Sonography in Benign and Malignant Renal Masses Raj Mohan Paspulati, MD a, *, Shweta Bhatt, MD b Ultrasonography is often the initial modality for imaging of the kidneys, although contrast-en- hanced CT is the established imaging modality for the diagnosis of renal tumors. Despite technical limitations, a large percentage of renal tumors can be characterized by ultrasonography. Cystic and solid renal parenchymal mass lesions can be well differentiated by ultrasonography. Technical ad- vances in the gray-scale and color-flow Doppler (CFD) ultrasound have improved the sensitivity in detection of small renal tumors. Gray-scale and CFD ultrasonography can demonstrate the vascular invasion in selected groups of patients who have renal cell carcinoma (RCC). Contrast-enhanced Doppler ultrasonography appears promising as a cost-effective, noninvasive imaging technique in the characterization and follow-up of indetermi- nate renal mass lesions. As nephron-sparing surgery is being increasingly used in the management of small RCC, intraoperative ultrasound (US) has be- come a useful tool in guiding the surgeon. This ar- ticle reviews the gray-scale and CFD features of benign and malignant renal masses encountered in radiology practice. RADIOLOGIC CLINICS OF NORTH AMERICA Radiol Clin N Am 44 (2006) 787–803 This article was originally published in Ultrasound Clinics 1:1, January 2006. a Department of Radiology, University Hospitals of Cleveland, Case Western Reserve University, 11100 Euclid Avenue, Cleveland, OH 44106, USA b Department of Radiology, University of Rochester Medical Center, 601 Elmwood Avenue, Box 648, Roches- ter, NY 14642, USA * Corresponding author. E-mail address: [email protected] (R.M. Paspulati). - Normal sonographic anatomy of the kidney - Sonographic technique - Pseudolesions of kidney Dromedary hump Persistent fetal lobulation Prominent column of Bertin (hypertrophy) Junctional parenchymal defect Hypoechoic renal sinus Inflammatory mass lesions - Benign renal tumors Angiomyolipoma Renal adenoma Oncocytoma Leiomyoma Hemangioma Juxtaglomerular tumor (reninoma) Hemangiopericytoma - Renal cell carcinoma Hereditary renal cell carcinoma Clinical presentation of renal cell carcinoma Imaging strategies of renal cell carcinoma Sonographic findings of renal cell carcinomas - Malignant uroepithelial tumors of the renal collecting system Transitional carcinoma of renal pelvis Squamous cell carcinoma and adenocarcinoma - Renal metastases - Renal lymphoma - Summary - References 787 0033-8389/06/$ – see front matter ª 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.rcl.2006.10.002 radiologic.theclinics.com
Ultrasonography is often the initial modality forimaging of the kidneys, although contrast-en-hanced CT is the established imaging modality forthe diagnosis of renal tumors. Despite technicallimitations, a large percentage of renal tumors canbe characterized by ultrasonography. Cystic andsolid renal parenchymal mass lesions can be welldifferentiated by ultrasonography. Technical ad-vances in the gray-scale and color-flow Doppler(CFD) ultrasound have improved the sensitivity indetection of small renal tumors. Gray-scale andCFD ultrasonography can demonstrate the vascular
0033-8389/06/$ – see front matter ª 2006 Elsevier Inc. All rightradiologic.theclinics.com
invasion in selected groups of patients who haverenal cell carcinoma (RCC). Contrast-enhancedDoppler ultrasonography appears promising asa cost-effective, noninvasive imaging technique inthe characterization and follow-up of indetermi-nate renal mass lesions. As nephron-sparing surgeryis being increasingly used in the management ofsmall RCC, intraoperative ultrasound (US) has be-come a useful tool in guiding the surgeon. This ar-ticle reviews the gray-scale and CFD features ofbenign and malignant renal masses encounteredin radiology practice.
This article was originally published in Ultrasound Clinics 1:1, January 2006.a Department of Radiology, University Hospitals of Cleveland, Case Western Reserve University, 11100 EuclidAvenue, Cleveland, OH 44106, USAb Department of Radiology, University of Rochester Medical Center, 601 Elmwood Avenue, Box 648, Roches-ter, NY 14642, USA* Corresponding author.E-mail address: [email protected] (R.M. Paspulati).
The kidneys are bean-shaped retroperitoneal organswith their medial aspects parallel to the lateralmargin of the adjacent psoas muscles. The normalorientation of the kidneys is such that the upperpole is medial and anterior to the lower pole. Theright kidney is 1 to 2 cm inferior in position ascompared with the left kidney because of the loca-tion of the liver superior to the right kidney. Therenal size varies with the age, sex, and body habi-tus. The measurement of renal volume is a moreeffective way of assessing the renal size, thoughmeasurement of renal length is more practical inregular practice [1]. The normal adult kidney mea-sures 10 to 12 cm in length, 4 to 5 cm in width,and 2.5 to 3 cm in thickness. A discrepancy ofmore than 2 cm between the lengths of two kidneysis considered significant and needs further evalua-tion. The liver and hepatic flexure of the colonare situated anterior to the right kidney. The spleenlies anterosuperior to the left kidney and the rest ofthe left kidney is related anteriorly with the colon.
On ultrasonography of a normal kidney, there isgood differentiation of the renal capsule, cortex,medulla, and central sinus complex [Fig. 1]. The re-nal capsule is visible as an echogenic line because ofthe interface between the echogenic perinephric fatand renal cortex. The renal parenchyma is com-posed of outer cortex and inner medulla (pyra-mids). The renal cortex is echogenic as comparedwith the medulla, but is iso- to hypoechoic as com-pared with the normal hepatic or splenic paren-chyma. The extension of renal cortex toward therenal sinus between the renal pyramids forms thecolumns of Bertin. The central sinus is composedof fat, fibrous tissue, renal vessels, and lymphaticvessels. It has highest echogenicity because of theadipose tissue, and its size increases with the ageof the person.
Sonographic technique
Sonographic evaluation of the right kidney is ide-ally performed from an anterior oblique approach
Fig. 1. Normal kidney. Longitudinal (A) and transverse (B) gray-scale US of the normal right kidney (calipers showthe maximum longitudinal dimension of the kidney). (C) The schematic representation of the sagittal section ofthe kidney. L, liver.
Renal Mass Sonography 789
using liver as an acoustic window, whereas the leftkidney is scanned through a posterior oblique ap-proach. The lower pole of the right kidney maybe imaged using a more posterior approach. Theupper pole of the left kidney is often best seenthrough an intercostal approach using spleen as awindow. In addition to supine position, decubitus,prone, or upright positions may provide betterimages of the kidneys [2]. An appropriate trans-ducer frequency ranging from 2.5 to 5 MHz shouldbe used, depending on the body habitus. Time gaincompensation and adjustment of other scanningparameters will allow a uniform acoustic patternthroughout the image [3]. Renal echogenicityshould be compared with the echogenicity ofthe liver and spleen [2]. The renal parenchyma ofa normal adult kidney is hypoechoic to the liverand spleen. The sonographic examination of thekidneys should include long axis and transverseviews of the upper poles, midportions, and thelower poles, with assessment of the cortex andcentral sinus. Maximum measurement of renallength should be recorded for both kidneys [2].Kidneys and the perirenal regions should be as-sessed for abnormalities. CFD and Power Doppler(PD) are used to differentiate vascular from non-vascular structures.
Pseudolesions of kidney
There are various developmental variants of the kid-ney that need to be identified on sonography toavoid misdiagnosis as renal neoplasm or other re-nal pathology [Table 1].
Dromedary hump
Dromedary hump is a common renal variation usu-ally seen as a focal bulge on the lateral border of theleft kidney [Fig. 2]. It is a result of adaptation of the
renal surface to the adjacent spleen. It can be easilydifferentiated from a renal mass because of its sim-ilar echotexture to that of adjacent renal paren-chyma on gray-scale ultrasound. CFD and PD willdemonstrate similar perfusion to that of adjacentrenal parenchyma.
Persistent fetal lobulation
Persistent fetal lobulation is another common renalvariant that can be mistaken for renal scarring, a con-sequence of chronic infective process of the kidneys.Persistent fetal lobulation can be differentiatedfrom scarred kidneys by the location of the renalsurface indentations, which do not overlie the med-ullary pyramids as in true renal scarring [4], butoverlie the space between the pyramids [Fig. 3].The underlying medulla and the cortex are normal.
Prominent column of Bertin (hypertrophy)
Prominent column of Bertin is a prominent corticaltissue that is present between the pyramids and pro-jects into the renal sinus [Fig. 4]. If not identified asa normal variant, it may be mistaken for an intrare-nal tumor. Sonography can accurately identify it bydepicting its continuity with the renal cortex anda similar echo pattern as the renal parenchyma.CFD and PD imaging can further assist by depictinga similar vascular pattern as that of normal renal tis-sue [5,6]. Prominent columns of Bertin are usuallyseen in the middle third of the kidney and are morecommon on the left side [5].
Junctional parenchymal defect
Junctional parenchymal defect (JPD) is another var-iant commonly mistaken for a cortical scar or a hy-perechogenic renal tumor. JPD is a linear ortriangular hyperechoic structure in the anterosupe-rior or posteroinferior surface of the kidney [Fig. 5].
Table 1: Renal pseudotumors
Pseudotumor Diagnostic imaging features
Congenital normal variantsDromedary hump Focal bulge in the lateral contour of left kidney with echotexture
similar to renal parenchymaPersistent fetal lobulation Renal surface indentations overlying the space between the pyramidsProminent column of Bertin Continuity with the normal cortex; echotexture and vascular
perfusion similar to the normal cortexJunctional parenchymal
defectCharacteristic location in the anterosuperior and posteroinferiorsurface of the kidney and demonstration of continuity with thecentral sinus
Hypoechoic renal sinus fat No distinct margin and normal vessels traversing the sinusInflammatory lesions Diagnosis is based on the proper clinical context
Focal bacterial nephritisRenal abscess
Paspulati & Bhatt790
These are caused by normal extensions of the renalsinus at the junction of the embryonic renunculi.These are differentiated from pathologic lesions bytheir characteristic location and demonstrate conti-nuity with the central sinus by an echogenic linecalled interrenicular septum [7–9].
Hypoechoic renal sinus
The echogenicity of the renal sinus may vary fromechogenic to anechoic. Hypoechoic renal sinusmay mimic a mass lesion [10]. Absence of a well-defined margin and demonstration of normalvessels traversing the renal sinus by CFD will aidin differentiating a hypoechoic renal sinus froma mass lesion [10].
Inflammatory mass lesions
Acute focal bacterial nephritis and renal abscessmay present as renal mass lesions indistinguishablefrom a renal tumor by ultrasonography and contrast-enhanced CT. The clinical presentation will aid in
Fig. 2. Dromedary hump. Longitudinal gray-scalesonogram of the left kidney demonstrates the drom-edary hump (arrow). SP, spleen.
differentiating these inflammatory pseudotumorsfrom RCC [11–13].
Benign renal tumors
Angiomyolipoma
Angiomyolipoma (AML) is a hamartoma and hasvariable amounts of mature adipose tissue, smoothmuscle, and thick-walled blood vessels. Eighty per-cent of the AMLs are sporadic in occurrence and20% of them are associated with tuberous sclerosis(TS). Presence of subependymal nodules and giantcell astrocytoma are sine qua non of TS, but notAML. On the contrary, 80% of the patients whohave TS develop AMLs [14,15]. Patients whohave TS develop AMLs at a much younger age,and these tend to be multiple, bilateral, and largerthan in sporadic cases. AMLs in patients who haveTS are more likely to grow and become symptom-atic [15,16]. The presence of estrogen and proges-terone receptors in angiolipomas has beenreported, and such AMLs are more common inwomen and in TS. These AMLs tend to grow dur-ing pregnancy and present with hemorrhage[17,18]. Small AMLs are asymptomatic and are in-cidental findings on imaging. AMLs smaller than4 cm are symptomatic and are at increased riskfor spontaneous hemorrhage [16,19]. Massiveretroperitoneal hemorrhage from AML, alsoknown as Wunderlich’s syndrome, has been foundin 10% of patients.
The characteristic sonographic appearance ofAML is a well-defined hyperechoic mass [Fig. 6].This increased echogenicity is attributed to the fatcontent, multiple interfaces, heterogeneous cellulararchitecture, and multiple vessels within the tumor[20,21]. However, there is significant overlap be-tween the imaging features of AML and RCC. Small
Fig. 3. Persistent fetal lobulations. Longitudinal (A) gray-scale sonogram of the right kidney demonstratespersistent fetal lobulation (arrow). L, liver. Schematic (B) appearance of persistent fetal lobulations (note fetallobulations may be single or multiple).
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Fig. 4. Prominent column of Bertin. (A) Longitudinal gray-scale US of the left kidney demonstrates a prominentcolumn of Bertin (arrows) mimicking an isoechoic renal mass. MRI was performed to confirm ultrasound findings.T1 flash fat-sat (B) and gadolinium-enhanced (C) MRI images of the kidneys reveal a prominent column of Bertin(arrows) seen in continuity with the renal cortex. (D) Schematic drawing of a prominent column of Bertin.
RCCs can be hyperechoic and indistinguishablefrom an AML on sonography. Acoustic shadowing,hypoechoic rim, and intratumoral cystic changesare some of the sonographic features found to behelpful in differentiating an AML from RCC. Hypo-echoic rim and intratumoral cystic changes are seenonly in RCC, whereas acoustic shadowing is
observed with AML [Fig. 7] [22–24]. PD of AMLmay reveal focal intratumoral flow and a penetratingflow pattern [25]. The demonstration of intratu-moral fat on CT confirms the diagnosis of anAML. The CT appearance of an AML also dependson the relative proportion of smooth muscle andvascular components of the tumor. Rarely, RCCs
Fig. 5. Junctional parenchymal defect. (A) Longitudinal gray-scale US of the right kidney demonstrates a notch inthe lateral border (arrow). L, liver. (B) Contrast-enhanced CT of the kidneys in another patient demonstrates thejunctional parencymal defect (arrow).
Paspulati & Bhatt792
Fig. 6. Angiomyolipoma. (A) Longitudinal gray-scale US of the right kidney demonstrates an echogenic mass(arrow) with posterior acoustic shadowing (arrowheads). (B) Corresponding CT (excretory phase) confirms thislesion to be an angiomyolipoma, seen as a fat attenuation lesion (arrow) with a household unit of -8.
can demonstrate fat attenuation caused by entrap-ment of the perinal or renal sinus fat, lipid necrosis,or osseous metaplasia [26]. The characteristic intra-tumoral fat cannot be detected in 4.5% of AMLs,and will have high attenuation on an unenhancedCT scan. This finding has been attributed to mini-mal fat content or immature fat [25,27]. TheseAMLs with low fat content demonstrate homoge-neous and prolonged enhancement on a contrast-enhanced scan, which distinguishes them from anRCC [25,28]. These AMLs with minimal fat are iso-echoic with renal parenchyma on sonography [25].The demonstration of micro- or macroaneurysms atangiography is reported to be characteristic of anAML [29].
The risk for spontaneous rupture and hemor-rhage of an AML is related to the tumor size and thesize of microaneurysms. AMLs larger than 4 cm andthose with microaneurysms larger than 5 mm arereported to be at increased risk for spontaneousrupture [16,30,31]. Management options of AMLs
Fig. 7. RCC. Longitudinal gray-scale sonogram of theleft kidney demonstrates a hyperechoic mass (M) aris-ing from the lower pole with areas of intra tumoralcystic changes (asterisk).
include observation, embolization, and partial ortotal nephrectomy. Prophylactic transcatheterembolization of AMLs larger than 4 cm is reportedto prevent tumor growth and spontaneous rupture[19,31,32]. Kothary and colleagues [33] have de-scribed a high recurrence rate of AMLs after embo-lization in patients who have TS, and recommendlong-term surveillance of these patients followingembolization.
Renal adenoma
Renal cell adenoma is considered to be a benigncounterpart of RCC, though the true nature and po-tential of this tumor is a subject of much debate.The size criterion used by many pathologists in dis-tinguishing an adenoma from RCC is based on theinitial observation by Bell, that renal cortical glan-dular tumors of smaller than 3 cm rarely metasta-size [34,35]. There are no histopathologic,histochemical, immunologic, or imaging character-istics that distinguish a benign adenoma from anRCC [36]. Most pathologists consider these smallrenal cortical tumors to be premalignant or poten-tially malignant and believe that tumor size is nota valid differentiating criterion [37]. The wide-spread use of US and CT has resulted in the inciden-tal detection of these tumors.
Oncocytoma
Renal oncocytoma is a benign tumor of renal tu-bular origin (renal tubular epithelium is also calledoncocyte). It has the male predominance and ageincidence similar to RCC. They are asymptomaticand are discovered as incidental findings on im-aging [38]. They are well-defined tumors of variablesize and can be as large as 20 cm [35]. The preoper-ative differentiation of oncocytomas from RCC isinvaluable, but is often difficult because of overlap
Renal Mass Sonography 793
of imaging features. The characteristic central stel-late scar on cross-sectional imaging and spoke-wheel pattern of enhancement on an angiogramare infrequently seen in oncocytomas and can alsobe seen in RCC [39–41]. Oncocytomas can be hypo-echoic, isoechoic, or hyperechoic to the renal paren-chyma on sonography. MRI is reported to besuperior to CT and US in identifying the imagingfeatures of a small tumor [42]. The radiologic fea-tures, which are helpful in distinguishing an onco-cytoma from RCC, include well-defined margins,homogeneous enhancement without hemorrhage,calcification or necrosis, presence of a central stel-late scar, and spoke-wheel pattern of arterial en-hancement. There are few reports of bilateral andmulticentric oncocytomas [43,44]. Renal oncocyto-ma and RCC can coexist in the same or contralateralkidney [45,46]. Dechet and colleagues [47] have re-ported coexistent RCC in 10% of a total 138 cases ofoncocytomas. Imaging-guided biopsy of renal tu-mors is indicated whenever there is radiologic suspi-cion of an oncocytoma [48–50].
Leiomyoma
Renal leiomyoma is a rare benign tumor of smoothmuscle origin. These tumors are either peripheral,arising from the renal capsule, or central in parapel-vic location. They are more common in women be-tween the second and fifth decades of life. Mostrenal leiomyomas are asymptomatic, with inciden-tal detection on routine diagnostic imaging. Theseare well-defined tumors and are indistinguishablefrom RCC by imaging. Renal leiomyomas have vari-able appearance on imaging: from that of an en-tirely solid, to a mixed solid/cystic, to an entirelycystic lesion. Renal leiomyomas appear as well-defined hypoechoic solid mass lesions on ultraso-nography. The peripheral lesions may extend intothe retroperitoneum and can resemble primary ret-roperitoneal sarcomas. The central lesions will havea mass effect over the collecting system and renalvasculature. They are most often avascular or hypo-vascular on angiogram [51–53].
Hemangioma
Hemangiomas are uncommon benign tumors ofthe kidney that can present with macroscopic he-maturia. They are commonly located in the renalpyramids and renal pelvis, and are classified intocapillary and cavernous hemangiomas. The vascu-lar spaces are small in capillary hemangioma andlarge in cavernous hemangiomas. They are predom-inantly smaller than 1 cm, but occasionally presentas large mass lesions [54]. Gray-scale US featuresa nonspecific solid mass, and CT demonstratesa well-defined low-density mass without significantenhancement [55,56]. Larger lesions may cause
displacement of the renal vessels and collectingsystem. Angiography may demonstrate a hypovascu-lar or hypervascular mass [57,58].
Juxtaglomerular tumor (reninoma)
Juxtaglomerular tumors are benign, renin-produc-ing tumors of the kidney that arise from the afferentarterioles of the glomerulus. These tumors were firstdescribed by Robertson and colleagues in 1967[59]. They are twice as common in women as inmen. In a young patient who has hypertension,the presence of a renal mass, elevated serum reninlevels, and hypokalemia should raise a suspicionof reninoma. The tumor is either hypo- or hypere-choic on sonography and appears as a well-definedhypodense solid mass on a contrast-enhanced CT.Angiography demonstrates a hypovascular masswith normal renal arteries. Renal vein samplingdemonstrates elevated renin levels in reninomas,but renin is also elevated in renal artery stenosis.Surgical resection of the tumor results in reversalof hypertension and hypokalemia [60–62].
Hemangiopericytoma
Hemangiopericytomas are rare renal tumors witha malignant potential that arise from the pericytes.Tumor-induced hypoglycemia is characteristic ofhemangiopericytoma and has been attributed tothe production of insulin-like growth factors bythe tumor. There are no distinguishing radiologicfeatures of hemangiopericytoma from RCC or othermesenchymal tumors of the kidney [63–65].
Renal cell carcinoma
RCC is the most common primary malignancy ofthe kidney. It accounts for 2% of all malignancies.There has been a steady increase of 38% in the in-cidence of RCC between 1974 and 1990 [66]. Thesurvival rates have also improved from 52% be-tween 1974 and 1976 to 58% between 1983 and1996 [66]. This trend has been attributed to the im-proved imaging technique and early diagnosis.Smith and colleagues [67] have reported that only5.3% of the tumors between 1974 and 1977 were3 cm or smaller as compared with 25.4% during1982 to 1985. Of these small tumors in the latergroup, 96.7% were incidentally discovered by ultra-sonography and CT. Most RCCs that are amenablefor surgical cure by either partial nephrectomy ornephron-sparing surgery are incidentally detectedby the increased use of cross-sectional imaging. Ul-trasonography, being the primary imaging modalityof the kidneys, is useful for screening and detectionof small RCCs [68,69].
The RCCs are classified histologically into fourmain types [Table 2]. These include clear cell
Chromophobe type 4%–5% –Collecting duct type <1% Aggressive tumor with poor prognosis –Medullary carcinoma <1% Aggressive tumor with poor prognosis
and common in sickle cell trait–
carcinoma, papillary carcinoma, chromophobe car-cinoma, and collecting duct carcinoma. The clearcell carcinomas are the most common type, ac-counting for 70% of the RCCs. The papillary typeis the second most common type, accounting for10% to 15% of the RCCs. The papillary type is sub-classsified into type 1 and type 2 tumors. The type 2papillary tumors are more aggressive than type 1.Clear cell and papillary tumors arise from the prox-imal tubular epithelium. The chromophobe carci-nomas account for 5% of the RCCs and arisefrom cells of distal tubule. The collecting duct carci-nomas are the least common type, arise fromcollecting duct epithelium, and are the mostaggressive of all RCCs. The medullary carcinomais a subtype of collecting duct carcinoma that ismore common in patients who have sickle cell trait.Imaging cannot differentiate the different histologictypes of RCC. The incidence of RCC is increased inacquired cystic disease of the kidney (ACDK). Clearcell carcinoma is the most common type of RCC as-sociated with ACDK. The incidence of papillary
type of RCC in ACDK is also higher than in the gen-eral population [70,71].
Hereditary renal cell carcinoma
RCCs are predominantly sporadic in occurrenceand only 4% of them are familial in nature. The dif-ferent types of hereditary RCCs are displayed inTable 3. The hereditary RCCs are characterized byautosomal dominant inheritance, presentation ata young age (third to fifth decades), and multifocaland bilateral tumors [72].
Clinical presentation of renal cell carcinoma
The classic clinical triad of hematuria, abdominalpain, and abdominal mass is seen in less than10% of patients. About 20% to 40% present withparaneoplastic syndrome, which includes anemia,fever, hypertension, hypercalcemia, and hepaticdysfunction [73–75]. RCC can be associated withStauffer syndrome, which is characterized by non-metastatic intrahepatic cholestasis. This syndromeis a tumor-induced inflammatory response and is
Table 3: Hereditary renal cell carcinoma
Syndrome InheritancePredominant renaltumor
Other renallesions
Associatedabnormalities
Von Hippel-Lindau AD Clear cell carcinoma Cysts HemangioblastomasRetinal angiomasPancreatic cystsNeuroendocrinetumors of pancreasPhaeochromocytoma
Hereditary papillary RCC AD Papillary type 1 None NoneHereditary leiomyomaRCC
AD Papillary type 2 None Cutaneous anduterine leiomyomas
reversible after resection of the tumor [76–78].About 2% of the male patients present with left-sided varicocele because of renal vein involvement[79].
Imaging strategies of renal cell carcinoma
The goal of imaging is detection, diagnosis, andstaging of RCC. Ultrasonography, CT, and MRIhave variable sensitivity in detecting and stagingRCC. Ultrasonography is less sensitive in detectingsmall renal lesions, especially those that do not de-form the contour of the kidney. The sensitivity ofCT and ultrasonography for detection of lesions3 cm and less is 94% and 79%, respectively [80].CT and MRI have nearly 100% accuracy in thediagnosis of RCC [81]. Ultrasonography is alsoless accurate than CT and MRI in staging of RCC.The accuracy of CT and MRI in staging of RCCranges from 67% to 96%. Catalano and colleagues[82] have reported 96% sensitivity, 93% specificity,and 95% accuracy of multidetector CT (MDCT) inevaluating Robson stage I RCC. Robson and tumor,nodes, and metastases (TNM) staging of RCC areoutlined in Table 4.
Despite these limitations, ultrasonography is stillthe initial imaging modality for screening and char-acterization of renal mass lesions. Ultrasonographyis also useful in characterizing indeterminate renalmass lesions detected by CT, such as atypical cystic
Table 4: Staging of renal cell carcinoma
Robsonstage Tumor description
TNMstage
I Tumor confined withinrenal capsuleTumor <2.5 cm T1Tumor >2.5 cm T2
II Tumor extension toperinephric fat or adrenalgland
lesions, hypovascular solid mass lesions, and AMLswith minimal fat component [83].
Sonographic findings of renal cell carcinomas
The sonographic spectrum of RCCs varies from hy-poechoic to hyperechoic solid mass lesions [Fig. 8].RCCs 3 cm and smaller are predominantly hypere-choic and must be differentiated from AMLs[84,85]. The hyperechoic appearance is reportedto be caused by papillary, tubular, or microcystic ar-chitecture; minute calcification; intratumoral hem-orrhage; cystic degeneration; or fibrosis [24]. Thepresence of an anechoic rim caused by a pseudocap-sule and intratumoral cystic changes can aid in dif-ferentiation of hyperechoic RCC from AML [24,86].Several investigators have reported acoustic shad-owing as a useful sign of AML [22,23]. Small isoe-choic RCCs and those located at the poles can bemissed by ultrasonography [26]. The isoechoicRCCs must be differentiated from pseudotumors,which include prominent column of Bertin, drom-edary hump, persistent fetal lobulation, and com-pensatory hypertrophy. Careful attention to themorphology on gray-scale US will differentiatepseudotumors from a mass lesion. Power Dopplerand contrast-enhanced sonography are useful indifferentiating pseudotumors from true renal masslesions by demonstrating similar vascularity of thepseudotumors to that of adjacent normal renal cor-tex [87,88]. Power Doppler and contrast-enhancedsonography will demonstrate the vascularity of a re-nal mass, but cannot differentiate an RCC from anAML [87,88].
Approximately 15% of the RCCs are cystic in na-ture and may result from extensive necrosis of a tu-mor, or represent a primary cystic renal carcinoma[89]. Histologically, the cystic RCCs are predomi-nantly of clear cell type. RCCs with extensive necro-sis are more aggressive as compared with theprimary multilocular cystic RCCs [90,91]. Multiloc-ular cystic RCC (MCRCC) is an uncommon subtypeof RCC and constitutes about 3% of all RCCs.MCRCCs have a benign clinical course and maybenefit from nephron-sparing surgery [92]. Cross-sectional imaging with US and CT of MCRCC willdemonstrate well-defined, multilocular cystic masswith thin septations. Dystrophic calcification andmural nodules are less common and MCRCCshould be included in the differential diagnosis ofall multilocular cystic renal mass lesions in adults[93]. Small MCRCCs of less than 3 cm are hypere-choic on US and can mimic solid mass lesions,but show minimal enhancement on contrast-enhanced CT or MRI [94]. Contrast-enhancedDoppler US is reported to improve the diagnosticaccuracy of malignant cystic renal mass by
Paspulati & Bhatt796
Fig. 8. RCC. Longitudinal gray-scale (A) and CFD (B) sonography of the right kidney demonstrates an iso- tohypoechoic mass arising from the lower pole, which shows presence of vascularity consistent with a RCC.
demonstrating the vascularity of the intracystic sep-tations and mural nodules [95].
CT and MRI are the standard imaging methodsfor staging of RCC. However, US is useful in detect-ing the venous invasion and for demonstrating thecranial extent of the inferior vena cava (IVC) throm-bus. Overall accuracy, sensitivity, and specificity ofCFD for detecting the tumor involvement of renalvein and IVC is 93%, 81%, and 98%, respectively[96]. McGahan and colleagues [97] have reporteda 100% sensitivity in the detection of renal vein in-volvement as compared with 89% sensitivity forIVC involvement by CFD sonography. Hence, USmay be used as a complementary imaging modalitywhen CT findings are equivocal in the assessment ofvenous extension of the tumor. The tumor throm-bus is seen as an echogenic intraluminal mass caus-ing distension of the vein. CFD will demonstrateflow around a bland thrombus and vascularitywithin a tumor thrombus. Use of US contrast agentsis reported to improve the accuracy not only indemonstrating the extent of the thrombus butalso in differentiating a tumor from a blandthrombus.
The prognosis of RCC will depend on the stage,histologic type, and grade of the tumor. The5-year survival rates of TNM stages I, II, III, and IVare reported to be 91%, 74%, 67%, and 32%,
respectively [98]. The presence of a sarcomatoidcomponent is reported to have poor outcome [99].
Malignant uroepithelial tumors of the renalcollecting system
Malignant uroepithelial tumors of the renal pelvisconstitute about 5% of all the urinary tract neo-plasms [100]. 90% of them are transitional cell car-cinomas (TCC), 5% to 10% are squamous cellcarcinomas, and less than 1% are adenocarcinomas[101,102].
Transitional carcinoma of renal pelvis
TCCs of the renal pelvis have similar epidemiologicfeatures to those of bladder and ureter. The risk fac-tors include exposure to chemicals in petroleum,rubber, and dye industries; analgesic abuse; andchronic inflammations. TCC is one of the severalextracolonic manifestations of hereditary nonpoly-posis colorectal cancer (HNPCC)/Lynch syndrome[103]. The mean age of presentation of TCC is 68years with a higher rate of incidence in men thanwomen. Painless hematuria is the characteristicclinical presentation of TCC [104]. Three morpho-logic forms of TCC are described, including focalintraluminal mass, mural thickening with narrow-ing of lumen, and an infiltrating mass in the renal
Renal Mass Sonography 797
sinus [105–107]. The excretory urogram has beenthe primary imaging modality for the diagnosis ofTCC and is being replaced by CT or MR urogram[108,109]. These imaging modalities have the ad-vantage of evaluating the entire urinary tract, whichis crucial in the assessment of TCC.
Sonography demonstrates a poorly definedhypo- or hyperechoic mass in the renal sinus withor without pelvicaliectasis. The mass lesions are ini-tially intraluminal and later invade the renal sinusfat and renal parenchyma. Infiltrating tumors ofthe renal parenchyma tend to preserve the reniformshape of the kidney [106,110].
Squamous cell carcinomaand adenocarcinoma
Squamous cell carcinoma is the second most com-mon malignant uroepithelial tumor of the renalcollecting system [Fig. 9]. Chronic irritation of theuroepithelium is the etiologic factor, which leadsto squamous or columnar metaplasia of the transi-tional epithelium. Renal calculi with longstandinghydronephrosis and inflammation are important
predisposing factors for squamous cell carcinomaand adenocarcinoma of the renal pelvis[101,102,111]. The clinical presentation rangesfrom painless hematuria to nonspecific flank paincaused by hydronephrosis [101]. Squamous cell car-cinomas are more aggressive than TCC and the tu-mor manifests as an infiltrating mass involvingthe collecting system, renal sinus fat, and renal pa-renchyma [112]. It is often difficult to differentiatesquamous cell carcinoma of the renal pelvis fromxanthogranulomatous pyelonephritis by imaging[113,114].
Renal metastases
The frequency of renal metastases is reported tovary from 7% to 13% based on autopsy findings[115,116]. More frequent use of cross-sectional im-aging has resulted in an increase in the detectionof renal metastases [117,118]. In patients whohave a known history of malignancy, renal metasta-ses are three times more common than primary re-nal tumors and are usually asymptomatic [115,116].The tumors that most commonly metastasize to
Fig. 9. Squamous cell carcinoma. Longitudinal gray-scale (A) and CFD (B) US of the left kidney demonstrates anenlarged kidney with areas of chunky calcification (arrows) with posterior acoustic shadowing (arrowheads).There is increased vascularity in the mass with large areas of necrosis (asterisk). Corresponding contrast-enhanced coronal CT (C) confirms the presence of calcification (arrows) and necrosis (asterisk). This tumorwas pathologically confirmed to be a squamous cell carcinoma.
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Fig. 10. Hyperechoic renal metastasis. Patient is a known case of esophageal carcinoma. Longitudinal gray-scaleUS of the right (A) and the left (B) kidney demonstrate multiple hyperechoic mass lesions (arrows). Tranverse CFDimage (C) of the right kidney reveals increased vascularity.
the kidney are carcinoma of the lung, breast, andgastrointestinal tract, and melanoma [Fig. 10][115,119]. The most common manifestation is bi-lateral, multiple renal mass lesions, though theycan present with unilateral and solitary lesions. Re-nal metastases can be well-defined focal mass le-sions or infiltrating in nature [117,120].
The most common sonographic appearance ishypoechoic, cortical mass lesions withoutthrough-transmission [Fig. 11] [121,122]. CT hashigher sensitivity and accuracy than US in the
detection of renal metastases [121–123]. In patientswho have a known extrarenal primary malignancy,tissue sampling is necessary to differentiate metasta-ses from a synchronous primary RCC [124].
Renal lymphoma
Renal lymphoma is commonly secondary to hema-togeneous dissemination or contiguous extensionfrom a retroperitoneal nodal disease. Primary lym-phoma is rare as there is no lymphoid tissue in
Fig. 11. Hypoechoic renal metastasis. Longitudinal gray-scale ultrasound of the right (A) and left (B) kidneysdemonstrate multiple hypoechoic masses (arrows) in the renal parenchyma consistent with metastasis. L, liver.
Renal Mass Sonography 799
Fig. 12. Lymphoma. Longitudinal gray-scale US of the right (A) and the left (B) kidneys demonstrates bilaterallyenlarged kidneys (R, 15.6 cm; L, 14.8 cm). In addition, right kidney also demonstrates a focal mass (arrow) inpatient who has known non-Hodgkins lymphoma.
the kidney [125,126]. Though the reported inci-dence of renal involvement on autopsy rangesfrom 30% to 60%, the actual detection by imagingis only 3% to 8% [127]. The kidney is most com-monly involved by the non-Hodgkin’s B-cell typeof lymphoma [128–130]. There is a wide spectrumof renal involvement of lymphoma. Unilateral orbilateral multiple renal mass lesions are the mostcommon type of renal lymphoma. Bilateral renalinvolvement is reported to occur in 50% to 72%of lymphomas [Fig. 12]. Other manifestations in-clude solitary renal mass, diffuse infiltration of therenal parenchyma, contiguous invasion from retro-peritoneal disease, and isolated perinephric mass[127]. In the diffuse infiltrative form, there is prolif-eration of lymphoma within the interstitium of therenal parenchyma, resulting in enlarged kidneyswith preservation of the reniform shape [120].The renal mass lesions are homogenously hypoe-choic on ultrasonography and are hypodense ona contrast-enhanced CT [120,131,132]. Spontane-ous hemorrhage, cystic changes, and calcificationare uncommon and are usually secondary to priortreatment [127].
Summary
CT is the gold standard for the detection and char-acterization of renal mass lesions and in staging ofRCC. Despite its limitations, ultrasonography is of-ten the first imaging modality of the kidneys andplays an important role in the diagnosis of renal tu-mors. Technical advances in the gray-scale ultraso-nography have improved the detection of smallRCCs. CFD and contrast-enhanced Doppler ultra-sonography are useful in characterization of renaltumors and in the identification of pseudotumors.
As nephron-sparing surgery is now an establishedtechnique in the management of small RCC, intra-operative US has a key role in guiding the surgeon.
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