable information before surgery. This preoperative road map can aid in complete tumor excision with minimal morbidity. In most cases, conventional angiography for preoperative vascular mapping is no longer needed (Fig. 2). In patients who cannot receive iodinated intravenous contrast material because of allergy or renal insufficiency, MRI offers the advantage of evaluating patients without exposure to nephrotoxic contrast material. Gadolinium has been shown to be remarkably safe, can be administered to patients without concern for contrast-induced nephrotoxicity, and is well tolerated in patients with a history of iodinated contrast allergy [5–7]. Further advantages of MRI include the lack of ionizing radiation, the direct multiplanar capability that enables more accurate localization of masses, and the superior intrinsic soft tissue contrast augmented by the use of gadolinium. Over the past 10 years, the quality and reliability of MRI have improved to the point at which it is now considered at least equivalent to CT in the detection and characterization of renal masses. In a group of patients, MRI may be indicated in lieu of CT. This group includes patients with mildly reduced renal function
(patients with renal insufficiency who are not on dialysis) and patients with a solitary kidney. In addition, MRI should be the test of choice in patients who have undergone nephron-sparing surgery and who are in need of serial follow-up examinations. The increased load of potentially nephrotoxic iodinated contrast material delivered to the remaining renal tissue on a per nephron basis in these patients has made clinicians and radiologists hesitant to administer a full dose of contrast material [8]. Occasionally, MRI can characterize a lesion that cannot be diagnosed by CT. Nevertheless, MRI should not be performed because a poorquality CT scan was obtained. In these cases, the CT should be repeated and optimized. If the lesion is still indeterminate, MRI could then be considered. A high-quality MR examination incorporates high-performance gradients with a torso-phased array coil and a variety of breath hold sequences, which can be obtained in multiple planes. Although debate continues as to which sequences are necessary to characterize a renal mass, the authors recommend a combination of T1-weighted images obtained with and without frequency-selective fat suppression and T2weighted images. As is true for CT, it is critical to be able to demonstrate enhancement within a renal mass; therefore, images need to be obtained before and after gadolinium administration. The authors recommend a three-dimensional, fat-suppressed, T1-weighted sequence, which can be reconstructed in any plane desired and also used to perform MR angiography (Fig. 3). In many cases, it is possible to determine the enhancement of a lesion by performing a side-byside comparison of the precontrast and postcontrast acquisitions; however, in cases in which a lesion is hypovascular or hyperintense on the precontrast images, qualitative enhancement is difficult if not impossible to appreciate. In this instance, a subtraction algorithm can be applied to help assess enhancement characteristics [9]. Solid renal masses: differentiating surgical from nonsurgical lesions In general, any renal mass that enhances with intravenous contrast should be considered a renal cell carcinoma until proven otherwise; however, not all enhancing solid renal masses represent renal cell carcinomas. One must be able to differentiate masses that require surgery (renal
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Fig. 1. (A) Axial contrast-enhanced image from a CT scan in a 45-year-old man with a right renal tumor and bilateral renal calculi demonstrates a 2-cm enhancing renal neoplasm in the right kidney. In the axial plane, it is difficult to determine the relationship of the tumor to its surrounding structures. (B) Coronal maximum intensity projection image demonstrates the relationship of the neoplasm to the renal pelvis (P) and vasculature (arrow) supplying the kidney. Note the multiple calyceal and pelvic calculi. This projection aided the surgeon in planning and performing a successful partial nephrectomy for renal cell carcinoma.
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Fig. 2. Axial oblique three-dimensional volume rendered image in a 63-year-old woman with a renal cell carcinoma in the right kidney obtained from data acquired using a multidetector CT scan. An exophytic solid enhancing renal mass is demonstrated in the right kidney (short arrow). A single renal artery (open arrow) and vein (curved arrow) are clearly demonstrated. Note the defect (long arrow) in the kidney secondary to a small cyst.
cell carcinoma, oncocytoma) from those that do not (lymphoma, most angiomyolipomas, pseudotumors, and metastatic disease to the kidneys). Generally, a combination of the clinical history and imaging findings can be used to make this distinction; however, in some patients, such as in those who have lymphoma or a metastatic neoplasm to the kidney, percutaneous needle aspiration may be performed for confirmation. Renal cell carcinoma Renal cell carcinoma is the most common renal neoplasm, accounting for 80% to 85% of all malignant renal tumors and 2% of all cancers. The widespread use of cross-sectional imaging and the incidental detection of asymptomatic neoplasms have increased the incidence of renal cell carcinoma [10,11]. The increased detection
combined with the improved characterization of small renal lesions and subsequent earlier surgical intervention has resulted in a slight improvement in the 5-year survival for renal cell carcinoma. Computed tomography is the imaging modality of choice for evaluating patients with a renal mass and has been demonstrated to be accurate in the diagnosis and staging of renal cell carcinoma. With the recent advances in technology and the development of multidetector CT, it is now possible to obtain thinner slices (on the order of 1 mm) with near isotropic data sets in less time when compared with single-slice helical CT. This improvement should enhance the ability to characterize small renal masses accurately as partial volume averaging and respiratory motion are virtually eliminated. The prognosis of renal cell carcinoma is related to the tumor stage. CT and MRI are accurate in
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Fig. 3. (A) Coronal T2-weighted image in a 55-year-old woman with a renal cell carcinoma demonstrates an exophytic 5-cm complex cystic and solid neoplasm arising from the lower pole of the right kidney. (B) Coronal maximal intensity projection of an MR angiogram performed as part of the comprehensive MR examination shows a single renal artery supplying the right kidney with branches supplying the neoplasm.
the staging of renal cell carcinoma and are the examinations of choice in preoperative assessment [12]. Semelka et al [13] demonstrated that MRI was more accurate than CT for the evaluation of tumor extension into the renal vein and inferior vena cava; however, with the advent of multidetector CT scanning, it is unclear whether this proposed advantage still holds true. Regardless, it is important to demonstrate the most cephalad extent of thrombus in the inferior vena cava, because the surgical approach will be altered if thrombus extends into the right atrium (Fig. 4). The multiplanar capability of MRI and multidetector CT are ideally suited for this determination. In addition, it is often possible to differentiate tumor thrombus from bland thrombus, because tumor thrombus is vascular and bland thrombus is not (Fig. 5). Currently, CT and MRI cannot distinguish between metastatic and hyperplastic lymph nodes; therefore, the evaluation for metastatic adenopathy is size dependent, with lymph nodes greater than 1 cm considered abnormal and suspicious for
metastatic disease. Patients with renal cell carcinoma may have enlarged hyperplastic retroperitoneal lymph nodes, a known pitfall in the staging of renal cell carcinoma with CT or MRI [14]. Many small (1–2.5 cm) incidental neoplasms are found in the kidneys during routine abdominal scanning using sonography, CT, or MRI. When these lesions are found in young healthy patients, they are removed by open or laparoscopic partial nephrectomy. Nevertheless, not all of these small tumors need to be removed, and, in some instances, many of them can be managed by watchful waiting. As is true for other malignancies in the body, the discovery of very small renal cancers will not ensure that mortality from cancer will be reduced. The incidental discovery of tiny renal lesions might simply reflect the detection of early cancers that will be lethal in any case (lead-time bias), the detection of cancers that are growing slowly and are less likely to be lethal (length-time bias), or the detection of tumors with questionable malignant potential (overdiagnosis bias) [15].
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Fig. 4. (A) Coronal gadolinium-enhanced, T1-weighted image in a 55-year-old woman with a renal cell carcinoma demonstrates a complex enhancing neoplasm in the upper pole of the right kidney that on other images was shown to be clearly separate from the liver. (B) Two consecutive gadolinium-enhanced, T1-weighted images demonstrate thrombus within the right renal vein (short arrow) and inferior vena cava (long arrows). The thrombus extends into the intrahepatic portion of the inferior vena cava but not into the right atrium. A flow artifact is seen in the superior aspect of the inferior vena cava. The patient underwent nephrectomy for renal cell carcinoma, but tumor recurrence in the postoperative bed and bone metastases occurred 2 years later.
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Fig. 5. Coronal gadolinium-enhanced, T1-weighted image in a 57-year-old woman with a large renal cell carcinoma demonstrates a large neoplasm that replaces the entire left kidney (T). Enhancing tumor thrombus is seen in the left renal vein (short arrows) and extends up the inferior vena cava (long arrow) to the level of the right hemidiaphragm. Nonenhancing bland thrombus is present in the infrarenal portion of the inferior vena cava (curved arrows). The patient underwent nephrectomy for renal cell carcinoma including removal of the tumor thrombus from the inferior vena cava and is alive 5 years later.
For all of these reasons and because of the previously reported data on tumor growth showing very slow growth in many of these small tumors [16], the use of watchful waiting in the management of some of these lesions seems reasonable (Fig. 6). The use of this approach will depend on the patient’s age, clinical condition, and associated diseases, and the size and imaging appearance of the tumor when first encountered. Patients who are elderly and fragile, who are poor surgical risks, or who have other significant disease and a well-marginated renal lesion less than 2.0 cm can be observed by watchful waiting. This approach can be extended to lesions up to 2.5 or 3.0 cm in size, particularly in elderly or poor-risk patients. These patients should
have follow-up studies to assess the growth rate of the lesion. Further decisions as to management can then be made periodically. If significant growth is detected and surgery is clinically possible, it should be performed. If the patient is not a surgical candidate, percutaneous ablation techniques can be used. If slow or no growth is demonstrated, further watchful waiting seems reasonable. Follow-up studies performed at 6-month or 1-year intervals should be carried out depending on the previous growth rate. With the introduction of laparoscopic partial nephrectomy (and if radiofrequency and cryoablation techniques prove safe and effective), watchful waiting will have less of a role in management and will be used in patients who are
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Fig. 6. (A) Axial contrast-enhanced CT image in a 75-year-old man with a small renal neoplasm shows a 1.5-cm enhancing renal mass (arrow) in the right kidney. The patient was a poor surgical candidate secondary to cardiac disease, and it was decided to use a watchful waiting approach in management. (B) Follow-up examination 5 years later demonstrates minimal growth of the neoplasm (arrow), which now measures 2 cm. Note that the adjacent renal cyst has grown slightly in the interval.
fragile or who have a clearly shortened projected lifespan. Nonsurgical lesions Angiomyolipoma Angiomyolipoma (renal hamartoma), a benign tumor, is composed of varying amounts of fat, smooth muscle, and blood vessels. It is an uncommon lesion with a prevalence of 0.3% to 3% and occurs more commonly in women than men. Angiomyolipomas occur in two different clinical scenarios. More commonly, they are sporadic (80%); however, they may be associated with tuberous sclerosis (20%), in which case they tend to be multiple and bilateral [17]. Patients are usually asymptomatic, and angiomyolipomas are usually incidentally discovered when the patient is imaged for another reason. In most instances, they do not require surgical intervention; however, when they are large, angiomyolipomas may exert mass effect on the adjacent organs and cause symptoms. In addition, patients with large angio-
myolipomas may present with acute flank pain caused by spontaneous hemorrhage. This event may be life threatening and require embolization to stop the bleeding or surgical intervention if angiographic embolization techniques are successful or available [18]. Angiomyolipoma is the only renal tumor that can be characterized based on its tissue composition. The relative amounts of fat, smooth muscle, and vessels within the tumor establish its CT and MRI appearance. The diagnosis of angiomyolipoma rests on demonstrating macroscopic fat within the lesion. CT and MRI can establish the presence of fat within a lesion, and the diagnosis of angiomyolipoma can be made in virtually every case [20]. On CT, the fatty portions of the lesion will measure ÿ30 HU or less. If a very small amount of fat is suspected in a renal mass, it may be necessary to perform very thin sections without intravenous contrast to maximize the ability to depict fat within the lesion [19]. On MRI, the fatty portion of an angiomyolipoma will demonstrate
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hyperintense signal on the T1-weighted images; however, other renal masses, including hemorrhagic cysts, may show similar signal characteristics. It is imperative to compare the T1-weighted images obtained with frequency-selective fat suppression with those obtained without fat suppression to establish definitively the presence or absence of macroscopic fat [20]. In the extremely rare instance in which these lesions do not contain any fat (angiomyoma), the preoperative diagnosis of hamartoma cannot be made, and the lesion is indistinguishable from a renal cell carcinoma. Recent case reports have demonstrated that if a renal mass contains calcification in addition to fat, the possibility of renal cell carcinoma must be seriously considered [21,22]. A clear diagnosis of angiomyolipoma cannot be made if calcium is present within a fat-containing lesion, and surgery is indicated in such cases. Likewise, a few cases of large bulky renal cell carcinomas containing small amounts of macroscopic fat have been reported [23]. These lesions over 10 cm in diameter (not associated with tuberous sclerosis) must also be evaluated surgically, because it is difficult to determine whether the lesion has engulfed renal sinus fat or perirenal fat, or has even engulfed a coexistent angiomyolipoma.
Lymphoma Lymphoma may involve the kidneys via hematogenous spread, in which a single mass or multiple bilateral masses are present, or by direct extension of retroperitoneal lymphoma. Generally, most patients with renal lymphoma have systemic involvement; therefore, the diagnosis should not be difficult given the appropriate clinical history. The imaging appearance of lymphoma is nonspecific. The most common appearance is that of multiple homogeneous solid masses that may be well defined but that tend to have infiltrative margins with the kidney. When lymphoma diffusely infiltrates a kidney, the kidney enlarges but maintains its reniform shape [24,25]. In patients with known lymphoma and in whom there is a single focal mass in the kidney that could represent lymphoma or a coexistent renal cell carcinoma, treatment of the lymphoma is indicated. If the lymphoma responds to treatment but the renal mass does not, renal cell carcinoma is the likely diagnosis. This diagnosis could be confirmed by needle biopsy if any doubt exists.
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Metastases to the kidney The most common tumor to metastasize to the kidney is carcinoma of the lung. Renal metastases tend to be multiple and bilateral and frequently are associated with metastases to other organs. Although they have nonspecific CT and MRI features, renal metastases commonly demonstrate infiltrative growth patterns. With the proper clinical history, the diagnosis should be obvious. In a patient with a history of malignancy (without other metastases) and a solitary renal mass, the renal mass is more likely to represent a renal cell carcinoma and not a metastasis [26]. Nevertheless, it is possible that a single renal metastasis could occur, and differentiation from a renal cell carcinoma may not be obvious. In this situation, a renal biopsy is indicated to determine proper management. Cystic renal masses Cystic renal masses are common, and, fortunately, it is not difficult to differentiate a simple renal cyst from a cystic renal cell carcinoma. Nevertheless, the differentiation of benign complex cystic masses from those that require surgical intervention can be difficult and is a common problem in clinical practice. The characterization of these lesions includes an evaluation of the thickness of the wall, the presence, number, and thickness of any septae, the presence and amount of calcification, the interface of the lesion with the kidney, and, most importantly, the presence or absence of any enhancing soft tissue components [27–30]. The Bosniak classification of renal cysts introduced in 1986 is a useful tool to evaluate these lesions and decide on clinical management [27]. This classification has been accepted worldwide as a way of approaching these lesions. Although there is interobserver variation in some cases, there is a general agreement in most instances [31– 35]. The classification enables urologists and radiologists to communicate about these cases and creates a framework for decisions on management of these lesions. Cystic lesions are classified into several categories based on the imaging findings. Category I masses are simple benign cysts with thin walls. They measure water density, do not contain septations or calcification, and do not enhance. Category II masses are minimally complicated benign cystic lesions. They may contain a few hairline thin septations and fine calcification in their walls or septae (Fig. 7). They
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Fig. 7. Axial contrast-enhanced CT image in a 52-year-old woman demonstrates a minimally complicated category II cyst in the left kidney that contains fine calcification in thin septae.
do not enhance. Small (<3 cm) hyperdense cysts are included in this category. An important subset of category II lesions are category IIF lesions (‘‘F’’ for follow-up). These lesions are slightly more complex than most category II lesions and do not fall neatly into category II. They do not require surgical exploration but do require follow-up examinations to
prove that they are not growing or becoming more complex. These lesions are thought to be benign but require further follow-up to be certain of this diagnosis. They may contain increased numbers of thin septae or a slightly thickened but smooth wall or septum (Fig. 8). They may also contain calcification, which may be thicker and nodular [36]. Completely intrarenal high-density cysts
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Fig. 8. Axial contrast-enhanced CT image in a 43-year-old man demonstrates a 4-cm category IIF left renal cyst that has a smooth but slightly thickened and calcified wall (straight arrow) and septation (curved arrow). Follow-up examinations performed up to 4 years later (not shown) demonstrated no change in the lesion, consistent with a benign cyst.
larger than 3 cm also fall into this group. Follow-up examinations demonstrating stability enable a diagnosis of benignity to be established. Any increase in the thickness of the wall or septae would indicate a lesion that needs to be explored surgically. Follow-up studies should initially be performed at 6 months and then at 1-year intervals. Category III lesions are indeterminate cystic masses and have some findings that are present in
malignant lesions. Consequently, they cannot be distinguished clearly from malignancy by imaging studies alone and are surgical lesions. They may have grossly thickened irregular walls or septations and may contain either small or large amounts of calcium. Enhancement of the wall or septae can be clearly appreciated (Figs. 9 and 10). Surgical exploration is necessary unless clinical aspects necessitate a follow-up approach. The
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Fig. 9. Axial contrast-enhanced CT image in a 55-year-old man shows a 6-cm category III left renal cystic mass with a grossly thickened and enhancing wall. This lesion was indeterminate although highly suspicious for malignancy. The lesion proved to represent a necrotic renal cell carcinoma at nephrectomy.
surgical approach can range from exploration and biopsy to enucleation to partial nephrectomy to radical nephrectomy. Although the surgeon ultimately decides which type of surgery will be performed, a determination can often be made as to whether the lesion is more likely to be benign or malignant. This determination along with the other clinical aspects of the patient, the size of the lesion, and its position in the kidney will help in the selection of the surgical approach. These lesions are frequently indeterminate on gross inspection at surgical exploration, and a final determination of whether they are benign or malignant may only be made by histologic study. In general, approximately 50% of category III
lesions are benign (hemorrhagic cysts, infected cysts, complex septated cysts, or multilocular cystic nephroma), and 50% represent cystic neoplasms. Category IV lesions are malignant cystic masses that contain enhancing soft tissue components independent of the thickened wall (Fig. 11) and rarely cause difficulty in diagnosis. A tumor in the wall of a cyst and a necrotic cystic malignancy are included in this category and are usually readily diagnosed. Although this classification scheme is based on CT criteria, the same approach provides a useful framework for MRI [37]. Nevertheless, there is not always a clear correlation between the findings
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Fig. 10. Axial contrast-enhanced CT image in a 42-year-old woman shows a 7-cm category III right renal cystic lesion with an enhancing thick and irregular wall that contains a small amount of calcification. The patient presented with fever, leukocytosis, right flank pain, and a history of chronic urinary tract infections. Needle aspiration was performed, and pus was recovered, confirming the diagnosis of an infected renal cyst. The patient was treated with catheter drainage.
at MRI and the CT images, and further work is needed to identify these differences. Just as the internal architecture of cystic renal masses may be appreciated more readily with ultrasound than with CT, the findings on MRI may need further study using another modality. Pseudoenhancement of simple renal cysts The enhancement pattern of cystic renal masses is critical in their proper characterization and treatment. Using CT equipment available in the 1980s and early 1990s, an increase in attenuation of at least 10 HU was suggested as evidence of enhancement [27]. With the advent of faster helical scanners, some simple cysts (category I) may demonstrate enhancement of greater than 10 HU [38,39], referred to as ‘‘pseudoenhancement’’; therefore, an increase in attenuation of approximately 15 to 20 HU has now been proposed as evidence of enhancement [39]. The
presence of enhancement must be unequivocal and is the most important determination of whether a lesion is benign or malignant. Pseudoenhancement has tremendous clinical implications and may lead to the erroneous characterization of a simple cyst as a renal neoplasm, resulting in unnecessary surgery. One must be aware of this phenomenon and know when to suspect it. Many of these cysts are small and completely intraparenchymal. If pseudoenhancement of a lesion is suspected, another examination such as ultrasound or MRI should be performed for definite characterization of the lesion as a simple cyst. Pseudotumors and tumorlike masses of the kidney Occasionally, a benign condition may present as a renal mass, and it may be difficult to differentiate it from a renal neoplasm. This group of lesions includes congenital anomalies,
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Fig. 11. Axial contrast-enhanced CT image in a 71-year-old man demonstrates a 6-cm complex category IV right renal cystic mass that contains enhancing soft tissue components indicative of a cystic renal cell carcinoma.
inflammatory masses, hematoma, hydrocalyx, and vascular structures. Careful attention to the imaging characteristics of the lesion combined with an appreciation of its clinical context will result in the correct diagnosis in almost every case. Renal pseudotumors refer to normal renal tissue that may mimic a renal neoplasm and that may be congenital or acquired in nature. Congenital normal variants, including prominent renal columns of Bertin, renal dysmorphism, and dromedary humps, may all be mistaken for a renal mass. In addition, hypertrophy of a portion of the renal parenchyma adjacent to cortical scarring may assume a tumorlike appearance. An awareness of the possibility that a benign condition may mimic a neoplastic process and careful evaluation of the findings of a high-quality examination should reveal the true nature of these lesions. The
key in the proper diagnosis of these pseudotumors is to demonstrate that the lesion enhances identically to the renal parenchyma. In these situations, it may be advantageous to scan not only during the nephrographic phase of enhancement but also during the corticomedullary phase to demonstrate corticomedullary differentiation in the suspect lesion. Pyelonephritis and renal abscess may be mistaken for a renal neoplasm on imaging studies; however, when placed in the proper clinical context, the correct diagnosis usually becomes evident. Nevertheless, differentiating cystic neoplasms from a subacute or chronic renal abscess when the typical clinical findings of infection are not present can be difficult. If a remote history of fever, leukocytosis, or urinary tract infection is obtained, the possibility of a chronic renal
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infection should be considered. This situation is one of the few times that needle aspiration of a cystic mass is prudent. If pus is recovered, percutaneous drainage can be performed. If blood or necrotic debris is recovered, surgical removal is indicated. A renal artery aneurysm or arteriovenous fistula may present as an enhancing renal mass. CT or MR examination performed with a good bolus of contrast material will clearly demonstrate the vascular nature of the lesion. If the contrast material is not administered as a bolus or if the contrast-enhanced portion of the examination is performed during the excretory phase of enhancement, the true nature of the lesion may not be evident. Summary The development and expansion of CT and MRI technology have enhanced the detection and characterization of renal lesions. Although these advancements should lead to earlier diagnosis of renal cell carcinoma with subsequent improved cure rates, the increased imaging has also uncovered many cases that are problematic not only in diagnosis but in management as well. The performance of high-quality examinations combined with growing experience should improve the ability to diagnose and manage these cases successfully. Continued advances in CT and MR technology combined with the current trend toward minimally invasive surgery will continue to expand the role of preoperative imaging and, it is hoped, improve the cure rate of renal cancer. References [1] Birnbaum BA, Jacobs JE, Ramchandani P. Multiphasic renal CT: comparison of renal mass enhancement during the corticomedullary and nephrographic phases. Radiology 1996;200:753–8. [2] Cohan RH, Sherman LS, Korobkin M, et al. Renal masses: assessment of corticomedullary-phase and nephrographic-phase CT scans. Radiology 1995; 196:445–51. [3] Coll DM, Herts BR, Davros WJ, et al. Preoperative use of 3D volume rendering to demonstrate renal tumors and renal anatomy. Radiographics 2000; 20:431–8. [4] Sheth S, Scatarige JC, Horton KM, et al, Current concepts in the diagnosis and management of renal cell carcinoma: role of multidetector CT and three-dimensional CT. Radiographics 2001;21: S237–54.
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[5] Nelson KL, Gifford LM, Lauber-Huber C, et al. Clinical safety of gadopentetate dimeglumine. Radiology 1995;196:439–43. [6] Prince MR, Arnoldus C, Frisoli JK. Nephrotoxicity of high-dose gadolinium compared with iodinated contrast. J Magn Reson Imaging 1996;1:162–6. [7] Rofsky NM, Weinreb JC, Bosniak MA, et al. Renal lesion characterization with gadolinium-enhanced MR imaging: efficacy and safety in patients with renal insufficiency. Radiology 1991;180:85–9. [8] Bosniak MA, Rofsky NM. Problems in the detection and characterization of small renal masses. Radiology 1996;198:638–41. [9] Suto Y, Caner BE, Tamagawa Y, et al. Subtracted synthetic images in GD-DTPA enhanced MR. J Compt Assist Tomogr 1989;13:925–8. [10] Motzer RJ, Bander NH, Nanus DM. Renal cell carcinoma. N Engl J Med 1996;335:865–75. [11] Smith SJ, Bosniak MA, Megibow AJ, et al. Renal cell carcinoma: earlier discovery and increased detection. Radiology 1989;170:699–703. [12] Zagoria RJ. Imaging of small renal masses: a medical success story. AJR Am J Roentgenol 2000;175:945–55. [13] Semelka RC, Shoenlut JP, Magro CM, et al. Renal cancer staging: comparison of contrast-enhanced CT and gadolinium-enhanced fat-suppressed spinecho and gradient-echo MR imaging. J Magn Reson Imaging 1993;3:597–602. [14] Studer UE, Scherz S, Scheidegger J, et al. Enlargement of regional lymph nodes in renal cell carcinoma is often not due to metastases. J Urol 1990; 144:243–5. [15] Black WC, Ling A. Is earlier diagnosis really better? The misleading effects of lead time and length biases. AJR Am J Roentgenol 1990;155:625–30. [16] Bosniak MA, Birnbaum BA, Krinsky GA, et al. Small renal parenchymal neoplasms: further observations on growth. Radiology 1995;197:589–97. [17] Helenon O, Serran S, Paraf F, et al. Unusual fatcontaining tumors of the kidney: a diagnostic dilemma. Radiographics 1997;17:129–44. [18] Han YM, Kim JK, Roh BS, et al. Renal angiomyolipoma: selective arterial embolization-effectiveness and changes in angiomyogenic components in long-term follow-up. Radiology 1997;204:65–70. [19] Bosniak MA. Angiomyolipoma (hamartoma) of the kidney: a preoperative diagnosis is possible in virtually every case. Urol Radiol 1981;3:135–42. [20] Bosniak MA, Megibow AJ, Hulnick DH, et al. CT diagnosis of renal angiomyolipoma: The importance of detecting small amounts of fat. Am J Roentgenol 1998;151:491–501. [21] Helenon O, Chretien Y, Paraf F, et al. Renal cell carcinoma containing fat: demonstration with CT. Radiology 1993;188:429–30. [22] Strotzer M, Lehner KB, Becker K. Detection of fat in a renal cell carcinoma mimicking angiomyolipoma. Radiology 1993;188:427–8.
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[23] D’Angelo PC, Gash JR, Horn AW, et al. Fat in renal cell carcinoma that lack associated calcifications. AJR Am J Roentgenol 2002;178:931–2. [24] Ambos MA, Bosniak MA, Madayag MA, et al. Infiltrating neoplasms of the kidney. AJR Am J Roentgenol 1977;129:859–64. [25] Pickhardt PJ, Lonergan GJ, Davis CJ, et al. From the archives of the AFIP. Infiltrative renal lesions: radiologic-pathologic correlation. Radiographics 2000;20:215–43. [26] Bosniak MA. Problems in the radiologic diagnosis of renal parenchymal tumors. Urol Clin North Am 1993;20:217–30. [27] Bosniak MA. The current radiological approach to renal cysts. Radiology 1986;158:1–10. [28] Bosniak MA. Difficulties in classifying cystic lesion of the kidney. Urol Radiol 1991;13:91–3. [29] Bosniak MA. Diagnosis and management of patients with complicated cystic lesions of the kidney. AJR Am J Roentgenol 1997;169:819–21. [30] Bosniak MA. The use of the Bosniak classification system for renal cysts and cystic tumors. J Urol 1997;157:1852–3. [31] Aronson S, Frazier HA, Balwah JD, Hartman DS, Christenson PJ. Cystic renal masses: usefulness of the Bosniak classification. Urol Radiol 1991;13: 83–90.
[32] Curry NS, Cochran ST, Bissada NK, Cystic renal masses: accurate Bosniak classification requires adequate renal CT. AJR Am J Roentgenol 2000; 175:339–42. [33] Koga S, Nishikido M, Inuzuka S, et al. An evaluation of Bosniak’s radiological classification of cystic renal masses. BJU Int 2000;86:607–9. [34] Levy P, Helenon O, Merran S, et al. Cystic tumors of the kidney in adults: radio-histopathologic correlations. J Radiol 1999;80:121–33. [35] Siegel CL, McFarland EG, Brink JA, Fisher AJ, Humphrey P, Heiken JP. CT of cystic renal masses: analysis of diagnostic performance and interobserver variation. AJR Am J Roentgenol 1997;169: 813–8. [36] Israel GM, Bosniak MA. Calcification in cystic renal masses: is it important in diagnosis? Radiology 2003;226:47–52. [37] Balci NC, Semelka RC, Patt RH, et al. Complex renal cysts: findings on MR imaging. AJR Am J Roentgenol 1999;172:1495–500. [38] Bae KT, Heiken JP, Siegel CL, et al. Renal cysts: is attenuation artificially increased on contrastenhanced CT images? Radiology 2000;216:792–6. [39] Maki DD, Birnbaum BA, Chakraborty DP, et al. Renal cyst pseudoenhancement: beam-hardening effects on CT numbers. Radiology 1999;213:468–72.
Renal imaging for diagnosis and staging of renal cell carcinoma Gary M. Israel, MD*, Morton A. Bosniak, MD Division of Abdominal Imaging, Department of Radiology, New York University Medical Center, 560 First Avenue, New York, NY 10016, USA
Radiologic evaluation of renal masses has rapidly progressed from conventional angiography and intravenous urography to CT and MRI. As more patients are imaged for various reasons, the incidental detection of renal masses has increased. Although most of these lesions are benign cysts, neoplastic disease is also uncovered. Some of these lesions require surgery, whereas others do not. The ability to characterize these lesions accurately, to determine their appropriate management, and to stage renal malignancies has been afforded by continuously improving imaging techniques. This article reviews the use of CT and MRI in the diagnosis and staging of malignant renal disease and its differentiation from nonsurgical renal masses. Technique and quality of the radiologic examination The radiologic diagnosis of a renal mass is dependent on high-quality imaging studies, independent of the imaging modality used. In clinical practice, many lesions are labeled ‘‘indeterminate’’ not secondary to their character but owing to the quality of the examination. Although radiologists are responsible for performing and optimizing each examination, referring urologists must also be cognizant of what constitutes an acceptable examination. In this way, urologists can assume a more informed role in the diagnosis
* Corresponding author. E-mail address: [email protected] (G.M. Israel).
and can realize when the examination is suboptimal. Many renal masses are discovered during routine abdominal CT scanning performed for other reasons. In many of these examinations, the diagnosis of the renal mass is obvious and clear cut. Most of these lesions are simple cysts, but some large neoplasms are found as well. Often, the diagnosis is not certain, and a repeat CT scan dedicated to the kidneys is necessary to diagnose accurately the lesion in question. A repeat scan also may be needed when a complex lesion is detected on sonography and needs further evaluation. A high-quality CT scan dedicated to the kidneys includes images obtained before and after intravenous contrast administration. An adequate amount of contrast material given as a rapid bolus with a power injector is necessary to ensure that a high blood concentration of contrast is present at the time of scanning. The slice collimation can be variable depending on the size of the lesion in question. The authors recommend starting with 5mm sections, followed by thinner sections (3 mm or less) through the lesion or the suspect portion of the lesion if necessary. The scan delay (the time after the contrast is given at which the CT scan is started) is critical. After contrast administration, images are acquired after a minimum of 90 seconds (the nephrographic phase of enhancement). This delay ensures that the images are obtained when renal enhancement and renal neoplasm enhancement are maximal. Examinations obtained too early after the administration of intravenous contrast (during the corticomedullary phase of enhancement) are not adequate for optimal characterization of renal neoplasms [1,2].
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The scan parameters of the precontrast and postcontrast examinations must be constant to ensure that the most accurate Hounsfield unit (HU) measurements are obtained. More recently, multidetector CT has been introduced into clinical practice. This modality allows for the acquisition of large volumetric data sets with nearly isotropic voxels during a single breath hold. The increased speed of multidetector CT allows for dynamic multiphase image acquisition, which enables the radiologist to create meaningful multiplanar reformations and to perform three-dimensional volume rendering. With the advent of nephron-sparing surgery and the recent surgical advances in laparoscopic techniques, multidetector CT provides important preoperative information to the surgeon. In addition, it may be helpful in cases in which percutaneous ablation therapy is possible. One can create images that clearly demonstrate the relationship of a neoplasm to the surrounding renal parenchyma, perirenal fat, and neighboring organs, as well as three-dimensional images with angiographic and urographic renderings (Fig. 1) [3,4]. When the images are analyzed at a workstation in real time, the surgeon can view them in any orientation, depicting invaluable information before surgery. This preoperative road map can aid in complete tumor excision with minimal morbidity. In most cases, conventional angiography for preoperative vascular mapping is no longer needed (Fig. 2). In patients who cannot receive iodinated intravenous contrast material because of allergy or renal insufficiency, MRI offers the advantage of evaluating patients without exposure to nephrotoxic contrast material. Gadolinium has been shown to be remarkably safe, can be administered to patients without concern for contrast-induced nephrotoxicity, and is well tolerated in patients with a history of iodinated contrast allergy [5–7]. Further advantages of MRI include the lack of ionizing radiation, the direct multiplanar capability that enables more accurate localization of masses, and the superior intrinsic soft tissue contrast augmented by the use of gadolinium. Over the past 10 years, the quality and reliability of MRI have improved to the point at which it is now considered at least equivalent to CT in the detection and characterization of renal masses. In a group of patients, MRI may be indicated in lieu of CT. This group includes patients with mildly reduced renal function
(patients with renal insufficiency who are not on dialysis) and patients with a solitary kidney. In addition, MRI should be the test of choice in patients who have undergone nephron-sparing surgery and who are in need of serial follow-up examinations. The increased load of potentially nephrotoxic iodinated contrast material delivered to the remaining renal tissue on a per nephron basis in these patients has made clinicians and radiologists hesitant to administer a full dose of contrast material [8]. Occasionally, MRI can characterize a lesion that cannot be diagnosed by CT. Nevertheless, MRI should not be performed because a poorquality CT scan was obtained. In these cases, the CT should be repeated and optimized. If the lesion is still indeterminate, MRI could then be considered. A high-quality MR examination incorporates high-performance gradients with a torso-phased array coil and a variety of breath hold sequences, which can be obtained in multiple planes. Although debate continues as to which sequences are necessary to characterize a renal mass, the authors recommend a combination of T1-weighted images obtained with and without frequency-selective fat suppression and T2weighted images. As is true for CT, it is critical to be able to demonstrate enhancement within a renal mass; therefore, images need to be obtained before and after gadolinium administration. The authors recommend a three-dimensional, fat-suppressed, T1-weighted sequence, which can be reconstructed in any plane desired and also used to perform MR angiography (Fig. 3). In many cases, it is possible to determine the enhancement of a lesion by performing a side-byside comparison of the precontrast and postcontrast acquisitions; however, in cases in which a lesion is hypovascular or hyperintense on the precontrast images, qualitative enhancement is difficult if not impossible to appreciate. In this instance, a subtraction algorithm can be applied to help assess enhancement characteristics [9]. Solid renal masses: differentiating surgical from nonsurgical lesions In general, any renal mass that enhances with intravenous contrast should be considered a renal cell carcinoma until proven otherwise; however, not all enhancing solid renal masses represent renal cell carcinomas. One must be able to differentiate masses that require surgery (renal
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Fig. 1. (A) Axial contrast-enhanced image from a CT scan in a 45-year-old man with a right renal tumor and bilateral renal calculi demonstrates a 2-cm enhancing renal neoplasm in the right kidney. In the axial plane, it is difficult to determine the relationship of the tumor to its surrounding structures. (B) Coronal maximum intensity projection image demonstrates the relationship of the neoplasm to the renal pelvis (P) and vasculature (arrow) supplying the kidney. Note the multiple calyceal and pelvic calculi. This projection aided the surgeon in planning and performing a successful partial nephrectomy for renal cell carcinoma.
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Fig. 2. Axial oblique three-dimensional volume rendered image in a 63-year-old woman with a renal cell carcinoma in the right kidney obtained from data acquired using a multidetector CT scan. An exophytic solid enhancing renal mass is demonstrated in the right kidney (short arrow). A single renal artery (open arrow) and vein (curved arrow) are clearly demonstrated. Note the defect (long arrow) in the kidney secondary to a small cyst.
cell carcinoma, oncocytoma) from those that do not (lymphoma, most angiomyolipomas, pseudotumors, and metastatic disease to the kidneys). Generally, a combination of the clinical history and imaging findings can be used to make this distinction; however, in some patients, such as in those who have lymphoma or a metastatic neoplasm to the kidney, percutaneous needle aspiration may be performed for confirmation. Renal cell carcinoma Renal cell carcinoma is the most common renal neoplasm, accounting for 80% to 85% of all malignant renal tumors and 2% of all cancers. The widespread use of cross-sectional imaging and the incidental detection of asymptomatic neoplasms have increased the incidence of renal cell carcinoma [10,11]. The increased detection
combined with the improved characterization of small renal lesions and subsequent earlier surgical intervention has resulted in a slight improvement in the 5-year survival for renal cell carcinoma. Computed tomography is the imaging modality of choice for evaluating patients with a renal mass and has been demonstrated to be accurate in the diagnosis and staging of renal cell carcinoma. With the recent advances in technology and the development of multidetector CT, it is now possible to obtain thinner slices (on the order of 1 mm) with near isotropic data sets in less time when compared with single-slice helical CT. This improvement should enhance the ability to characterize small renal masses accurately as partial volume averaging and respiratory motion are virtually eliminated. The prognosis of renal cell carcinoma is related to the tumor stage. CT and MRI are accurate in
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Fig. 3. (A) Coronal T2-weighted image in a 55-year-old woman with a renal cell carcinoma demonstrates an exophytic 5-cm complex cystic and solid neoplasm arising from the lower pole of the right kidney. (B) Coronal maximal intensity projection of an MR angiogram performed as part of the comprehensive MR examination shows a single renal artery supplying the right kidney with branches supplying the neoplasm.
the staging of renal cell carcinoma and are the examinations of choice in preoperative assessment [12]. Semelka et al [13] demonstrated that MRI was more accurate than CT for the evaluation of tumor extension into the renal vein and inferior vena cava; however, with the advent of multidetector CT scanning, it is unclear whether this proposed advantage still holds true. Regardless, it is important to demonstrate the most cephalad extent of thrombus in the inferior vena cava, because the surgical approach will be altered if thrombus extends into the right atrium (Fig. 4). The multiplanar capability of MRI and multidetector CT are ideally suited for this determination. In addition, it is often possible to differentiate tumor thrombus from bland thrombus, because tumor thrombus is vascular and bland thrombus is not (Fig. 5). Currently, CT and MRI cannot distinguish between metastatic and hyperplastic lymph nodes; therefore, the evaluation for metastatic adenopathy is size dependent, with lymph nodes greater than 1 cm considered abnormal and suspicious for
metastatic disease. Patients with renal cell carcinoma may have enlarged hyperplastic retroperitoneal lymph nodes, a known pitfall in the staging of renal cell carcinoma with CT or MRI [14]. Many small (1–2.5 cm) incidental neoplasms are found in the kidneys during routine abdominal scanning using sonography, CT, or MRI. When these lesions are found in young healthy patients, they are removed by open or laparoscopic partial nephrectomy. Nevertheless, not all of these small tumors need to be removed, and, in some instances, many of them can be managed by watchful waiting. As is true for other malignancies in the body, the discovery of very small renal cancers will not ensure that mortality from cancer will be reduced. The incidental discovery of tiny renal lesions might simply reflect the detection of early cancers that will be lethal in any case (lead-time bias), the detection of cancers that are growing slowly and are less likely to be lethal (length-time bias), or the detection of tumors with questionable malignant potential (overdiagnosis bias) [15].
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Fig. 4. (A) Coronal gadolinium-enhanced, T1-weighted image in a 55-year-old woman with a renal cell carcinoma demonstrates a complex enhancing neoplasm in the upper pole of the right kidney that on other images was shown to be clearly separate from the liver. (B) Two consecutive gadolinium-enhanced, T1-weighted images demonstrate thrombus within the right renal vein (short arrow) and inferior vena cava (long arrows). The thrombus extends into the intrahepatic portion of the inferior vena cava but not into the right atrium. A flow artifact is seen in the superior aspect of the inferior vena cava. The patient underwent nephrectomy for renal cell carcinoma, but tumor recurrence in the postoperative bed and bone metastases occurred 2 years later.
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Fig. 5. Coronal gadolinium-enhanced, T1-weighted image in a 57-year-old woman with a large renal cell carcinoma demonstrates a large neoplasm that replaces the entire left kidney (T). Enhancing tumor thrombus is seen in the left renal vein (short arrows) and extends up the inferior vena cava (long arrow) to the level of the right hemidiaphragm. Nonenhancing bland thrombus is present in the infrarenal portion of the inferior vena cava (curved arrows). The patient underwent nephrectomy for renal cell carcinoma including removal of the tumor thrombus from the inferior vena cava and is alive 5 years later.
For all of these reasons and because of the previously reported data on tumor growth showing very slow growth in many of these small tumors [16], the use of watchful waiting in the management of some of these lesions seems reasonable (Fig. 6). The use of this approach will depend on the patient’s age, clinical condition, and associated diseases, and the size and imaging appearance of the tumor when first encountered. Patients who are elderly and fragile, who are poor surgical risks, or who have other significant disease and a well-marginated renal lesion less than 2.0 cm can be observed by watchful waiting. This approach can be extended to lesions up to 2.5 or 3.0 cm in size, particularly in elderly or poor-risk patients. These patients should
have follow-up studies to assess the growth rate of the lesion. Further decisions as to management can then be made periodically. If significant growth is detected and surgery is clinically possible, it should be performed. If the patient is not a surgical candidate, percutaneous ablation techniques can be used. If slow or no growth is demonstrated, further watchful waiting seems reasonable. Follow-up studies performed at 6-month or 1-year intervals should be carried out depending on the previous growth rate. With the introduction of laparoscopic partial nephrectomy (and if radiofrequency and cryoablation techniques prove safe and effective), watchful waiting will have less of a role in management and will be used in patients who are
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Fig. 6. (A) Axial contrast-enhanced CT image in a 75-year-old man with a small renal neoplasm shows a 1.5-cm enhancing renal mass (arrow) in the right kidney. The patient was a poor surgical candidate secondary to cardiac disease, and it was decided to use a watchful waiting approach in management. (B) Follow-up examination 5 years later demonstrates minimal growth of the neoplasm (arrow), which now measures 2 cm. Note that the adjacent renal cyst has grown slightly in the interval.
fragile or who have a clearly shortened projected lifespan. Nonsurgical lesions Angiomyolipoma Angiomyolipoma (renal hamartoma), a benign tumor, is composed of varying amounts of fat, smooth muscle, and blood vessels. It is an uncommon lesion with a prevalence of 0.3% to 3% and occurs more commonly in women than men. Angiomyolipomas occur in two different clinical scenarios. More commonly, they are sporadic (80%); however, they may be associated with tuberous sclerosis (20%), in which case they tend to be multiple and bilateral [17]. Patients are usually asymptomatic, and angiomyolipomas are usually incidentally discovered when the patient is imaged for another reason. In most instances, they do not require surgical intervention; however, when they are large, angiomyolipomas may exert mass effect on the adjacent organs and cause symptoms. In addition, patients with large angio-
myolipomas may present with acute flank pain caused by spontaneous hemorrhage. This event may be life threatening and require embolization to stop the bleeding or surgical intervention if angiographic embolization techniques are successful or available [18]. Angiomyolipoma is the only renal tumor that can be characterized based on its tissue composition. The relative amounts of fat, smooth muscle, and vessels within the tumor establish its CT and MRI appearance. The diagnosis of angiomyolipoma rests on demonstrating macroscopic fat within the lesion. CT and MRI can establish the presence of fat within a lesion, and the diagnosis of angiomyolipoma can be made in virtually every case [20]. On CT, the fatty portions of the lesion will measure ÿ30 HU or less. If a very small amount of fat is suspected in a renal mass, it may be necessary to perform very thin sections without intravenous contrast to maximize the ability to depict fat within the lesion [19]. On MRI, the fatty portion of an angiomyolipoma will demonstrate
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hyperintense signal on the T1-weighted images; however, other renal masses, including hemorrhagic cysts, may show similar signal characteristics. It is imperative to compare the T1-weighted images obtained with frequency-selective fat suppression with those obtained without fat suppression to establish definitively the presence or absence of macroscopic fat [20]. In the extremely rare instance in which these lesions do not contain any fat (angiomyoma), the preoperative diagnosis of hamartoma cannot be made, and the lesion is indistinguishable from a renal cell carcinoma. Recent case reports have demonstrated that if a renal mass contains calcification in addition to fat, the possibility of renal cell carcinoma must be seriously considered [21,22]. A clear diagnosis of angiomyolipoma cannot be made if calcium is present within a fat-containing lesion, and surgery is indicated in such cases. Likewise, a few cases of large bulky renal cell carcinomas containing small amounts of macroscopic fat have been reported [23]. These lesions over 10 cm in diameter (not associated with tuberous sclerosis) must also be evaluated surgically, because it is difficult to determine whether the lesion has engulfed renal sinus fat or perirenal fat, or has even engulfed a coexistent angiomyolipoma.
Lymphoma Lymphoma may involve the kidneys via hematogenous spread, in which a single mass or multiple bilateral masses are present, or by direct extension of retroperitoneal lymphoma. Generally, most patients with renal lymphoma have systemic involvement; therefore, the diagnosis should not be difficult given the appropriate clinical history. The imaging appearance of lymphoma is nonspecific. The most common appearance is that of multiple homogeneous solid masses that may be well defined but that tend to have infiltrative margins with the kidney. When lymphoma diffusely infiltrates a kidney, the kidney enlarges but maintains its reniform shape [24,25]. In patients with known lymphoma and in whom there is a single focal mass in the kidney that could represent lymphoma or a coexistent renal cell carcinoma, treatment of the lymphoma is indicated. If the lymphoma responds to treatment but the renal mass does not, renal cell carcinoma is the likely diagnosis. This diagnosis could be confirmed by needle biopsy if any doubt exists.
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Metastases to the kidney The most common tumor to metastasize to the kidney is carcinoma of the lung. Renal metastases tend to be multiple and bilateral and frequently are associated with metastases to other organs. Although they have nonspecific CT and MRI features, renal metastases commonly demonstrate infiltrative growth patterns. With the proper clinical history, the diagnosis should be obvious. In a patient with a history of malignancy (without other metastases) and a solitary renal mass, the renal mass is more likely to represent a renal cell carcinoma and not a metastasis [26]. Nevertheless, it is possible that a single renal metastasis could occur, and differentiation from a renal cell carcinoma may not be obvious. In this situation, a renal biopsy is indicated to determine proper management. Cystic renal masses Cystic renal masses are common, and, fortunately, it is not difficult to differentiate a simple renal cyst from a cystic renal cell carcinoma. Nevertheless, the differentiation of benign complex cystic masses from those that require surgical intervention can be difficult and is a common problem in clinical practice. The characterization of these lesions includes an evaluation of the thickness of the wall, the presence, number, and thickness of any septae, the presence and amount of calcification, the interface of the lesion with the kidney, and, most importantly, the presence or absence of any enhancing soft tissue components [27–30]. The Bosniak classification of renal cysts introduced in 1986 is a useful tool to evaluate these lesions and decide on clinical management [27]. This classification has been accepted worldwide as a way of approaching these lesions. Although there is interobserver variation in some cases, there is a general agreement in most instances [31– 35]. The classification enables urologists and radiologists to communicate about these cases and creates a framework for decisions on management of these lesions. Cystic lesions are classified into several categories based on the imaging findings. Category I masses are simple benign cysts with thin walls. They measure water density, do not contain septations or calcification, and do not enhance. Category II masses are minimally complicated benign cystic lesions. They may contain a few hairline thin septations and fine calcification in their walls or septae (Fig. 7). They
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Fig. 7. Axial contrast-enhanced CT image in a 52-year-old woman demonstrates a minimally complicated category II cyst in the left kidney that contains fine calcification in thin septae.
do not enhance. Small (<3 cm) hyperdense cysts are included in this category. An important subset of category II lesions are category IIF lesions (‘‘F’’ for follow-up). These lesions are slightly more complex than most category II lesions and do not fall neatly into category II. They do not require surgical exploration but do require follow-up examinations to
prove that they are not growing or becoming more complex. These lesions are thought to be benign but require further follow-up to be certain of this diagnosis. They may contain increased numbers of thin septae or a slightly thickened but smooth wall or septum (Fig. 8). They may also contain calcification, which may be thicker and nodular [36]. Completely intrarenal high-density cysts
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Fig. 8. Axial contrast-enhanced CT image in a 43-year-old man demonstrates a 4-cm category IIF left renal cyst that has a smooth but slightly thickened and calcified wall (straight arrow) and septation (curved arrow). Follow-up examinations performed up to 4 years later (not shown) demonstrated no change in the lesion, consistent with a benign cyst.
larger than 3 cm also fall into this group. Follow-up examinations demonstrating stability enable a diagnosis of benignity to be established. Any increase in the thickness of the wall or septae would indicate a lesion that needs to be explored surgically. Follow-up studies should initially be performed at 6 months and then at 1-year intervals. Category III lesions are indeterminate cystic masses and have some findings that are present in
malignant lesions. Consequently, they cannot be distinguished clearly from malignancy by imaging studies alone and are surgical lesions. They may have grossly thickened irregular walls or septations and may contain either small or large amounts of calcium. Enhancement of the wall or septae can be clearly appreciated (Figs. 9 and 10). Surgical exploration is necessary unless clinical aspects necessitate a follow-up approach. The
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Fig. 9. Axial contrast-enhanced CT image in a 55-year-old man shows a 6-cm category III left renal cystic mass with a grossly thickened and enhancing wall. This lesion was indeterminate although highly suspicious for malignancy. The lesion proved to represent a necrotic renal cell carcinoma at nephrectomy.
surgical approach can range from exploration and biopsy to enucleation to partial nephrectomy to radical nephrectomy. Although the surgeon ultimately decides which type of surgery will be performed, a determination can often be made as to whether the lesion is more likely to be benign or malignant. This determination along with the other clinical aspects of the patient, the size of the lesion, and its position in the kidney will help in the selection of the surgical approach. These lesions are frequently indeterminate on gross inspection at surgical exploration, and a final determination of whether they are benign or malignant may only be made by histologic study. In general, approximately 50% of category III
lesions are benign (hemorrhagic cysts, infected cysts, complex septated cysts, or multilocular cystic nephroma), and 50% represent cystic neoplasms. Category IV lesions are malignant cystic masses that contain enhancing soft tissue components independent of the thickened wall (Fig. 11) and rarely cause difficulty in diagnosis. A tumor in the wall of a cyst and a necrotic cystic malignancy are included in this category and are usually readily diagnosed. Although this classification scheme is based on CT criteria, the same approach provides a useful framework for MRI [37]. Nevertheless, there is not always a clear correlation between the findings
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Fig. 10. Axial contrast-enhanced CT image in a 42-year-old woman shows a 7-cm category III right renal cystic lesion with an enhancing thick and irregular wall that contains a small amount of calcification. The patient presented with fever, leukocytosis, right flank pain, and a history of chronic urinary tract infections. Needle aspiration was performed, and pus was recovered, confirming the diagnosis of an infected renal cyst. The patient was treated with catheter drainage.
at MRI and the CT images, and further work is needed to identify these differences. Just as the internal architecture of cystic renal masses may be appreciated more readily with ultrasound than with CT, the findings on MRI may need further study using another modality. Pseudoenhancement of simple renal cysts The enhancement pattern of cystic renal masses is critical in their proper characterization and treatment. Using CT equipment available in the 1980s and early 1990s, an increase in attenuation of at least 10 HU was suggested as evidence of enhancement [27]. With the advent of faster helical scanners, some simple cysts (category I) may demonstrate enhancement of greater than 10 HU [38,39], referred to as ‘‘pseudoenhancement’’; therefore, an increase in attenuation of approximately 15 to 20 HU has now been proposed as evidence of enhancement [39]. The
presence of enhancement must be unequivocal and is the most important determination of whether a lesion is benign or malignant. Pseudoenhancement has tremendous clinical implications and may lead to the erroneous characterization of a simple cyst as a renal neoplasm, resulting in unnecessary surgery. One must be aware of this phenomenon and know when to suspect it. Many of these cysts are small and completely intraparenchymal. If pseudoenhancement of a lesion is suspected, another examination such as ultrasound or MRI should be performed for definite characterization of the lesion as a simple cyst. Pseudotumors and tumorlike masses of the kidney Occasionally, a benign condition may present as a renal mass, and it may be difficult to differentiate it from a renal neoplasm. This group of lesions includes congenital anomalies,
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Fig. 11. Axial contrast-enhanced CT image in a 71-year-old man demonstrates a 6-cm complex category IV right renal cystic mass that contains enhancing soft tissue components indicative of a cystic renal cell carcinoma.
inflammatory masses, hematoma, hydrocalyx, and vascular structures. Careful attention to the imaging characteristics of the lesion combined with an appreciation of its clinical context will result in the correct diagnosis in almost every case. Renal pseudotumors refer to normal renal tissue that may mimic a renal neoplasm and that may be congenital or acquired in nature. Congenital normal variants, including prominent renal columns of Bertin, renal dysmorphism, and dromedary humps, may all be mistaken for a renal mass. In addition, hypertrophy of a portion of the renal parenchyma adjacent to cortical scarring may assume a tumorlike appearance. An awareness of the possibility that a benign condition may mimic a neoplastic process and careful evaluation of the findings of a high-quality examination should reveal the true nature of these lesions. The
key in the proper diagnosis of these pseudotumors is to demonstrate that the lesion enhances identically to the renal parenchyma. In these situations, it may be advantageous to scan not only during the nephrographic phase of enhancement but also during the corticomedullary phase to demonstrate corticomedullary differentiation in the suspect lesion. Pyelonephritis and renal abscess may be mistaken for a renal neoplasm on imaging studies; however, when placed in the proper clinical context, the correct diagnosis usually becomes evident. Nevertheless, differentiating cystic neoplasms from a subacute or chronic renal abscess when the typical clinical findings of infection are not present can be difficult. If a remote history of fever, leukocytosis, or urinary tract infection is obtained, the possibility of a chronic renal
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infection should be considered. This situation is one of the few times that needle aspiration of a cystic mass is prudent. If pus is recovered, percutaneous drainage can be performed. If blood or necrotic debris is recovered, surgical removal is indicated. A renal artery aneurysm or arteriovenous fistula may present as an enhancing renal mass. CT or MR examination performed with a good bolus of contrast material will clearly demonstrate the vascular nature of the lesion. If the contrast material is not administered as a bolus or if the contrast-enhanced portion of the examination is performed during the excretory phase of enhancement, the true nature of the lesion may not be evident. Summary The development and expansion of CT and MRI technology have enhanced the detection and characterization of renal lesions. Although these advancements should lead to earlier diagnosis of renal cell carcinoma with subsequent improved cure rates, the increased imaging has also uncovered many cases that are problematic not only in diagnosis but in management as well. The performance of high-quality examinations combined with growing experience should improve the ability to diagnose and manage these cases successfully. Continued advances in CT and MR technology combined with the current trend toward minimally invasive surgery will continue to expand the role of preoperative imaging and, it is hoped, improve the cure rate of renal cancer. References [1] Birnbaum BA, Jacobs JE, Ramchandani P. Multiphasic renal CT: comparison of renal mass enhancement during the corticomedullary and nephrographic phases. Radiology 1996;200:753–8. [2] Cohan RH, Sherman LS, Korobkin M, et al. Renal masses: assessment of corticomedullary-phase and nephrographic-phase CT scans. Radiology 1995; 196:445–51. [3] Coll DM, Herts BR, Davros WJ, et al. Preoperative use of 3D volume rendering to demonstrate renal tumors and renal anatomy. Radiographics 2000; 20:431–8. [4] Sheth S, Scatarige JC, Horton KM, et al, Current concepts in the diagnosis and management of renal cell carcinoma: role of multidetector CT and three-dimensional CT. Radiographics 2001;21: S237–54.
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