DOI: 10.1148/rg.274065147
RadioGraphics 2007;27:1131-1143
© RSNA, 2007
Hyperattenuating Renal Masses: Etiologies, Pathogenesis, and Imaging Evaluation1
Stuart G. Silverman, MD,
Koenraad J. Mortele, MD,
Kemal Tuncali, MD,
Masahiro Jinzaki, MD, and
Edmund S. Cibas, MD
1 From the Division of Abdominal Imaging and Intervention, Department of Radiology (S.G.S., K.J.M., K.T.), and Division of Cytology, Department of Pathology (E.S.C.), Brigham and Women’s Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115; and Department of Diagnostic Radiology, Keio University School of Medicine, Tokyo, Japan (M.J.). Recipient of a Certificate of Merit award for an education exhibit at the 2005 RSNA Annual Meeting. Received August 4, 2006; revision requested August 28 and received October 5; accepted October 10. S.G.S. is a consultant with Galil Medical, Yokneam, Israel, and with Siemens Medical Solutions, Forchheim, Germany; all remaining authors have no financial relationships to disclose.
Address correspondence to S.G.S. (e-mail: sgsilverman{at}partners.org).
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Abstract
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Some renal masses have higher attenuation than the surrounding renal parenchyma at computed tomography (CT). Their hyperattenuation is usually the result of proteinaceous fluid or densely packed cells. Most hyperattenuating renal masses are benign hemorrhagic or protein-aceous cysts. However, solid enhancing hyperattenuating renal masses may have malignant as well as benign causes. Possible malignant causes include renal cell carcinoma and lymphoma; benign causes include angiomyolipoma with minimal fat. It is important to identify the cause of a hyperattenuating renal mass so as to avoid unnecessary surgical resection or ablation. CT may be useful for diagnosing benign hyperattenuating renal cysts, hematomas, and vascular anomalies that appear masslike. However, some solid, enhancing, hyperattenuating masses cannot be diagnosed confidently with CT alone: Small (
3-cm-diameter), homogeneously enhancing, hyperattenuating renal masses depicted on CT images may be either benign angiomyolipomas with minimal fat or renal cell carcinomas. Magnetic resonance (MR) imaging may be helpful for differentiating between angiomyolipomas with minimal fat and clear cell renal cell carcinomas; however, differentiation between angiomyolipomas with minimal fat and papillary renal cell carcinomas often is not possible on the basis of MR imaging. In such cases, a percutaneous biopsy may be useful for diagnosis. If the results of MR imaging and percutaneous biopsy are not definitive, surgery is warranted.
© RSNA, 2007
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Introduction
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Although most hyperattenuating renal masses are benign proteinaceous cysts, such masses have varied etiologies that include both benign and malignant causes (1). Benign proteinaceous cysts are common and may be diagnosed with confidence on the basis of their small size (diameter of 3 cm or less) and lack of contrast enhancement. On the other hand, small, solid, hyperattenuating renal masses that enhance at computed tomography (CT) after the administration of a contrast agent may be either benign or malignant. In fact, a substantial fraction of small solid renal masses are benign (2–9). In a study of 2770 surgical resections of solid renal masses, investigators found that 25% of the masses smaller than 3 cm were benign (6). Furthermore, it was shown that the smaller the mass in the kidney, the more likely it was to be benign. Homogeneously enhancing, hyperattenuating renal masses are a subset of solid renal masses that may be benign (angiomyolipomas with minimal fat, oncocytomas) or malignant (renal cell carcinomas) (10). Solid renal masses that can be diagnosed confidently as benign do not typically need treatment, and this fact provides an incentive for learning to differentiate between benign and malignant masses at imaging.
The article provides a review of the etiologies and pathogenesis of hyperattenuating renal masses, the best imaging methods for their evaluation, and their characteristic features at CT and magnetic resonance (MR) imaging.
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Definition and Basic Pathogenesis
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Hyperattenuating renal masses consist entirely or predominantly of materials with a CT attenuation that is higher than that of the surrounding renal parenchyma; therefore, they appear dense on un-enhanced CT images. The attenuation of the renal parenchyma typically ranges from 30 to 40 HU; that of hyperattenuating renal masses usually is at least 40 HU but no higher than 90 HU (1).
A hemorrhage and a high concentration of protein in a renal cyst are two basic pathologic processes that frequently cause the appearance of a hyperattenuating mass on unenhanced CT scans (11,12). Other possible causes include an elevation of iron content, colloid formation, infection, and transient iodine accumulation in a simple cyst (13,14). Masses that are composed either entirely or partially of solid elements may show hyperattenuation. Attenuation at CT is affected primarily by Compton interactions and scattering, which are related to electron density. Since electron density is related to physical density, densely packed cells may be one reason why the CT attenuation of some solid tumors is higher than that of others; diffuse microcalcifications are another possible explanation (13).
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Hematoma
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Hyperattenuating masslike renal lesions may be due to a spontaneous or traumatic hematoma. Most hematomas, however, are perinephric, crescentic in shape, and surrounded by fat stranding (Fig 1). Occasionally, a traumatic hematoma may have a masslike appearance. Some hematomas may not be discovered until long after the traumatic event and may simulate renal cell carcinoma. Chronic hematomas do not enhance, whereas almost all renal cell carcinomas do; therefore, hematoma should be considered a possible cause of a nonenhancing hyperattenuating renal mass, particularly in the presence of calcification and perinephric fat stranding (Fig 2).
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Figure 1a. Renal hematoma in a 59-year-old man with lung cancer. (a) Unenhanced CT scan shows a 4 x 2.5-cm slightly heterogeneously hyperattenuating (50 HU) exophytic renal mass (arrowhead) and mild perinephric fat stranding (arrow). (b) Contrast material–enhanced CT scan shows no enhancement of the mass. A percutaneous biopsy, performed because the lesion did not fulfill the criteria for diagnosis of a benign hyperattenuating cyst, yielded fibrous tissue with evidence of chronic inflammation and no malignant cells.
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Figure 1b. Renal hematoma in a 59-year-old man with lung cancer. (a) Unenhanced CT scan shows a 4 x 2.5-cm slightly heterogeneously hyperat-tenuating (50 HU) exophytic renal mass (arrowhead) and mild perinephric fat stranding (arrow). (b) Contrast material–enhanced CT scan shows no enhancement of the mass. A percutaneous biopsy, performed because the lesion did not fulfill the criteria for diagnosis of a benign hyperattenuating cyst, yielded fibrous tissue with evidence of chronic inflammation and no malignant cells.
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Figure 2a. Renal hematoma in a 69-year-old man with abdominal pain. (a) Unenhanced CT scan shows a 10-cm-diameter heterogeneously hyperattenuating (46 HU) renal mass (arrows) with linear calcifications at its center and periphery. (b) Contrast-enhanced CT scan obtained during the nephrographic phase reveals no enhancement of the mass (arrows). A renal hematoma was found at nephrectomy.
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Figure 2b. Renal hematoma in a 69-year-old man with abdominal pain. (a) Unenhanced CT scan shows a 10-cm-diameter heterogeneously hyperattenuating (46 HU) renal mass (arrows) with linear calcifications at its center and periphery. (b) Contrast-enhanced CT scan obtained during the nephrographic phase reveals no enhancement of the mass (arrows). A renal hematoma was found at nephrectomy.
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Vascular Aneurysms and Malformations
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Hyperattenuating masslike lesions in the kidney may be caused by vascular abnormalities such as arteriovenous malformations, pseudoaneurysms, and aneurysms. These rare entities may have particularly high attenuation if they contain a thrombus. Features indicative of a vascular abnormality include a saccular or tubular shape, draining veins that emanate from the mass, and attenuation or signal intensity similar to that of nearby blood vessels during the various phases of a dynamic CT or MR imaging examination (Fig 3).
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Figure 3a. Vascular malformation in a 50-year-old woman. (a) Unenhanced CT scan shows a 1.3-cm-diameter hyperattenuating (42 HU) renal mass (arrow). (b) Contrast-enhanced CT scan obtained during the nephrographic phase reveals enhancement (72 HU) of the mass and shows nearby dilated veins (arrowheads). (c) Contrast-enhanced MR angiogram shows that the mass (arrow) is connected to a draining vein (arrowhead) in the perinephric space.
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Figure 3b. Vascular malformation in a 50-year-old woman. (a) Unenhanced CT scan shows a 1.3-cm-diameter hyperattenuating (42 HU) renal mass (arrow). (b) Contrast-enhanced CT scan obtained during the nephrographic phase reveals enhancement (72 HU) of the mass and shows nearby dilated veins (arrowheads). (c) Contrast-enhanced MR angiogram shows that the mass (arrow) is connected to a draining vein (arrowhead) in the perinephric space.
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Figure 3c. Vascular malformation in a 50-year-old woman. (a) Unenhanced CT scan shows a 1.3-cm-diameter hyperattenuating (42 HU) renal mass (arrow). (b) Contrast-enhanced CT scan obtained during the nephrographic phase reveals enhancement (72 HU) of the mass and shows nearby dilated veins (arrowheads). (c) Contrast-enhanced MR angiogram shows that the mass (arrow) is connected to a draining vein (arrowhead) in the perinephric space.
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Benign Hyperattenuating Cysts
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Benign cysts are overwhelmingly the most common type of hyperattenuating renal mass (Fig 4). Benign hyperattenuating renal cysts are also known as hyperdense renal cysts. They are frequently found in patients with either acquired cystic renal disease or autosomal dominant polycystic renal disease (15) (Fig 5), and they can be diagnosed with confidence if they fulfill the following criteria described by Bosniak (1): They should be small (with a diameter of 3 cm or less), round, well marginated, and homogeneously hyperattenuating even when a narrow CT window setting is used. This latter feature is important; heterogeneity at unenhanced CT should be considered suggestive of a solid mass (16). Occasionally, heterogeneity may be due to a recent hemorrhage in a simple cyst. At least one quarter of a hyperattenuating cyst’s wall should abut perinephric fat so that its wall can be assessed as thin and smooth. Hyperattenuating cysts should not enhance and, therefore, cannot be diagnosed with confidence by using unenhanced CT alone. A proper CT examination includes scanning both before and after the administration of intravenous contrast material, with a maximum section thickness of 5 mm. Masses that increase in attenuation by 10 HU or less are considered nonenhancing (1). Masses that increase in attenuation by more than 10 HU are considered enhancing (1). However, because the standard deviation of attenuation measurements may be more than 10 HU, an attenuation difference of 20 HU or more is a more specific criterion of enhancement (17). If a hyperattenuating mass is depicted only on enhanced CT images, delayed (by at least 15 minutes) CT can be performed to search for the diminution of attenuation measurements over time ("deenhancement") (18). Deenhancement by 15 HU or more indicates that, excluding a vascular abnormality, the mass is solid; a lack of deenhancement indicates that the mass is probably a hyperattenuating cyst (18). Ultrasonography (US) may be helpful, but cysts that hyperattenuate at CT typically produce internal echos that may make it difficult to differentiate them from solid masses.
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Figure 4a. Hyperattenuating cyst in a 46-year-old woman. (a) Unenhanced CT scan shows a 1.2-cm-diameter hyperattenuating (80 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows no enhancement of the mass.
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Figure 4b. Hyperattenuating cyst in a 46-year-old woman. (a) Unenhanced CT scan shows a 1.2-cm-diameter hyperattenuating (80 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows no enhancement of the mass.
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Figure 5. Hyperattenuating cyst in a 55-year-old woman with autosomal dominant polycystic renal disease. Unenhanced CT scan shows a 2.5-cm-diameter hyperattenuating (65 HU) right-sided renal mass (arrow).
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Malignant Cysts
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Although hyperattenuating cysts may be considered reliably benign, a hyperattenuating mass that fulfilled all of the Bosniak criteria for a benign cyst was found to be a cystic renal cell carcinoma (14). The mass contained straw-colored nonhemorrhagic fluid. The wall of the mass was lined by a single layer of neoplastic cells. This case represents an exception to the general rule that a hyperattenuating mass that fulfills the criteria described by Bosniak can be reliably considered benign.
Multiseptate cystic masses may be hyperattenuating. The differential diagnosis of multiseptate renal masses includes benign multilocular cyst, multilocular cystic nephroma, and cystic renal cell carcinoma (Fig 6). Because imaging alone does not allow accurate differentiation between these entities, surgical resection should be considered.
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Figure 6a. Multilocular cystic renal cell carcinoma in a 52-year-old woman. (a) Unenhanced CT scan shows a 3-cm-diameter lobulated heterogeneously hyperattenuating (40 HU) renal mass (arrows). (b) Contrast-enhanced CT scan shows no enhancement of the mass. (c) Contrast-enhanced CT scan at a lower level shows enhancing septa (arrows). (d) Histologic section viewed at low power (original magnification, x 100; hematoxylin-eosin stain) shows several cystic areas separated by septa (arrow). (e) Histologic section viewed at high power (original magnification, x 400; hematoxylin-eosin stain) shows a single layer of renal cell carcinoma cells of the clear cell type (arrow) lining one of the septa.
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Figure 6b. Multilocular cystic renal cell carcinoma in a 52-year-old woman. (a) Unenhanced CT scan shows a 3-cm-diameter lobulated heterogeneously hyperattenuating (40 HU) renal mass (arrows). (b) Contrast-enhanced CT scan shows no enhancement of the mass. (c) Contrast-enhanced CT scan at a lower level shows enhancing septa (arrows). (d) Histologic section viewed at low power (original magnification, x 100; hematoxylin-eosin stain) shows several cystic areas separated by septa (arrow). (e) Histologic section viewed at high power (original magnification, x 400; hematoxylin-eosin stain) shows a single layer of renal cell carcinoma cells of the clear cell type (arrow) lining one of the septa.
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Figure 6c. Multilocular cystic renal cell carcinoma in a 52-year-old woman. (a) Unenhanced CT scan shows a 3-cm-diameter lobulated heterogeneously hyperattenuating (40 HU) renal mass (arrows). (b) Contrast-enhanced CT scan shows no enhancement of the mass. (c) Contrast-enhanced CT scan at a lower level shows enhancing septa (arrows). (d) Histologic section viewed at low power (original magnification, x 100; hematoxylin-eosin stain) shows several cystic areas separated by septa (arrow). (e) Histologic section viewed at high power (original magnification, x 400; hematoxylin-eosin stain) shows a single layer of renal cell carcinoma cells of the clear cell type (arrow) lining one of the septa.
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Figure 6d. Multilocular cystic renal cell carcinoma in a 52-year-old woman. (a) Unenhanced CT scan shows a 3-cm-diameter lobulated heterogeneously hyperattenuating (40 HU) renal mass (arrows). (b) Contrast-enhanced CT scan shows no enhancement of the mass. (c) Contrast-enhanced CT scan at a lower level shows enhancing septa (arrows). (d) Histologic section viewed at low power (original magnification, x 100; hematoxylin-eosin stain) shows several cystic areas separated by septa (arrow). (e) Histologic section viewed at high power (original magnification, x 400; hematoxylin-eosin stain) shows a single layer of renal cell carcinoma cells of the clear cell type (arrow) lining one of the septa.
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Figure 6e. Multilocular cystic renal cell carcinoma in a 52-year-old woman. (a) Unenhanced CT scan shows a 3-cm-diameter lobulated heterogeneously hyperattenuating (40 HU) renal mass (arrows). (b) Contrast-enhanced CT scan shows no enhancement of the mass. (c) Contrast-enhanced CT scan at a lower level shows enhancing septa (arrows). (d) Histologic section viewed at low power (original magnification, x 100; hematoxylin-eosin stain) shows several cystic areas separated by septa (arrow). (e) Histologic section viewed at high power (original magnification, x 400; hematoxylin-eosin stain) shows a single layer of renal cell carcinoma cells of the clear cell type (arrow) lining one of the septa.
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Solid Hyperattenuating Masses
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When the predominant part of a renal mass enhances, the mass is considered solid and likely neoplastic. However, vascular anomalies, aneurysms, and focal inflammatory processes should be excluded before a neoplasm is considered. Vascular anomalies and aneurysms show enhancement similar to that of arteries or veins and typically greater than that of solid masses. Focal inflammatory processes include changes due to infection, trauma, and infarction. Focal bacterial pyelonephritis usually can be diagnosed by noting that the mass has ill-defined margins (19) and is associated with abundant perinephric fat stranding. A history of an ongoing urinary tract infection usually can be elicited from the patient. Similarly, a history of renal trauma is nearly always present in the case of a renal contusion. Renal infarction may mimic infection but usually can be diagnosed in the proper clinical setting. When infection, trauma, and vascular abnormality have been excluded, an enhancing hyperattenuating renal mass is neoplastic.
Although enhancing hyperattenuating renal masses in adults may be malignant, benign neoplasms such as angiomyolipomas also may be hyperattenuating (6). Angiomyolipomas consist of variable quantities of blood vessels, smooth muscle, and fat. Most angiomyolipomas can be diagnosed by identifying portions of the mass with an attenuation of –10 HU or less, which is indicative of fat (20). However, approximately 4%–5% of angiomyolipomas either do not contain any fat cells or contain an insufficient amount of fat cells to allow a diagnosis based on imaging (10,21); these angiomyolipomas with minimal fat consist mostly of smooth muscle and are typically hyperattenuating (Figs 7, 8). Of 175 resected renal masses that were thought to be renal cell carcinomas, six were angiomyolipomas. Each of those six was uniformly hyperattenuating and enhancing at CT and isoechoic at US (10). Of the 100 resected renal cell carcinomas, 2% were hyperattenuating and enhancing, but none was isoechoic (Fig 9). Therefore, US may be useful for differentiating hyperattenuating angiomyolipomas from hyperattenuating renal cell carcinomas.
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Figure 7a. Angiomyolipoma with minimal fat in a 53-year-old man. (a) Unenhanced CT scan obtained during a CT-guided fine-needle biopsy shows a 2.5-cm-diameter hyperattenuating (45 HU) renal mass (arrows). (b) Smear from the aspirate (original magnification, x 400; hemacolor stain) shows densely packed spindle cells without significant atypia, a finding that represents a myoid component. No fat or vessels are seen in this field. The morphologic impression of angiomyolipoma was confirmed at immunocytochemical analysis.
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Figure 7b. Angiomyolipoma with minimal fat in a 53-year-old man. (a) Unenhanced CT scan obtained during a CT-guided fine-needle biopsy shows a 2.5-cm-diameter hyperattenuating (45 HU) renal mass (arrows). (b) Smear from the aspirate (original magnification, x 400; hemacolor stain) shows densely packed spindle cells without significant atypia, a finding that represents a myoid component. No fat or vessels are seen in this field. The morphologic impression of angiomyolipoma was confirmed at immunocytochemical analysis.
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Figure 8a. Angiomyolipoma with minimal fat in a 50-year-old man. (a) Unenhanced CT scan shows a 1-cm-diameter exophytic hyperattenuating (58 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 96 HU. The diagnosis was made at surgical resection.
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Figure 8b. Angiomyolipoma with minimal fat in a 50-year-old man. (a) Unenhanced CT scan shows a 1-cm-diameter exophytic hyperattenuating (58 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 96 HU. The diagnosis was made at surgical resection.
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Figure 9a. Renal cell carcinoma in a 43-year-old woman. (a) Unenhanced CT scan reveals a 2-cm-diameter hyperattenuating (70 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 96 HU. (c) Photograph of a gross specimen cross section shows a clear cell renal cell carcinoma (arrow) with internal hemorrhage.
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Figure 9b. Renal cell carcinoma in a 43-year-old woman. (a) Unenhanced CT scan reveals a 2-cm-diameter hyperattenuating (70 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 96 HU. (c) Photograph of a gross specimen cross section shows a clear cell renal cell carcinoma (arrow) with internal hemorrhage.
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Figure 9c. Renal cell carcinoma in a 43-year-old woman. (a) Unenhanced CT scan reveals a 2-cm-diameter hyperattenuating (70 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 96 HU. (c) Photograph of a gross specimen cross section shows a clear cell renal cell carcinoma (arrow) with internal hemorrhage.
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Alternatively, when an enhancing hyperattenuating renal mass is observed at CT, MR imaging may be performed. The MR imaging examination should include the application of T1- and T2-weighted sequences and of a frequency-selective fat-suppressed dynamically acquired T1-weighted sequence before and after intravenous gadolinium chelate administration. Fat suppression techniques generally are unhelpful for detecting fat in angiomyolipomas with minimal fat, because such masses contain little or no fat and often appear isointense to the renal parenchyma on T1-weighted images (10). If a mass exhibits areas of signal suppression, a fat-containing angiomyolipoma should be considered a strong possibility. Most angiomyolipomas can be diagnosed with confidence by identifying the presence of fat cells within a noncalcified renal mass (21). There have been case reports of fat within calcified renal cell carcinomas (22–25), noncalcified renal cell carcinomas (26), and atypical Wilms tumors (27). In addition, large malignant renal tumors that engulf the fat-containing renal sinus or a region of perirenal fat may appear to contain intratumoral fat (28,29). Chemical shift imaging may be used to detect angiomyolipomas that contain fat cells (30). On chemical shift images, the presence of a sharp black line (also known as an india ink artifact) at the interface of a macroscopic fat-containing renal mass and the adjacent water-containing renal parenchyma may be considered suggestive of an angiomyolipoma (30). However, there are two reasons to be cautious about relying on features depicted on chemical shift images for a diagnosis of angiomyolipoma. First, as noted above, renal cell carcinomas rarely do contain fat cells. Second, the neoplastic cells of clear cell renal cell carcinoma may contain intracytoplasmic lipid and, as a result, areas of signal suppression may appear on opposed-phase gradient-echo MR images (31). In fact, a renal mass that is suppressed focally (excluding india ink artifact) or diffusely on opposed-phase gradient-echo images and that does not exhibit fat suppression should arouse suspicion about the possible presence of clear cell renal cell carcinoma. However, such a mass also may be an angiomyolipoma that contains microscopic amounts of fat cells dispersed throughout and identifiable only with chemical shift MR imaging.
Although more studies are needed, MR imaging features of angiomyolipoma with minimal fat and of renal cell carcinoma have been described (10,32–35). These data may be used to evaluate renal masses that hyperattenuate at CT. An angiomyolipoma with minimal fat typically appears hypointense on T2-weighted images, likely because of its smooth muscle content (10) (Fig 10). Clear cell renal cell carcinoma typically is isointense or hyperintense on T2-weighted images (32–34). Therefore, if an enhancing hyperattenuating renal mass is isointense or hyperintense on T2-weighted images, clear cell renal cell carcinoma is likely. Papillary renal cell carcinoma typically is hypointense on T2-weighted images, perhaps because of the iron-containing hemosiderin that sometimes is found in the cytoplasm of the tumor cells (33,35) (Fig 11). If an enhancing hyperattenuating renal mass appears hypointense on T2-weighted images, both angiomyolipoma with minimal fat and papillary renal cell carcinoma should be considered. Renal mass enhancement is considered unequivocal at MR imaging if the signal intensity measurements increase by more than 20% and borderline if the measurements increase by only 15% (36). Papillary renal cell carcinoma is typically a hypovascular neoplasm and shows mild contrast enhancement (37–39), whereas angiomyolipomas with minimal fat are generally hypervascular and show marked enhancement (20,40). However, the degree of enhancement ranges widely in angiomyolipomas in which there is no visible fat at unenhanced CT (20,40). Therefore, differentiating a papillary renal cell carcinoma from an angiomyolipoma with minimal fat on the basis of enhancement characteristics alone is difficult. Finally, susceptibility artifacts caused by abundant hemosiderin deposits may be indicative of papillary renal cell carcinoma on gradient-echo images (Fig 11).
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Figure 10a. Angiomyolipoma with minimal fat in a 65-year-old woman. (a) Unenhanced CT scan shows a 2.6-cm-diameter hyperattenuating (51 HU) renal mass (arrow). (b, c) T1-weighted (b) and T2-weighted (c) MR images show the mass as hypointense. (d) Gradient-echo MR image obtained after intravenous administration of a gadolinium chelate shows enhancement of the mass. (e) Histologic section (original magnification, x 400; hematoxylin-eosin stain) shows spindle cells and no fat cells. Immunocytochemical analysis helped confirm the diagnosis.
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Figure 10b. Angiomyolipoma with minimal fat in a 65-year-old woman. (a) Unenhanced CT scan shows a 2.6-cm-diameter hyperattenuating (51 HU) renal mass (arrow). (b, c) T1-weighted (b) and T2-weighted (c) MR images show the mass as hypointense. (d) Gradient-echo MR image obtained after intravenous administration of a gadolinium chelate shows enhancement of the mass. (e) Histologic section (original magnification, x 400; hematoxylin-eosin stain) shows spindle cells and no fat cells. Immunocytochemical analysis helped confirm the diagnosis.
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Figure 10c. Angiomyolipoma with minimal fat in a 65-year-old woman. (a) Unenhanced CT scan shows a 2.6-cm-diameter hyperattenuating (51 HU) renal mass (arrow). (b, c) T1-weighted (b) and T2-weighted (c) MR images show the mass as hypointense. (d) Gradient-echo MR image obtained after intravenous administration of a gadolinium chelate shows enhancement of the mass. (e) Histologic section (original magnification, x 400; hematoxylin-eosin stain) shows spindle cells and no fat cells. Immunocytochemical analysis helped confirm the diagnosis.
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Figure 10d. Angiomyolipoma with minimal fat in a 65-year-old woman. (a) Unenhanced CT scan shows a 2.6-cm-diameter hyperattenuating (51 HU) renal mass (arrow). (b, c) T1-weighted (b) and T2-weighted (c) MR images show the mass as hypointense. (d) Gradient-echo MR image obtained after intravenous administration of a gadolinium chelate shows enhancement of the mass. (e) Histologic section (original magnification, x 400; hematoxylin-eosin stain) shows spindle cells and no fat cells. Immunocytochemical analysis helped confirm the diagnosis.
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Figure 10e. Angiomyolipoma with minimal fat in a 65-year-old woman. (a) Unenhanced CT scan shows a 2.6-cm-diameter hyperattenuating (51 HU) renal mass (arrow). (b, c) T1-weighted (b) and T2-weighted (c) MR images show the mass as hypointense. (d) Gradient-echo MR image obtained after intravenous administration of a gadolinium chelate shows enhancement of the mass. (e) Histologic section (original magnification, x 400; hematoxylin-eosin stain) shows spindle cells and no fat cells. Immunocytochemical analysis helped confirm the diagnosis.
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Figure 11a. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11b. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11c. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11d. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11e. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11f. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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Figure 11g. Papillary renal cell carcinoma in a 76-year-old man. (a) Unenhanced CT scan shows a 2.4-cm-diameter hyperattenuating (60 HU) renal mass (arrow). (b) Contrast-enhanced CT scan shows enhancement of the mass to 88 HU. (c–f) T1-weighted (c), T2-weighted (d), and gradient-echo MR images obtained before (e) and after (f) intravenous gadolinium chelate administration show the mass as hypointense. (g) Smear (original magnification, x 400; Papanicolaou stain) from a fine-needle biopsy shows papillary renal cell carcinoma cells (arrows) that contain hemosiderin (arrowhead). The iron in hemosiderin leads to a loss of MR signal because of magnetic susceptibility effects.
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In addition to angiomyolipomas with minimal fat, other benign tumors may appear hyperattenuating and enhancing at CT and be hypointense at T2-weighted MR imaging. Metanephric adenoma, an uncommon benign renal tumor, may manifest as a hyperattenuating mass (37,41). The tumor typically shows mild enhancement at CT after the administration of an intravenous contrast material (37) and, in our experience, may be hypointense on T2-weighted images. It may be indistinguishable from a papillary renal cell carcinoma. Leiomyoma is a rare tumor of the renal capsule that also may appear hyperattenuating (42) (Fig 12). Its relatively high attenuation, similar to that of angiomyolipoma, is likely due to the presence of densely packed smooth muscle cells.
When we observe an enhancing hyperattenuating renal mass that appears hypointense on T2-weighted images and is not suppressed on frequency-selective fat-suppressed images or opposed-phased gradient-echo images, we perform a biopsy of the mass to be certain that it is not cancerous and to prevent resection of a benign tumor.
A biopsy is needed because the differential diagnosis includes both benign and malignant causes. Papillary renal cell carcinoma (33,35), angiomyolipoma with minimal fat (10), and metanephric adenoma (41) may display these imaging features and may be diagnosed with a percutaneous biopsy (43–48). Leiomyomas may be sampled percutaneously, but differentiating benign leiomyomas from malignant ones may be difficult on the basis of a needle biopsy alone (44).
Lymphoma is typically hypoattenuating but also may manifest as a hyperattenuating renal mass (Fig 13). Oncocytoma, like renal cell carcinoma, has a variable appearance on CT scans and MR images but also may be hyperattenuating (49). Oncocytoma cannot always be definitively diagnosed on the basis of a percutaneous biopsy, but findings at biopsy may be strongly suggestive of the diagnosis (44,50,51).
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Figure 13. Renal lymphoma in a 49-year-old woman. Unenhanced CT scan shows an exophytic hyperattenuating (58 HU) renal mass (arrow). A previous contrast-enhanced CT scan showed enhancement of the mass. The diagnosis was based on histologic analysis of a percutaneous biopsy specimen.
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This article does not provide an exhaustive description of hyperattenuating renal masses. Indeed, some masses that typically are hypoattenuating occasionally may be hyperattenuating (usually because of hemorrhage). Some rare tumors and metastases also may hyperattenuate (52).
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Diagnostic Approach
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The following algorithm is suggested for diagnosis of a mass that has a predominantly hyperattenuating appearance at unenhanced CT (Fig 14). When a hyperattenuating (40–90 HU) renal mass that does not contain fat is detected at unenhanced CT, contrast-enhanced CT should be performed in the same session to evaluate any enhancement of the mass. If the mass does not enhance and fulfills all the criteria outlined by Bosniak, it may be reliably considered a benign hyperattenuating renal cyst (1). If the mass demonstrates homogeneous enhancement, MR imaging should be performed to reevaluate its fat content by using frequency-selective fat suppression. If macroscopic fat is not found at thin-section CT (3–5-mm section thickness), it is unlikely to be found at MR imaging. Nevertheless, if the signal in any portion of the mass does appear suppressed on images obtained with frequency-selective fat suppression—particularly if the mass is not calcified—the mass is likely to be an angiomyolipoma. As described earlier, renal cell carcinoma rarely may contain fat cells. Chemical shift MR imaging techniques also should be applied. If a renal mass shows no internal signal suppression on images obtained by using a frequency-selective fat suppression technique but does show signal suppression on opposed-phase gradient-echo images, clear cell renal cell carcinoma should be considered, particularly if the pattern of suppression is diffuse. No precise recommendation can be given regarding the management of such masses, because there are insufficient data to indicate what proportion of them are renal cell carcinomas. If the mass appears isointense or hyperintense in comparison with the renal parenchyma on T2-weighted images, it may be a renal cell carcinoma, some other malignant tumor, or an oncocytoma. If the mass appears hypointense on T2-weighted images, both angiomyolipoma with minimal fat and papillary renal cell carcinoma should be considered. Metanephric adenoma also is a possibility, but it is rare. Excluding the rare capsular leiomyoma, if a renal mass that is hypointense on T2-weighted MR images demonstrates marked enhancement, an angiomyolipoma with minimal fat is more likely than a papillary renal cell carcinoma. However, MR imaging findings are not definitive. Since enhancing hyperattenuating renal masses that are hypointense on T2-weighted MR images may be either benign or malignant, a percutaneous biopsy of the renal mass should be performed (44). A biopsy can help distinguish renal cell carcinoma from angiomyolipoma with minimal fat and from metanephric adenoma. A definitive diagnosis of the latter two benign entities on the basis of histopathologic analysis of a biopsy specimen may help avoid unnecessary surgery or ablation. A biopsy also may be useful for identifying oncocytic renal cell carcinomas, and in some cases it may point the way toward a diagnosis of oncocytoma.
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Figure 14. Algorithm for diagnostic evaluation of hyperattenuating renal masses with CT and MR imaging. Before the algorithm is applied, inflammatory changes and vascular abnormalities should be excluded and masses should be evaluated with unenhanced CT to identify small amounts of fat (–10 HU). Other imaging features, described in the text, are needed to characterize fully the masses listed in the Figure. The algorithm is applicable to masses that are predominantly hyperattenuating (> 40 HU) at unenhanced CT. Each step refers to features that should be examined in any portion of the mass. The diagnoses listed are those that are most likely, given the specified findings. AML = angiomyolipoma, contrast = contrast-enhanced, MRI = MR imaging, OOPS = out-of-phase sequence at chemical shift MR imaging, RCC = renal cell carcinoma, Supp = suppression.
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Summary
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Most hyperattenuating renal masses are benign cysts. However, hematomas, vascular abnormalities, and solid neoplasms may be manifested at imaging as hyperattenuating renal masses. Hyperattenuating solid neoplasms may be benign or malignant. When hyperattenuating renal masses are small and show homogeneous enhancement, benign causes should be considered. CT, MR imaging, and percutaneous biopsy may be used to diagnose most such lesions and thereby avoid unnecessary treatment.
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References
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- Bosniak MA. The small (less than or equal to 3.0 cm) renal parenchymal tumor: detection, diagnosis, and controversies. Radiology 1991;179:307–317.[Free Full Text]
- Duchene DA, Lotan Y, Cadeddu JA, Sagalowsky AI, Koeneman KS. Hist
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Abstract
Introduction
