Kidneys and Urinary Tract
Jonathan R. Dillman
Kassa Darge
INTRODUCTION
Congenital and acquired abnormalities of the kidneys and urinary tract are common in the pediatric population. Accurate detection and characterization of such abnormalities are important, as many may be associated with substantial morbidity, such as infection and progressive kidney injury. In this chapter, imaging techniques for evaluating the kidneys and urinary tract are discussed, and normal anatomy is reviewed. In addition, selected disorders affecting the pediatric kidneys and urinary tract are presented, including clinical features, characteristic imaging findings, and treatment approaches.
IMAGING TECHNIQUES
Radiography
Radiography has a limited role in assessment of the kidneys and urinary tract. On occasion, renal enlargement (e.g., due to a mass or severe hydronephrosis) may be first detected by radiography-based mass effect with the epicenter in one of the renal fossae. Urinary tract calculi may be evident on radiographs, though sensitivity may be decreased for small or radiolucent stones. In addition, bowel contents may obscure or simulate stones. Radiography may also depict urinary tract anatomy when obtained after IV contrast administration for preceding computed tomography (CT).
Ultrasound
Ultrasound is generally the first-line imaging modality for assessing the kidneys and urinary tract in the pediatric population. Gray-scale imaging can be used to thoroughly assess the renal parenchyma, renal collecting system, and bladder. The ureters may also be evaluated when dilated. Images through the kidneys and bladder are typically acquired in the longitudinal and transverse planes. Color and power Doppler imaging are useful for detecting blood flow within the kidneys and mass-like abnormalities affecting the kidneys and urinary tract. Color Doppler imaging can also be used to detect urolithiasis based on the presence of “twinkling” artifact.1,2 Spectral Doppler imaging can be used to assess the renal arteries and veins and to detect conditions such as renovascular hypertension and renal vein thrombosis. Advantages of ultrasound include its widespread availability, low cost, portability, and lack of ionizing radiation.
The intravesical use of ultrasound microbubble contrast agents enables the performance of contrast-enhanced voiding urosonography (ceVUS) for detecting vesicoureteral reflux (VUR). This technique has been shown to be more sensitive for detecting VUR than fluoroscopic voiding cystourethrography (VCUG),3 and it can be combined with transperineal ultrasound of the urethra.
Computed Tomography
CT may be used to evaluate the kidneys and urinary tract in select situations in the pediatric population. Noncontrast CT is most often used to identify symptomatic urinary tract calculi that cannot be detected by ultrasound.4 CT imaging with intravenous (IV) contrast material is commonly used to assess renal trauma and masses as well as to evaluate for complications of infection, such as perinephric abscess. When excretory-phase imaging (CT urography) is desired, the IV contrast material bolus can be split into two injections (separated by about 10 minutes).5 This split-bolus technique provides
images with optimal renal parenchymal enhancement as well as excreted contrast material in the urinary tract. The recent development of iterative reconstruction techniques has the potential to allow for reduced dose CT imaging of the kidneys and urinary tracts with preserved image quality.6
images with optimal renal parenchymal enhancement as well as excreted contrast material in the urinary tract. The recent development of iterative reconstruction techniques has the potential to allow for reduced dose CT imaging of the kidneys and urinary tracts with preserved image quality.6
CT imaging is generally performed using helical technique. The acquisition of an isotropic data set allows for the reconstruction of axial CT images at a variety of section widths and the creation of two-dimensional (2D) multiplanar reformations and three-dimensional (3D) reconstructions. These 2D multiplanar and 3D reconstruction CT images are helpful for evaluation of complex renal and urinary tract anatomic structures and enhance diagnostic accuracy.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is an increasingly utilized imaging modality for evaluating the kidneys and urinary tract, particularly in children. MRI can be used to assess a wide variety of renal parenchymal abnormalities identified but incompletely characterized by ultrasound or CT, such as certain renal masses.
MR urography (MRU) examinations are tailored to assess the kidneys and urinary tract, and this technique can be used to decipher complex urinary tract anatomy, evaluate suspected urinary tract obstruction, and estimate differential renal function. Most MRU examinations utilize multiple T2-weighted pulse sequences to image fluid (urine) in the urinary tract (MR hydrography) as well as postcontrast T1-weighted pulse sequences. Dynamic postcontrast T1-weighted MR imaging (e.g., up to 50 or more 3D image volumes over 8 to 15 minutes) can be used to evaluate for urinary tract obstruction and estimate differential renal function. IV hydration and furosemide injection assist with the acquisition of high-quality MRU images, as these adjunct maneuvers improve urinary tract visualization by increasing distention, allow evaluation of the urinary tract under diuretic “stress,” and help minimize T2*-related signal loss of postcontrast images.7 Volumetric 3D T2-weighted and postcontrast T1-weighted MR image acquisitions allow for 2D multiplanar reformations of the kidneys and urinary tracts as well as 3D reconstructions.
Drawbacks to MRI in children include the need for sedation (or general anesthesia) in some children and long examination times. Also, gadolinium chelate contrast materials should not be administered to pediatric patients with suspected or known acute kidney injury or chronic kidney disease with an estimated glomerular filtration rate <30 mL/min because of risk of nephrogenic systemic fibrosis.8
Nuclear Medicine
Several nuclear medicine studies can be used to evaluate the kidneys and urinary tract. Tc-99m dimercaptosuccinic acid (DMSA) is a radiotracer that selectively binds renal cortex and allows detection of pyelonephritis and parenchymal scarring, as well as calculation of differential renal function based on parenchymal mass. Imaging is generally performed 2 to 3 hours after radiotracer injection, and both pinhole collimation and single-photon emission computed tomography technique can be used to acquire higher-quality images.
Dynamic renal scintigraphy is most often performed using Tc-99m mercaptoacetyltriglycine (MAG3) and can be used to estimate differential renal function (based on effective renal plasma flow to the kidneys) and evaluate suspected urinary tract obstruction (based on renal diuretic response following IV furosemide injection). Tc-99m pertechnetate nuclear cystography is highly sensitive for the detection of VUR and can be used to as a first-line imaging test for evaluating suspected VUR in girls or to follow-up known VUR in children of either gender.
The major drawback of nuclear cystography is its inability to clearly depict urinary tract anatomy. This is a particular disadvantage in boys with possible posterior urethral valves where assessment of the urethra is critical. All of these nuclear medicine studies require ionizing radiation for image creation, with DMSA renal scintigraphy imparting the greatest radiation dose.
Fluoroscopy
VCUG is frequently the first-line imaging modality used to evaluate suspected VUR. Water-soluble iodinated contrast material is instilled into the bladder by gravity drip, and fluoroscopic images of the urinary tract are obtained before, during, and after voiding. Routinely acquired VCUG images are presented in Table 17.1. VUR, when present, is graded based on whether refluxed contrast material reaches the renal collecting system, as well as the degree of upper urinary tract dilatation and tortuosity. VCUG can also be used to further assess bladder, urethra, and upper tract anatomy when VUR is present. The radiation dose of VCUG examinations can be substantially reduced by using state-of-the-art fluoroscopy equipment and radiation dose reduction techniques, such as pulsed fluoroscopy and last image hold (grab or capture).9
Although the pediatric urethra is usually evaluated using VCUG, retrograde urethrography (RUG) may be necessary when the clinical question pertains specifically to the urethra, particularly in older boys. RUG is most commonly used to
assess the male urethra in the setting of trauma (e.g., pelvic or perineal injury with hematuria) or suspected stricture.
assess the male urethra in the setting of trauma (e.g., pelvic or perineal injury with hematuria) or suspected stricture.
TABLE 17.1 Routine Pediatric Voiding Cystourethrogram Images Acquired in Children with Typical Renal and Urinary Tract Anatomya | ||
|---|---|---|
|
NORMAL ANATOMY AND VARIANTS
Kidneys
The kidneys typically reside in the retroperitoneum just below the liver and spleen (one on each side of the spine) and are surrounded by perinephric fat and Gerota fascia. The renal hila are normally oriented anteromedially, and the long axis of the kidney is typically parallel to the long axis of the ipsilateral psoas muscle. The kidneys should increase in length over time during childhood.10
Renal parenchyma is composed of peripheral cortex and deeper medulla. The renal cortex contains glomeruli and tubules, whereas the medulla (medullary pyramids) contains tubules and collecting ducts. Normal kidneys typically demonstrate corticomedullary differentiation at imaging (e.g., ultrasound and MRI), and loss of corticomedullary differentiation may be a sign of parenchymal abnormality (e.g., renal dysplasia or autosomal recessive polycystic kidney disease [ARPKD]). The tips of the renal medullary pyramids, or papillae, empty into the renal collecting system and may be susceptible to certain pathologic processes, such as papillary necrosis because of renal ischemia. Embryologically, the kidney develops from two structures: (1) the metanephric blastema, which gives rise to renal glomeruli and tubules of the nephron and (2) the mesonephric (wolffian) duct (ureteric bud), which gives rise to the renal collecting system and collecting ducts.
Upper Urinary Tract
The renal collecting system and ureter comprise the upper urinary tract. The renal collecting system is made up of numerous calyces, with well-defined papillary impressions and sharp fornices, which empty into a renal pelvis. On occasion, the renal pelvis may extend beyond the contour of the kidney giving rise to an extrarenal pelvis, commonly considered a normal variant. The ureter is muscular-walled tubular structure connecting the renal pelvis to the bladder, normally inserting into the ipsilateral bladder trigone region. The mucosa of the renal collecting system and ureter is difficult to appreciate by imaging unless thickened because of infection, inflammation, or obstruction. Like the renal collecting system, the ureter also arises from the ureteral bud of the mesonephric duct.
Urinary Bladder
The bladder serves as a reservoir for urine produced by the kidneys. Normal bladder volume increases with age during childhood. The Koff formula11 is commonly employed for estimating bladder capacity and is based on patient age (in years), stating:
Capacity in mL = (Age + 2)×30
Appearance and thickness of the bladder wall depend in part on the degree of distention with urine. In utero, the bladder dome communicates with the allantois membrane via a tubular channel coursing through the space of Retzius (the urachus). Much of the bladder develops from the urogenital sinus, whereas the trigone arises from the caudal portions of the mesonephric ducts.
Urethra
The urethra is a tubular channel that transports urine from the bladder to outside the body. Micturition normally occurs after relaxation of the internal (involuntary) and external (voluntary) sphincter mechanisms. The female urethra is short, typically opens just above the vaginal orifice, and is rarely abnormal (other than in developmental abnormalities of the urogenital sinus, such as bladder exstrophy or cloacal malformation). The male urethra is substantially longer and has four parts (Fig. 17.1). The pelvic portion of the urogenital sinus gives rise to the entire female urethra as well as the prostatic and membranous portions of the male urethra. The anterior male urethra arises from the phallic portion of the urogenital sinus.
SPECTRUM OF RENAL DISORDERS
Congenital and Developmental Anomalies
Congenital and developmental anomalies of the kidneys are common. While often isolated, they also regularly occur in the presence of other congenital anomalies. For example, renal anomalies can be associated with congenital heart disease, mullerian anomalies, anorectal malformations, abnormalities of the urogenital sinus, and a variety of chromosomal anomalies (e.g., Turner syndrome [45,X]). In such conditions, detection may occur during the antenatal period, and renal sonography is typically performed early in life. Renal anomalies are also part of the nonrandom VACTERL association of congenital anomalies (Vertebral, Anorectal, Cardiovascular, TracheoEsophageal, Renal, Limb anomalies).12
Agenesis
Renal agenesis, or congenital absence of the kidney, is due to in utero failure of development, and it can be either unilateral or bilateral. Bilateral renal agenesis is incompatible with life, a cause of Potter sequence, and results in fetal demise or death very soon after birth, in part due to pulmonary insufficiency. Unilateral renal agenesis can be detected by antenatal ultrasound, and it is commonly associated with other congenital anomalies, including VACTERL association, mullerian anomalies in girls, and seminal vesicle cysts in boys.13 Renal agenesis is a part of the MURCS association (Mullerian anomaly, Renal agenesis, Cervicothoracic Somite dysplasia).14 No identifiable renal tissue is found in the ipsilateral renal fossa at ultrasound imaging. Careful imaging assessment of the retroperitoneum and pelvis must be undertaken to exclude the possibility of an ectopic, sometimes dysplastic kidney. The ipsilateral adrenal gland usually appears abnormally elongated (“lying down” appearance)
on longitudinal ultrasound images early in life (Fig. 17.2).15 Contralateral renal parenchymal hypertrophy is often present, even early in life. CT and MRI demonstrate similar findings and can be used to search for residual renal tissue, if clinically necessary. There is no specific medical or surgical therapy for renal agenesis, and prognosis is generally good assuming the contralateral kidney is normal. Care should be taken to preserve contralateral kidney function.
on longitudinal ultrasound images early in life (Fig. 17.2).15 Contralateral renal parenchymal hypertrophy is often present, even early in life. CT and MRI demonstrate similar findings and can be used to search for residual renal tissue, if clinically necessary. There is no specific medical or surgical therapy for renal agenesis, and prognosis is generally good assuming the contralateral kidney is normal. Care should be taken to preserve contralateral kidney function.
 FIGURE 17.1 Anatomy of the male urethra. |
Hypoplasia and Dysplasia
Abnormal development of the kidney in utero can cause a variety of parenchymal abnormalities. Renal hypoplasia refers to an abnormally small kidney that contains fewer nephrons than expected. This condition is uncommon and in some cases may be the result of decreased blood flow or congenital VUR. At imaging, hypoplastic kidneys appear smaller than expected for patient age but are otherwise morphologically normal without overt evidence of dysplasia (Fig. 17.3).16
 FIGURE 17.2 1-day-old girl with right renal agenesis. No kidney is identified in the right renal fossa, and the right adrenal gland (arrows) appears abnormally elongated. |
Renal dysplasia commonly occurs in the setting of in utero urinary tract obstruction (e.g., due to ectopic
ureteric insertion, ureterocele, or severe upper urinary tract narrowing) with resultant disordered assembly of renal tissue elements. While sometimes isolated, renal dysplasia can be associated with a variety of predisposing conditions, including upper urinary tract duplication (upper moiety parenchyma is most often dysplastic), posterior urethral valves, prune-belly syndrome, and mullerian anomalies.16,17 Microscopically, immature renal tubules embedded in primitive stroma, islands of cartilage, epithelium-lined cysts, and large aberrant blood vessels may be seen (Fig. 17.4).
ureteric insertion, ureterocele, or severe upper urinary tract narrowing) with resultant disordered assembly of renal tissue elements. While sometimes isolated, renal dysplasia can be associated with a variety of predisposing conditions, including upper urinary tract duplication (upper moiety parenchyma is most often dysplastic), posterior urethral valves, prune-belly syndrome, and mullerian anomalies.16,17 Microscopically, immature renal tubules embedded in primitive stroma, islands of cartilage, epithelium-lined cysts, and large aberrant blood vessels may be seen (Fig. 17.4).
 FIGURE 17.3 7-year-old boy with hypoplastic (vs. mildly dysplastic) left kidney due to congenital vesicoureteral reflux. Axial T2-weighted fat-saturated MR image shows a small but otherwise morphologically normal left kidney (arrows). There is slightly diminished left kidney corticomedullary differentiation and mild pelvicaliectasis. Right kidney is enlarged due to compensatory hypertrophy. |
 FIGURE 17.4 Hypoplastic/dysplastic kidney resected from a 9-year-old boy with a history of prune-belly syndrome (left). Microscopically (right), it showed disorganized renal elements characteristic of renal dysplasia, including nodules of abortive tubular structures surrounded by collarettes of stroma, large aberrant blood vessels, and islands of cartilage (arrow) ( hematoxylin and eosin, original magnification, 200×). |
At imaging, renal dysplasia can have a spectrum of appearances. Milder forms of dysplasia are associated with abnormal morphologic appearance of renal parenchyma and collecting system, small parenchymal cysts, and loss of corticomedullary differentiation (Fig. 17.5).18 The most severe form of renal dysplasia, multicystic dysplastic kidney (MCDK), presents as numerous variably sized non-communicating cysts with no normal intervening renal parenchyma (Fig. 17.6). This entity may be associated with contralateral ureteropelvic junction (UPJ) obstruction and VUR, and it rarely can occur in the setting of pelvic kidney, horseshoe kidney, or crossed renal ectopia. MCDKs commonly decrease in size over time and may involute (based on ultrasound evaluation); rarely, MCDKs may enlarge.19,20 Asymptomatic dysplastic kidneys are often managed conservatively, whereas symptomatic dysplastic kidneys (e.g., those causing recurrent infections or hypertension) are often surgically removed.21 Based on recent studies, MCDKs are likely at no significantly increased risk
of malignancy.20,21 In the setting of unilateral renal dysplasia, care should be taken to preserve contralateral kidney function.
of malignancy.20,21 In the setting of unilateral renal dysplasia, care should be taken to preserve contralateral kidney function.
 FIGURE 17.5 6-month-old girl with left kidney dysplasia. Coronal T2-weighted fat-saturated MR image shows a poorly formed left kidney with absent corticomedullary differentiation and scattered tiny parenchymal cysts (white arrows). The left ureter is obstructed due to an ectopic insertion (black arrow) and is markedly dilated. |
 FIGURE 17.6 2-month-old girl with right multicystic dysplastic kidney. A: Transverse gray-scale ultrasound image shows multiple variably sized, noncommunicating cysts (arrows) in the right renal fossa. Residual right kidney parenchyma is abnormally echogenic. B: Sagittal T2-weighted fat-saturated MR image shows similar findings (arrows). |
Abnormal Rotation, Ectopia, and Fusion
Abnormal Rotation
As the fetal kidneys ascend from the pelvis in utero, they normally undergo medial rotation in the transverse plane. As a result, the renal hila and pelves are normally oriented anteromedially. When a kidney fails to completely rotate, it is said to be either nonrotated or malrotated (Fig. 17.7). Much less commonly, the renal hilum and pelvis may be oriented posteriorly or laterally due to either hyperrotation or reverse rotation. Although kidneys that appropriately ascend can be abnormally rotated, this phenomenon is more commonly associated with ectopic or fused kidneys.22 Abnormal renal rotation can be detected by a variety of imaging modalities (e.g., ultrasound, CT, MRI, and VCUG) and is most often by itself of no clinical significance.
Ectopia
As previously mentioned, the kidneys normally ascend in utero from the pelvis to their expected locations in the retroperitoneum. Consequently, an ectopic kidney can be identified anywhere along its expected course of ascent. Less commonly, an ectopic kidney may cross the midline (crossed ectopia) or may be located superior to the expected location of the renal fossa (e.g., in the setting of a Bochdalek-type congenital diaphragmatic hernia). The majority of ectopic kidneys are located in the pelvis, whereas thoracic kidneys are very rare and most often left sided.23 Ectopic kidneys in children are usually identifiable using ultrasound and may show absence of the renal sinus echo complex.24 They can be easily
identified by CT and MRI. Suspicion should be high when imaging reveals an empty renal fossa. Ectopic kidneys are commonly nonrotated or malrotated, and they may have mild pelvicaliectasis, even in the absence of urinary tract obstruction or VUR (Fig. 17.8). The arterial supply and venous drainage of ectopic kidneys are usually anomalous, with renal arteries arising from nearby major arterial structures and renal veins draining to nearby major systemic venous structures. Ectopic kidneys are often asymptomatic but may be complicated by ipsilateral VUR, UPJ obstruction, and urolithiasis.25 Small, poorly functioning malpositioned kidneys rarely can present with urinary dribbling and/or incontinence due to an associated ectopic ureter.26
identified by CT and MRI. Suspicion should be high when imaging reveals an empty renal fossa. Ectopic kidneys are commonly nonrotated or malrotated, and they may have mild pelvicaliectasis, even in the absence of urinary tract obstruction or VUR (Fig. 17.8). The arterial supply and venous drainage of ectopic kidneys are usually anomalous, with renal arteries arising from nearby major arterial structures and renal veins draining to nearby major systemic venous structures. Ectopic kidneys are often asymptomatic but may be complicated by ipsilateral VUR, UPJ obstruction, and urolithiasis.25 Small, poorly functioning malpositioned kidneys rarely can present with urinary dribbling and/or incontinence due to an associated ectopic ureter.26
 FIGURE 17.7 3-year-old girl with nonrotated right kidney. Axial postcontrast T1-weighted fat-saturated MR image shows nonrotation of the right kidney, with an anteriorly oriented renal pelvis (arrow). The left renal fossa is empty. |
Abnormal Fusion
Anomalies of renal fusion can take on several configurations. The horseshoe kidney is fused in the midline, most often at the lower poles. Both renal moieties are lower in position than expected (as ascent is hindered by the inferior mesenteric artery), and they are abnormally oriented with their lower poles directed medially. Horseshoe kidneys occur with increased frequency in Turner syndrome (45,X).27 Cross-fused renal ectopia is diagnosed when both kidneys are located on the same side of the midline and fused, whereas pancake kidney refers to bilateral pelvic kidneys that are fused at both the upper and lower poles.
 FIGURE 17.8 11-year-old boy with pelvic kidney. Sagittal contrast-enhanced CT image shows that the left kidney (arrows) is located in the pelvis, posterior to the bladder (B). The kidney is nonrotated with its mildly dilated renal collecting system (asterisk) facing anteriorly. |
At ultrasound, abnormal long-axis orientation of the kidneys with poor visualization of the lower poles may be the first clue to the presence of a horseshoe kidney.22 Ultrasound imaging in the transverse and coronal planes can be used to identify either a parenchymal or fibrous isthmus located anterior to the lumbar spine between the abdominal aorta and inferior mesenteric artery (Fig. 17.9). The kidneys are frequently abnormally rotated, nonobstructed pelvicaliectasis may be present, and multiple bilateral renal arteries are typical (arising from the abdominal aorta, iliac arteries, and sometimes the inferior mesenteric artery).22 CT or MRI can be used to confirm the diagnosis of horseshoe kidney if ultrasound is inconclusive (Fig. 17.10). Horseshoe kidneys may be associated with VUR, UPJ obstruction, and urolithiasis, and they are at increased risk of traumatic injury due to their location adjacent to the spine (Fig. 17.11).28 Horseshoe kidneys have been reported to be associated with an increased incidence of Wilms tumor; a large study by the National Wilms Tumor Study Group demonstrated 41 of 8,617 (0.48%) Wilms tumors arose in a horseshoe kidney.29 Horseshoe kidneys require no specific treatment.
Crossed fused renal ectopia can also be readily identified by ultrasound. Both kidneys are located on the same side of the midline (with the ectopic kidney usually inferior). The ectopic kidney’s ureterovesical junction is usually orthotopic, on the contralateral side. A notch or indentation is frequently seen at the site of parenchymal fusion (Fig. 17.12).30 Cross-fused renal moieties are frequently abnormally rotated (e.g., anteriorly or laterally oriented), and they may take on a variety of configurations (e.g., S-shape or L-shape). Similar to horseshoe kidneys, renal arterial supply is typically anomalous, and these
kidneys may be associated with VUR, UPJ obstruction, and urolithiasis. Cross-fused renal ectopia also generally requires no specific treatment.
kidneys may be associated with VUR, UPJ obstruction, and urolithiasis. Cross-fused renal ectopia also generally requires no specific treatment.
 FIGURE 17.9 3-month-old boy with horseshoe kidney. Transverse gray-scale ultrasound image through the midabdomen shows fusion of the bilateral lower renal poles in the midline (arrow), anterior to the lumbar spine. |
 FIGURE 17.10 7-year-old boy with horseshoe kidney. A: Axial T2-weighted fat-saturated MR image shows abnormal rotation of the kidneys and mild left pelvicaliectasis (asterisk). B: More inferior MR image shows that the kidneys are fused in the midline with a thick parenchymal isthmus (arrow). (Case courtesy of J. Damien Grattan-Smith, MD, Children’s Healthcare of Atlanta, Atlanta, GA.) |
Infectious Disorders
Bacterial Pyelonephritis
Bacterial infection of the kidney, or pyelonephritis, characteristically presents with clinical findings of urinary tract infection. Fever and flank pain are common, but may not be present, particularly in very young children. The source of infection may be either ascending (due to cystitis related to perineal bacterial flora) or hematogenous. Escherichia coli is the most frequent causative organism. Urinary tract obstruction, high-grade VUR, and dysfunctional voiding are risk factors.
 FIGURE 17.11 13-year-old boy with injured horseshoe kidney due to motor vehicle accident. Axial contrast-enhanced CT image shows a lacerated horseshoe kidney with adjacent retroperitoneal hematoma (between arrows). Renal collecting system disruption was identified on delayed excretory-phase imaging. |
At imaging, pyelonephritis may be either focal or widespread and either unilateral or bilateral. Renal infection at ultrasound commonly appears as areas of increased parenchymal echogenicity and decreased corticomedullary differentiation with relatively decreased or absent color or power Doppler signal compared to adjacent normal kidney (Fig. 17.13).31 At contrast-enhanced CT, pyelonephritis can present as linear, geographic, or mass-like areas of decreased parenchymal attenuation (Fig. 17.13).32 The involved kidney is frequently enlarged, and perinephric fat stranding due to inflammation may be present.32 Acute pyelonephritis is usually less conspicuous on unenhanced CT images and may go undetected. Similar findings may be observed at MRI, including linear, geographic, or mass-like areas of signal intensity alteration and hypoenhancement. Interestingly, affected areas can hyperenhance on delayed postcontrast CT or MR imaging due to parenchymal edema and tubular obstruction.33 Tc-99m DMSA renal scintigraphy can also be used to detect pyelonephritis, appearing as areas of focal photopenia. Unlike MRI, DMSA imaging cannot differentiate acute pyelonephritis from scarring.34
Prompt recognition and treatment of children with pyelonephritis can prevent potential complications, such as renal scarring, loss of kidney function, and hypertension. Acute pyelonephritis may also be complicated by abscess formation within the kidney or adjacent perinephric space. Such abscesses appear as peripherally enhancing fluid collections on contrast-enhanced imaging and may be multiloculated, septated, or debris laden at ultrasound (Fig. 17.14).
Xanthogranulomatous pyelonephritis (XGP) is a form of chronic renal parenchymal infection that only rarely occurs in the pediatric population.35 Clinical presentation may be nonspecific, leading to delayed diagnosis. XGP is
often associated with infection by Proteus mirabilis, a ureasplitting bacterium that forms struvite calculi. At histology, XGP is characterized by the presence of chronic inflammation, including lipid-laden macrophages. At imaging, a large obstructing calculus, sometimes staghorn in appearance, classically is present in the renal pelvis. The affected kidney usually appears diffusely enlarged and contains numerous round areas of decreased echogenicity at ultrasound or decreased attenuation at CT that are due to severe hydronephrosis (“bear paw” sign) and parenchymal necrosis (Fig. 17.15). Extensive perinephric inflammatory changes are typically present, and abscess formation may occur in the perinephric space, adjacent psoas muscle, and even body wall. Delayed contrast-enhanced CT and renal scintigraphy reveal minimal or no renal function, and radical nephrectomy is generally indicated for definitive treatment. A minority of XGP cases are focal, involving only a portion of the kidney, sometimes appearing mass-like.36
often associated with infection by Proteus mirabilis, a ureasplitting bacterium that forms struvite calculi. At histology, XGP is characterized by the presence of chronic inflammation, including lipid-laden macrophages. At imaging, a large obstructing calculus, sometimes staghorn in appearance, classically is present in the renal pelvis. The affected kidney usually appears diffusely enlarged and contains numerous round areas of decreased echogenicity at ultrasound or decreased attenuation at CT that are due to severe hydronephrosis (“bear paw” sign) and parenchymal necrosis (Fig. 17.15). Extensive perinephric inflammatory changes are typically present, and abscess formation may occur in the perinephric space, adjacent psoas muscle, and even body wall. Delayed contrast-enhanced CT and renal scintigraphy reveal minimal or no renal function, and radical nephrectomy is generally indicated for definitive treatment. A minority of XGP cases are focal, involving only a portion of the kidney, sometimes appearing mass-like.36
 FIGURE 17.12 3-month-old boy with crossed fused renal ectopia. A: Voiding cystourethrogram image shows vesicoureteral reflux into two separate renal collecting systems (arrows), both located to the left of midline. B: Gray-scale ultrasound image confirms that the right kidney is ectopic and fused to the left kidney. A parenchymal notch (arrow) can be seen at the site of parenchymal fusion. |
 FIGURE 17.13 13-year-old boy with fever, right flank pain, and hematuria due to pyelonephritis. A: Longitudinal color Doppler ultrasound image of the right kidney shows a mass-like area of increased echogenicity (arrows) in the upper pole that has decreased blood flow compared to adjacent parenchyma. B: Axial contrast-enhanced CT image shows an ill-defined low attenuation area (arrows) in the right kidney upper pole due to parenchymal infection. This area of acute focal bacterial pyelonephritis resolved on follow-up ultrasound imaging after antibiotic therapy. |
Fungal Infection
Fungal infections of the kidneys and urinary tract are most often due to Candida species and commonly present with funguria. Involvement of the kidneys and collecting systems can be the result of hematogenous dissemination (often bilateral) or ascending spread from the bladder (often unilateral). Infection in very young children, including premature neonates, has the potential to be serious due to immature neutrophil function.37 Hematogenous spread of infection (candidemia) to the kidney may present as focal areas of parenchymal low attenuation on contrast-enhanced CT.
Ascending fungal infection within the renal collecting system is best appreciated by ultrasound, sometimes presenting as nonspecific echogenic debris in the urine or as an echogenic, circumscribed, sometimes mobile fungus ball (mycetoma) (Fig. 17.16).38 On occasion, ascending infection can infiltrate the renal medulla. Renal or perinephric abscesses due to fungal infection can rarely occur. Treatment is typically antifungal medical therapy, although, in certain children, asymptomatic funguria is due to colonization and does not require medical therapy. Rarely, such fungal infections can result in urinary tract obstruction or nonresolving abscesses that require percutaneous drainage or surgical management.39
Ascending fungal infection within the renal collecting system is best appreciated by ultrasound, sometimes presenting as nonspecific echogenic debris in the urine or as an echogenic, circumscribed, sometimes mobile fungus ball (mycetoma) (Fig. 17.16).38 On occasion, ascending infection can infiltrate the renal medulla. Renal or perinephric abscesses due to fungal infection can rarely occur. Treatment is typically antifungal medical therapy, although, in certain children, asymptomatic funguria is due to colonization and does not require medical therapy. Rarely, such fungal infections can result in urinary tract obstruction or nonresolving abscesses that require percutaneous drainage or surgical management.39
 FIGURE 17.14 13-year-old girl with fever and right flank pain. Axial contrast-enhanced CT image shows a heterogeneous abscess (arrow) in the anterior aspect of the mid right kidney. Perinephric fluid surrounds much of the right kidney. |
 FIGURE 17.15 1-year-old girl with recurrent fevers due to xanthogranulomatous pyelonephritis. A: Axial contrast-enhanced CT image shows marked enlargement of the right kidney. Multiple calculi (white arrow) are present in the right renal collecting system. Numerous areas of focal low attenuation in the right kidney are due to pelvicaliectasis and parenchymal necrosis. A right retroperitoneal abscess (black arrow) is present adjacent to the right psoas muscle. Enlarged retroperitoneal lymph nodes are reactive in etiology. B: Bivalved gross pathologic specimen shows extensive renal parenchymal necrosis. |
Neoplastic Disorders
Benign and Low-Malignant Potential Neoplasms
Congenital Mesoblastic Nephroma
Congenital mesoblastic nephroma is the most common neonatal renal neoplasm and may be present at birth. These tumors may be detected antenatally, and affected children sometimes have a history of maternal polyhydramnios.40 Ninety-percent of these lesions are diagnosed in children <1 year of age. Affected children can present with a palpable abdominal mass or less commonly hypertension and hematuria. This mesenchymal spindle cell tumor is divided into two histologic variants: (1) classic and (2) cellular.41 Both consist histologically of fibroblastic cells arranged in fascicles (Fig. 17.17). The cellular variant harbors the same t(12;15) chromosomal translocation as infantile fibrosarcoma and is currently viewed as infantile fibrosarcoma arising within the kidney.
 FIGURE 17.16 4-month-old girl with fungemia and funguria. Longitudinal gray-scale ultrasound image of the right kidney shows an echogenic, lobulated fungus ball (arrows) in upper pole collecting system. |
At imaging (ultrasound, CT, and MRI), these lesions commonly present as large, infiltrating masses replacing much of the affected kidney (Fig. 17.18). The classic variant usually appears solid, and it may have a peripheral hypoechoic ring at ultrasound.41 The cellular variant, which tends to present later in infancy, is often larger and more heterogeneous at imaging due to necrosis, hemorrhage, and/or cyst formation.42 This form can behave more aggressively with encasement of major vascular structures, recurrence in the setting of positive margins, or distant metastatic disease.42,43 Prognosis is generally excellent in the setting of nephrectomy with complete surgical resection of the lesion.
Ossifying Renal Tumor of Infancy
Ossifying renal tumor of infancy is a rare, benign pediatric renal neoplasm. This tumor presents with hematuria or, less often, as a palpable abdominal mass, and it occurs in young children under 3 years of age.44 Histopathologic examination reveals a mass composed of spindle cells and bone (including osteoid and osteoblasts) typically attached to a renal papilla; some have suggested a urothelial origin.45 At ultrasound, these lesions are typically echogenic with posterior acoustic shadowing, mimicking a large renal calculus. Renal collecting system dilatation can be present due to obstruction.46 CT typically reveals an ossified renal mass in the central kidney that may be associated with a hypoenhancing soft tissue mass (Fig. 17.19).47,48 Surgical resection is curative.
 FIGURE 17.17 Congenital mesoblastic nephroma, resected from a 2-month-old girl. The 9 cm mass expanded and occupied the majority of the kidney tissue, showing a poorly demarcated border with normal kidney (left). Microscopically, the tumor consisted of a cellular fibroblastic proliferation, showing intermixed residual nonneoplastic kidney including the benign medullary tubules depicted here (right, hematoxylin and eosin, original magnification, 200×). This tumor harbored an ETV6 gene rearrangement typical of cellular congenital mesoblastic nephroma, and the patient had a high serum calcium level attributed to paraneoplastic disease. |
 FIGURE 17.18 7-month-old girl with right congenital mesoblastic nephroma (cellular variant). Axial contrast-enhanced CT image demonstrates a very large, heterogeneous mass (arrows) arising from the right kidney with substantial mass effect upon adjacent structures. |
 FIGURE 17.19 10-month-old girl with ossifying renal tumor of infancy. Axial contrast-enhanced CT image shows a partially calcified mass (arrows) centered in the left renal collecting system. Left caliectasis and renal parenchymal thinning are due to chronic obstruction. (Image courtesy of Edward Y. Lee, MD, MPH, Boston Children’s Hospital and Harvard Medical School, Boston, MA.) |
 FIGURE 17.20 Cystic nephroma in a 13-year-old girl with a known DICER1 gene mutation and previous cervicovaginal rhabdomyosarcoma. The cut surface (left) shows a well-delineated 4.5 cm mass containing multiple cysts filled with clear fluid. Microscopically, the lesion shows epithelium-lined cysts with condensation of stromal cells immediately underlying the epithelium (right, hematoxylin and eosin, original magnification, 100×). |
Cystic Nephroma and Cystic Partially Differentiated Nephroblastoma
There are generally two forms of multilocular cystic neoplasm in children: (1) multilocular cystic nephroma (MLCN) and (2) cystic partially differentiated nephroblastoma (CPDN).49 Clinical presentations in affected children include palpable abdominal mass and hematuria. MLCN, or simply cystic nephroma, is a benign cystic lesion composed of epitheliumlined septated cysts; the childhood form is thought to be distinct from the type that occurs predominantly in adult women and commonly abuts pelvicalyceal structures (Fig. 17.20). CPDN is an intermediate lesion that is distinguished microscopically from MLCN by the presence of very primitive cells called blastema within the septa.49 Childhood CN is associated with the familial cancer syndrome caused by DICER1 gene mutations (a syndrome that includes pleuropulmonary blastoma, embryonal rhabdomyosarcoma, and Sertoli-Leydig cell tumor, among other tumors).
At ultrasound, both MLCN and CPDN appear similar, presenting as variably sized cystic renal masses containing numerous thin septations. On contrast-enhanced CT and MRI, mural and septal enhancement are generally present, and the lesion may bulge into the renal hilar region causing mass effect upon the collecting system (Fig. 17.21). These lesions normally have no associated solid elements. The radiologic differential diagnosis in the pediatric population includes the cystic forms of Wilms tumor and renal cell carcinoma.49 Surgical resection, either radical or partial nephrectomy based on size and location, is generally curative.
Angiomyolipoma
Angiomyolipomas (AMLs) are benign hamartoma-like renal masses that are classified as perivascular epithelioid cell tumors (PEComas) and contain variable amounts of smooth muscle, fat, and abnormal blood vessels.50 While sporadic lesions occur, many pediatric AMLs arise in the setting of tuberous sclerosis and may be bilateral and numerous.
 FIGURE 17.21 Cystic partially-differentiated nephroblastoma in an 11-month-old girl with a palpable abdominal mass. Axial contrast-enhanced CT image shows a large, cystic, heavily septated, centrally located mass (arrows) within the right kidney. |
At ultrasound, AMLs are variably sized and typically echogenic due to the presence of fat (Fig. 17.22). At CT, these lesions may heterogeneously enhance when large, and the presence of macroscopic fat is considered diagnostic (Hounsfield unit measurement less than -20). MRI can also be used to confirm the presence of an AML. Specific findings include loss of signal within the lesion when applying fat saturation and “India ink” artifact (black boundary artifact) on out-of-phase T1-weighted gradient-recalled echo imaging at fat-water interfaces within the mass or at its interface with the kidney (due to signal loss in voxels containing both lipid and water) (Fig. 17.23).51 However, up to about one-third of AMLs do not contain evidence of fat by imaging, making the diagnosis challenging.52 As AML may be complicated by life-threatening retroperitoneal hemorrhage (Wunderlich syndrome) (Fig. 17.24), serial ultrasound imaging is commonly performed to evaluate for enlarging lesions. Some recommend prophylactic embolization of large lesions (usually >4 cm) to minimize the risk of life-threatening hemorrhage.53
Malignant Neoplasms
Wilms Tumor and Nephroblastomatosis
Wilms tumor, or nephroblastoma, is the most common pediatric abdominal solid neoplasm, representing about 90% of pediatric renal malignancies. These tumors may clinically present with a palpable abdominal mass, abdominal pain, nausea and vomiting, hematuria, and/or hypertension. Histologically, they are triphasic, containing blastema, abortive epithelial tubular elements, and stroma (Fig. 17.25). It is common to see heterologous elements such as rhabdomyo-blastic, cartilaginous, osseous, and, less often, adipocytic and neuroglial differentiation. While most Wilms tumors are sporadic (about 75%), there are several predisposing conditions, including Beckwith-Wiedemann syndrome (macroglossia,
hemihypertrophy, macrosomia, midline abdominal wall defects, ear anomalies, and/or neonatal hypoglycemia), sporadic aniridia, WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, and mental Retardation), and Denys-Drash syndrome (gonadal dysgenesis, mesangial renal sclerosis leading to nephrotic syndrome and chronic kidney disease). All of these predisposing conditions relate to abnormalities of chromosome 11 and the WT1 and WT2 genes.54,55
hemihypertrophy, macrosomia, midline abdominal wall defects, ear anomalies, and/or neonatal hypoglycemia), sporadic aniridia, WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, and mental Retardation), and Denys-Drash syndrome (gonadal dysgenesis, mesangial renal sclerosis leading to nephrotic syndrome and chronic kidney disease). All of these predisposing conditions relate to abnormalities of chromosome 11 and the WT1 and WT2 genes.54,55
 FIGURE 17.22 7-year-old girl with tuberous sclerosis, numerous bilateral renal angiomyolipomas (AMLs), and left renal cell carcinoma. A: Longitudinal gray-scale ultrasound image of the right kidney shows many small echogenic parenchymal lesions, consistent with AMLs. B: Longitudinal gray-scale ultrasound image through the left kidney shows multiple punctate echogenic AMLs as well as a 3.5 cm dominant echogenic mass (arrows) in the upper pole. Image-guided core needle biopsy of the large upper pole lesion revealed renal cell carcinoma, which was confirmed at surgical pathology. |
Ultrasound typically reveals a large, heterogeneous mass arising from the kidney. Doppler ultrasound can be used to confirm the presence of blood flow within the mass as well as assess for extension of tumor into the renal vein and inferior vena cava. Contrast-enhanced CT and MRI typically demonstrate a large, heterogeneously enhancing renal mass that may rarely contain fat or calcification (reflecting heterologous adipose or osseous tumor components).56 Both of these imaging modalities can also be used to assess for tumor thrombus in the renal vein and inferior vena cava (Fig. 17.26).57 The presence of renal parenchyma wrapping around a portion of the mass (“claw sign”) indicates that the lesion is likely originating from the kidney. Tumor may spread to retroperitoneal lymph nodes, lung, and, less commonly, liver. The preoperative detection of Wilms tumor rupture by imaging is challenging. A recent report from the Children’s Oncology Group showed that CT has low sensitivity (54% to 70%)
and moderate specificity for identifying preoperative Wilms tumor rupture, with extension of ascites beyond the pelvic cul-de-sac being the best indicator.58
and moderate specificity for identifying preoperative Wilms tumor rupture, with extension of ascites beyond the pelvic cul-de-sac being the best indicator.58
 FIGURE 17.23 13-year-old girl with tuberous sclerosis and numerous bilateral renal angiomyolipomas. Axial out-of-phase T1-weighted gradient-recalled echo MR image demonstrates numerous areas of signal loss (arrows) in the kidneys (“India ink” artifact) due to the presence of lipid and water in the same voxel. |
 FIGURE 17.24 17-year-old girl with life-threatening left retroperitoneal hemorrhage. Multiple bilateral low-attenuation and enhancing renal lesions representing a combination of cysts and angiomyolipomas. High-attenuation perinephric hematoma (arrows) is due to a bleeding angiomyolipoma. |
 FIGURE 17.25 3-year-old boy with bilateral Wilms tumors. The left image shows triphasic differentiation, including blastemal, stromal, and epithelial elements, which here are forming abortive tubules and glomeruli (hematoxylin and eosin, original magnification, 200×). These tumors arose in a background of nephroblastomatosis (right), illustrated here by a perilobar nephrogenic rest (hematoxylin and eosin, original magnification, 20×). |
Treatment for unilateral Wilms tumor is generally radical nephrectomy with or without chemotherapy and radiation therapy.59 Preoperative chemotherapy is generally indicated in the following settings: bilateral Wilms tumor, unilateral Wilms tumor with two or more clearly separated masses, Wilms tumor in a solitary kidney, extension of tumor thrombus above the level of the hepatic veins, tumor involving contiguous vital structures, respiratory failure due to extensive pulmonary metastases, and tumor rupture.60 Partial nephrectomies or wedge resections may be performed in children with bilateral Wilms tumors or a solitary kidney. With appropriate therapy, 5-year survival is around 90%. Children with the predisposing conditions mentioned above typically undergo ultrasound screening for Wilms tumor every 3 months until ˜8 years of age.55
 FIGURE 17.26 14-month-old boy with a palpable abdominal mass due to left Wilms tumor. A: Axial contrast-enhanced CT image shows a large, heterogeneously enhancing mass arising from the left kidney extending into the left renal vein (arrows). The “claw sign” is present. B: Coronal contrast-enhanced CT image shows that the mass extends into the inferior vena cava (white arrow). There are multiple pulmonary metastases (black arrows). |
Nephrogenic rests have been defined as foci of persistent benign embryonal cells that have the potential to develop into Wilms tumor, and they are found in nearly all bilateral Wilms tumors.61 Nephroblastomatosis refers to multiple, sometimes diffuse, nephrogenic rests in one or both kidneys. There are two types of nephrogenic rests: (1) intralobar and (2) perilobar. While the intralobar form is at highest risk for giving rise to Wilms tumor due to its association with WT1 and WTX gene mutations, it is much less common than the perilobar form.
At ultrasound, nephroblastomatosis commonly presents as multiple hypoechoic nodules within the kidneys. However, CT or MRI evaluation is preferred, as ultrasound has limited sensitivity for detecting nephrogenic rests under about 1 cm in size. Nephrogenic rests are typically hypoenhancing on contrast-enhanced CT and MR images (Fig. 17.27). Diffuse nephroblastomatosis may present with nephromegaly and a
rind-like soft tissue abnormality replacing the renal cortex (Fig. 17.28).61,62
rind-like soft tissue abnormality replacing the renal cortex (Fig. 17.28).61,62
 FIGURE 17.27 11-month-old girl with Beckwith-Wiedemann syndrome, bilateral nephroblastomatosis, and presumed bilateral Wilms tumor. Coronal contrast-enhanced CT image shows multiple bilateral low-attenuation renal masses (asterisks) due to nephrogenic rests and multifocal Wilms tumor. |
Management is typically conservative with close radiologic observation, as most children with nephroblastomatosis do not develop Wilms tumor. Chemotherapy may be administered in the setting of new, enlarging, or increasingly heterogeneous nephrogenic rests, as such lesions are suspicious for Wilms tumor.61
Renal Cell Carcinoma
Renal cell carcinoma is the most common renal malignancy in children over the age of 10 years.63,64 Common clinical presentations include palpable abdominal mass, flank pain, retroperitoneal hemorrhage, hypertension, and hematuria. The nomenclature of epithelial tumors in the kidney has transformed as increasing knowledge of tumor genetics has been integrated into classification schemes. Renal cell carcinomas have been increasingly subdivided based on their genetic aberrations or their genetic syndromic associations. Some of these “new” tumors occurring in childhood include MiT family translocation renal cell carcinoma (including t(6;11) renal cell carcinoma), succinic dehydrogenase B deficiency-associated renal cell carcinoma, ALK translocation renal cell carcinoma, and hereditary leiomyomatosis renal cell carcinoma syndrome-associated renal cell carcinoma. Von Hippel-Lindau disease and tuberous sclerosis are other syndromes known for conferring a predisposition to renal cell carcinoma.65,66
 FIGURE 17.28 2-week-old boy with multiple congenital anomalies. Axial contrast-enhanced CT image shows right nephromegaly and rind-like parenchymal thickening (arrows) due to diffuse nephroblastomatosis. The patient was later treated with chemotherapy for presumed Wilms tumor as this abnormality enlarged during infancy. |
Although pediatric renal cell carcinomas average about 6 cm in size, they can be smaller or larger.63 Ultrasound may demonstrate a solid or complex cystic renal mass containing internal blood flow upon Doppler evaluation. At CT and MRI, most renal cell carcinomas appear heterogeneous and show the “claw sign.” These lesions may contain calcification (about 40%) and show evidence of intralesional or perinephric hemorrhage (about 50%) (Fig. 17.29).63 Areas of hemorrhage are usually hyperintense on T1-weighted MR images. Renal cell carcinomas and associated metastases frequently avidly enhance on arterial-phase postcontrast CT and MR imaging. Metastatic disease most often involves regional lymph nodes, liver, lung, brain, and bone.
Treatment is typically radical nephrectomy, although partial nephrectomy may be performed in children with solitary kidney, very small lesions, or a predisposing syndrome. Unfortunately, these tumors are generally chemotherapy resistant, resulting in poor outcomes in children with metastatic disease.67
 FIGURE 17.29 11-year-old girl with left flank pain due to renal cell carcinoma. Axial contrast-enhanced CT image shows a heterogeneously enhancing mass (arrows) arising from the left kidney. A large amount of high attenuation fluid (asterisks) in the left perinephric space is due to hemorrhage. Mildly enlarged retroperitoneal lymph nodes were proven to contain metastatic neoplasm. |
Clear Cell Sarcoma of the Kidney
Clear cell sarcoma of the kidney (CCSK), formerly known as bone metastasizing renal tumor of childhood, is a rare pediatric primary renal malignancy. This tumor affects young children, on average 36 months of age.68 Clinical presentation may be similar to Wilms tumor, including palpable abdominal mass, hematuria, and hypertension. Microscopically, undifferentiated cells arranged in cords and nests are divided by numerous small blood vessels. A subset of these tumors have been characterized by a t(10;17) translocation resulting in a YWHAE-FAM22 gene fusion.
This renal tumor is usually very large at presentation (11 cm mean diameter) and indistinguishable from Wilms tumor at ultrasound, CT, and MRI, appearing as a heterogeneous renal mass (Fig. 17.30).69 The “claw sign” is typically present, and both the renal vein and inferior vena cava should be assessed for tumor thrombus. CCSK has a propensity to metastasize to regional lymph nodes, bone, brain, and lung.54 Bone scintigraphy and contrast-enhanced CT and MRI of the brain are performed at the time of initial diagnosis. Staging is similar to Wilms tumor. Treatment may include radical nephrectomy, chemotherapy, and radiation therapy and can differ from Wilms tumor, as CCSK has a poorer prognosis with higher recurrence and mortality rates.69
Rhabdoid Tumor of the Kidney
Rhabdoid tumor of the kidney (RTK) is a rare, highly malignant primary renal neoplasm that is usually diagnosed under the age of 2 years (11 months mean age).70 This neoplasm is the most aggressive renal mass of childhood and portends the worst prognosis. Affected children may present with a palpable abdominal mass, hematuria, fever, and hypercalcemia.70 RTK is associated with the development of synchronous or metachronous posterior fossa brain tumors, ususally atypical teratoid/rhabdoid tumors, in 10% to 15% of children (rhabdoid tumor predisposition syndrome, due to germline SMARC gene alterations).
 FIGURE 17.30 1-year-old boy with palpable abdominal mass due to clear cell sarcoma of the kidney. Axial contrast-enhanced CT image shows a large, heterogeneous mass (arrows) arising from the left kidney. The “claw sign” is present. |
 FIGURE 17.31 1-year-old girl with rhabdoid tumor of the kidney. Coronal contrast-enhanced CT image shows a heterogeneously enhancing mass (arrows) in the upper pole of the left kidney. There is a large subcapsular fluid collection (asterisks) that is causing mass effect on normal left kidney parenchyma. (Image courtesy of Edward Y. Lee, MD, MPH, Boston Children’s Hospital and Harvard Medical School, Boston, MA.) |
At imaging, these lesions are typically large, heterogeneous, and infiltrative. They can be centrally located in the kidney. Calcification, subcapsular fluid collections due to hemorrhage, and vascular invasion can be present in RTK (Fig. 17.31).71 Metastatic disease is frequently present at the time of diagnosis or soon thereafter, is commonly multicentric, and often involves regional lymph nodes, lung, brain, and bone. Staging is similar to Wilms tumor. Treatment may include radical nephrectomy, chemotherapy, and radiation. Age at diagnosis has prognostic implications, with children <6 months old having only an 8.8% 4-year overall survival.72
Medullary Carcinoma of the Kidney
Renal medullary carcinoma is a rare primary renal malignancy that occurs in adolescent and young adult black individuals with sickle cell trait (or less often hemoglobin SC disease). Common clinical presentations include palpable abdominal mass, flank pain, and hematuria. Individuals affected are typically between the age of 10 and 40 years, and, interestingly, only a small percentage of these tumors are left-sided.73,74
At imaging, renal medullary carcinomas are generally large (7 cm mean size), heterogeneous, and centrally located within the kidney, arising from the medullary papilla (possibly the collecting duct) (Fig. 17.32). They commonly infiltrate the renal medulla and fill the renal collecting system, may cause calyceal obstruction, and often demonstrate venous and
lymphatic invasion. Renal parenchymal satellite lesions are also common.73 This neoplasm has an extremely poor prognosis with survival typically <6 months, and it is frequently metastatic at the time of diagnosis.74
lymphatic invasion. Renal parenchymal satellite lesions are also common.73 This neoplasm has an extremely poor prognosis with survival typically <6 months, and it is frequently metastatic at the time of diagnosis.74
 FIGURE 17.32 21-year-old young man with sickle cell trait and right kidney medullary carcinoma. Axial contrast-enhanced CT image shows a large, heterogeneous, infiltrative mass (arrows) located centrally in the right kidney. Coronal CT images (not shown) revealed extension of tumor into the proximal right ureter. |
Ewing Sarcoma/Primitive Neuroectodermal Tumor
Ewing sarcoma (or primitive neuroectodermal tumor [PNET]) of the kidney is another extremely rare, very aggressive pediatric malignancy. It is a fairly common childhood tumor of bone and soft tissue, and the advent of molecular tissue diagnostics has aided its recognition in visceral organs, such as the kidney. These tumors most commonly arise in adolescents and young adults, although cases have been described in very young children and the elderly. With primary kidney involvement, reported clinical presentations include flank pain and hematuria.75 This small round blue cell neoplasm classically shows CD99 (MIC2) antigen positivity along tumor cell membranes, and it characteristically contains chromosomal rearrangements in which the EWSR1 gene is fused to a member of the ETS family of transcription factor genes.76
Imaging generally reveals a large, heterogeneous, infiltrative mass arising from the kidney.77 Tumor may extend to involve the renal vein and inferior vena cava (Fig. 17.33).78,79 Up to two-thirds of affected individuals have metastatic disease at diagnosis, often involving the lungs and bone marrow.75 These tumors may mimic other primary renal malignancies of childhood, including Wilms tumor. Preoperative neoadjuvant chemotherapy is commonly administered if Ewing sarcoma of the kidney is diagnosed prior to radical nephrectomy. Median overall survival is about 24 months in patients with metastatic disease.75
Renal Lymphoma and Leukemia
Lymphoma
Involvement of the kidneys by lymphoma is relatively common and typically secondary to hematogenous dissemination, or less often, contiguous extension of retroperitoneal disease. Renal involvement in children is most often seen in the Burkitt subtype of non-Hodgkin lymphoma.
 FIGURE 17.33 14-year-old girl with flank pain and hematuria due to right kidney Ewing sarcoma/primitive neuroectodermal tumor (PNET). Axial contrast-enhanced CT image shows a large, enhancing mass arising from the right kidney. The mass extends into the right renal vein and inferior vena cava (arrows). |
At ultrasound and CT, bilateral hypoechoic or low-attenuation renal masses, respectively, are the most common imaging appearance (Fig. 17.34). Less common presentations
include a solitary renal mass or large retroperitoneal mass engulfing the kidney.80 Nearby retroperitoneal lymph node enlargement is common. Renal lymphomatous deposits typically regress with chemotherapy.
include a solitary renal mass or large retroperitoneal mass engulfing the kidney.80 Nearby retroperitoneal lymph node enlargement is common. Renal lymphomatous deposits typically regress with chemotherapy.
 FIGURE 17.34 5-year-old boy with renal involvement by Burkitt lymphoma, identified after presentation with a palatine tonsillar mass. Coronal contrast-enhanced CT image shows bilateral hypoenhancing solid renal masses (asterisks) due to lymphomatous deposits. |
 FIGURE 17.35 12-year-old boy with acute leukemia. Axial contrast-enhanced CT image shows very large areas of geographic low attenuation in both kidneys due to leukemia. The kidneys are enlarged, and abnormal soft tissue (arrows) encases the abdominal aorta. |
Leukemia
The kidneys may also be a site of infiltration by leukemic disease, most often in the setting of acute lymphoblastic leukemia in children. Renal leukemic involvement is relatively infrequently observed by imaging, as most pediatric patients with acute leukemia do not undergo routine CT staging or surveillance (unlike lymphoma).
Multiple bilateral hypoechoic (at ultrasound) or low-attenuation (at CT) masses are the most common imaging appearance of renal leukemia. Other presentations include a solitary renal mass and geographic (or wedge-shaped) renal parenchymal low attenuation at contrast-enhanced CT (Fig. 17.35).81 Bilateral renal enlargement is also common, even when focal renal parenchymal lesions are not apparent. Similar to lymphoma, renal leukemic deposits usually regress with chemotherapy.
Traumatic Disorders
Renal trauma in the pediatric population most often occurs in the setting of blunt abdominal trauma (e.g., motor vehicle accident or fall from a height); penetrating renal trauma is less common. Grading of renal trauma is commonly performed using the American Association for the Surgery of Trauma classification system based upon extent of parenchymal injury as well as involvement of the collecting system and renal hilum (Table 17.2).82
A variety of findings may be seen at ultrasound and contrast-enhanced CT, including focal parenchymal contusion, subcapsular hematoma, and laceration. Renal contusion may appear as focally altered renal parenchymal echogenicity or attenuation at ultrasound and CT, respectively. Lacerations are usually linear parenchymal defects that may contain fluid (urine and/or blood). When lacerations extend into the central kidney, delayed postcontrast CT imaging should be performed to assess for extravasation of contrast material from the renal collecting system into the perinephric space (urinoma) and to determine if the UPJ is intact (Fig. 17.36). “Shattered” kidney and complete devascularization of the kidney due to hilar injury are the most severe forms of renal injury (Fig. 17.37).
TABLE 17.2 American Association for the Surgery of Trauma Kidney Injury Grading Scale | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||
In hemodynamically stable children, management is most often conservative.83 Recent studies in children have shown that nonoperative management is highly successful,84 with at least partial renal preservation in most patients. Radical nephrectomy is rarely indicated in pediatric renal trauma (e.g., in the setting of irreparable vascular injury and severe hemodynamic instability).
Postprocedure Complications
A variety of complications may be diagnosed following renal percutaneous interventional procedures (e.g., percutaneous needle biopsy, percutaneous nephrostomy) in children. Bleeding along the surface of the kidney into the perinephric space can be easily detected by ultrasound, appearing as a focal fluid collection. Echogenicity of the fluid collection depends on the age of the hematoma. Perinephric hematomas are very common, most often clinically insignificant, and
vary in size from very small to quite large.85,86 Extravasation of intravascular contrast material on contrast-enhanced CT suggests active bleeding that may require treatment (e.g., arterial embolization). On occasion, hemorrhage may be confined between the renal capsule and underlying parenchyma. Subcapsular hematomas are generally crescentic or elliptical in shape and exert mass effect upon underlying renal parenchyma (Fig. 17.38). Acute and subacute subcapsular hematomas demonstrate higher attenuation than water at CT. On occasion, subcapsular hematomas and associated renal compression may cause renal failure and renin-mediated hypertension (so-called Page kidney).87
vary in size from very small to quite large.85,86 Extravasation of intravascular contrast material on contrast-enhanced CT suggests active bleeding that may require treatment (e.g., arterial embolization). On occasion, hemorrhage may be confined between the renal capsule and underlying parenchyma. Subcapsular hematomas are generally crescentic or elliptical in shape and exert mass effect upon underlying renal parenchyma (Fig. 17.38). Acute and subacute subcapsular hematomas demonstrate higher attenuation than water at CT. On occasion, subcapsular hematomas and associated renal compression may cause renal failure and renin-mediated hypertension (so-called Page kidney).87
 FIGURE 17.36 12-year-old boy in bicycle accident with right kidney laceration. A: Axial contrast-enhanced nephrographic phase CT image shows a large area of posterior right kidney hypoenhancement (between arrows). A large amount of fluid (blood and/or urine) is present in the right retroperitoneum. B: Axial contrast-enhanced excretory-phase CT image shows extravasation of excreted contrast material (arrows) into the right perinephric space due to collecting system injury. |
 FIGURE 17.37 13-year-old boy with blunt abdominal trauma and left renal artery injury. Axial contrast-enhanced excretory-phase CT image shows complete absence of left kidney parenchymal enhancement (asterisks). A contrast-filled pseudoaneurysm (arrow) is present in the left renal hilar region. |
Arteriovenous fistulas occur when there is injury to an adjacent artery and vein within the kidney with resultant abnormal communication. These lesions are usually diagnosed by ultrasound and best appreciated on color and spectral Doppler evaluation. Color Doppler imaging reveals focally increased renal vascularity with aliasing. Spectral Doppler typically shows increased arterial systolic velocity and diastolic
blood flow as well as “arterialization” of venous blood flow with increased turbulence (Fig. 17.39).85 Many of these lesions are small and involute without intervention, although large lesions may persist and require arterial embolization.86
blood flow as well as “arterialization” of venous blood flow with increased turbulence (Fig. 17.39).85 Many of these lesions are small and involute without intervention, although large lesions may persist and require arterial embolization.86
 FIGURE 17.38 8-year-old boy with obstructed right kidney due to large intra-abdominal rhabdomyosarcoma. Axial contrast-enhanced CT image shows a traumatic right kidney subcapsular hematoma (arrow) related to percutaneous nephrostomy tube placement. The hematoma has mass effect upon adjacent renal parenchyma. |
 FIGURE 17.39 11-year-old boy with left lower quadrant renal allograft status post percutaneous biopsy. A: Longitudinal spectral Doppler ultrasound image shows a focus of very high arterial peak systolic velocity with increased diastolic blood flow in the lower pole of the transplant kidney (arrow), consistent with arteriovenous fistula from recent biopsy. B: Interrogation of an interpolar arcuate artery shows a normal spectral Doppler waveform. |
Pseudoaneurysm is a rare (incidence <1%) complication of percutaneous interventional procedures that is due to focal arterial wall disruption.88 These lesions most often appear as intrarenal cyst-like structures on gray-scale ultrasound evaluation.88 However, color Doppler imaging shows swirling blood flow within the lesion, sometimes with a “yin-yang” appearance. Renal pseudoaneurysms can be treated with selective arterial embolization.
Nonneoplastic Cystic Disorders
Benign Epithelial Cysts
While prevalence increases with age, benign epithelial renal cysts are increasingly common in the pediatric population. This likely relates, at least in part, to the rising use of medical imaging in children as well as improvements in ultrasound image quality. At ultrasound, many of these cysts are solitary, simple, and anechoic with thin, imperceptible walls, demonstrating posterior acoustic through-transmission.
Simple renal cysts show no internal complexity or enhancement at CT or MRI (Figs. 17.40 and 17.41). Benign epithelial cysts can sometimes appear complicated, containing thin septations, mural calcification, and/or internal debris (Fig. 17.42). When renal cystic lesions have mural nodularity or solid elements, a diagnosis other than benign epithelial cyst should be considered. Although most benign renal cysts in children are asymptomatic, occasional lesions may be symptomatic due to hemorrhage or very large size. Image-guided percutaneous aspiration or sclerotherapy in children has been described.89
The Bosniak classification system for risk assessment, while not validated in the pediatric population, has been shown useful for directing the management of pediatric renal cysts.90
Autosomal Dominant Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is an inherited disorder that affects the kidneys as well as other organs. In some instances, this condition is noninherited due to a spontaneous genetic mutation. While ADPKD is most
often diagnosed during adulthood, it can present during childhood. ADPKD is classified as a “ciliopathy” and can be due to a mutation in either the PKD1 (85%) or PKD2 (15%) gene.91
often diagnosed during adulthood, it can present during childhood. ADPKD is classified as a “ciliopathy” and can be due to a mutation in either the PKD1 (85%) or PKD2 (15%) gene.91
 FIGURE 17.40 16-year-old girl with incidentally detected simple renal cyst. Coronal postcontrast T1-weighted fat-saturated MR image shows a 1.5-cm circumscribed, nonen-hancing lesion (arrow) in the right kidney upper pole. There is no internal complexity. |
 FIGURE 17.41 3-year-old girl with palpable abdominal mass due to a large left renal simple cyst. Axial T2-weighted fat-saturated MR image demonstrates a large left kidney cyst (arrows) without mural thickening or internal complexity. |
Imaging of the kidneys is often normal early in life. Characteristically, over time, the kidneys become enlarged and develop an increasing number of variably sized renal cysts (Fig. 17.43). Although most cysts are simple, they can be complicated, containing debris, septations, and calcifications. Hemorrhagic or infected cysts often appear hyperdense at CT. Ultrasound can demonstrate abnormally echogenic renal parenchyma with loss of corticomedullary differentiation in some children.92 Cysts may also be detected in numerous other organs, such as the liver, spleen, pancreas, testis, seminal vesicles, and prostate gland. Ultimately, most affected individuals eventually require dialysis or renal transplantation due to end-stage chronic kidney disease. Affected children should be screened for hypertension and early renal dysfunction.93 While there is an increased frequency of intracranial aneurysms in ADPKD patients, such aneurysms are only rarely symptomatic during childhood.94
 FIGURE 17.42 9-year-old girl with enuresis. Longitudinal gray-scale ultrasound image through the left kidney lower pole shows a complicated cyst (arrows) containing multiple septations. |
 FIGURE 17.43 7-year-old boy with autosomal dominant polycystic kidney disease. Coronal contrast-enhanced CT image shows multiple bilateral simple renal cysts. The left kidney is enlarged, and a very large cyst (arrows) arises from its upper pole. |
Autosomal Recessive Polycystic Kidney Disease
ARPKD is another inheritable ciliopathy that affects the kidneys as well as other organs.91 As this condition is autosomal recessive, it is much less common than ADPKD. ARPKD most often presents during the neonatal period or infancy and, less often, later in childhood. Antenatal detection has also been described. Common clinical presentations early in life include Potter sequence due to oligohydramnios (including pulmonary hypoplasia) and bilateral palpable abdominal masses.
At ultrasound, the kidneys appear enlarged and echogenic with loss of corticomedullary differentiation.95 The kidneys contain many tiny cystic structures (ectatic, nonobstructed tubules and ducts) that are best seen using a high-frequency linear transducer, although macrocysts may be observed in some children (Figs. 17.44 and 17.45). Renal cortical sparing has been described in some children, and scattered hyperechoic nonshadowing foci in the kidneys have been associated with the onset of renal failure.92,95
A variety of hepatic ductal plate abnormalities are observed in ARPKD. These liver abnormalities may be most severe in children that present with ARPKD later in childhood and that
have less severe renal involvement. Such abnormalities include congenital hepatic fibrosis and bile duct ectasia (Caroli syndrome), with associated splenomegaly, portosystemic varices, and ascites due to portal hypertension.96 Almost all individuals affected by ARPKD require dialysis or renal transplantation by adulthood, sometimes very early in life.
have less severe renal involvement. Such abnormalities include congenital hepatic fibrosis and bile duct ectasia (Caroli syndrome), with associated splenomegaly, portosystemic varices, and ascites due to portal hypertension.96 Almost all individuals affected by ARPKD require dialysis or renal transplantation by adulthood, sometimes very early in life.
 FIGURE 17.44 Diffusely enlarged (14.5 cm) kidney from a 6-day-old girl with autosomal recessive polycystic kidney disease who underwent bilateral nephrectomy to ameliorate respiratory distress. Close gross inspection (right) shows innumerable small cysts, some oriented with their long axis perpendicular to the kidney capsule. |
Renal Cysts Associated with Syndromes
Several other syndromes are associated with renal cysts. Children with von Hippel-Lindau disease and tuberous sclerosis, both autosomal dominant phakomatoses, often develop renal cysts, commonly bilateral and multiple (Figs. 17.46 and 17.47).97,98 Some renal cysts in von Hippel-Lindau patients are likely premalignant.99 Both von Hippel-Lindau disease and tuberous sclerosis are also associated with the development of renal cell carcinoma during childhood (Figs. 17.22 and 17.47), while tuberous sclerosis is also associated with renal AMLs. Renal cysts also arise in children with other syndromes, including Joubert syndrome, Meckel-Gruber syndrome, and asphyxiating thoracic dysplasia (Jeune syndrome), all of which are ciliopathies.100
 FIGURE 17.45 6-month-old boy who presented with cardiac arrest, respiratory failure, and marked abdominal distention. Longitudinal gray-scale ultrasound image through the left kidney reveals nephromegaly and loss of parenchymal corticomedullary differentiation. Numerous tiny cystic structures are present throughout the kidney. Findings are consistent with autosomal recessive polycystic kidney disease. The left renal collecting system is mildly dilated (asterisks). |
 FIGURE 17.46 19-year-old man with von Hippel-Lindau disease. Axial T2-weighted fat-saturated MR image shows a centrally located simple cyst in the left kidney (white arrow; two other small cysts were also present in the left kidney, not shown). Multiple small cystic lesions (black arrow) are also present in the pancreas. |
Vascular Disorders
Renal Artery Stenosis
Hypertension is uncommon in the pediatric population, with many cases having an identifiable secondary cause. Renin-mediated hypertension is responsible for 2% to 10% of cases, due to either aortic and/or renal artery narrowing.101 Clinically, renovascular hypertension is often severe and refractory to medical therapy, and it may be associated with the development of numerous complications, such as hypertensive encephalopathy, stroke, cardiac dysfunction, kidney injury, and retinopathy. There are numerous causes of renal artery narrowing in children, including developmental arteriopathy (sometimes referred to as fibromuscular dysplasia), vasculitis (e.g., Takayasu arteritis), thromboembolic complication of umbilical artery catheterization, and a variety of syndromes (e.g., neurofibromatosis, type 1, and Williams syndrome). Early detection of renovascular hypertension allows for potential endovascular or surgical cure, fewer hypertension-related complications, and decreased need for antihypertensive medications.
Catheter-based digitally subtracted angiography (CBDSA) is the gold standard for detecting and characterizing renal artery narrowings and allows therapeutic intervention in some children. This imaging modality can thoroughly assess extra- and intrarenal (including high-order branch) stenoses,
measure pressure gradients, as well as evaluate the aorta (Figs. 17.48 and 17.49). CBDSA has superior spatial resolution compared to other imaging modalities, detects accessory renal artery stenoses, and allows for simultaneous endovascular therapy of certain lesions.102 Renal artery stenoses are frequently intrarenal in children,102 and bilateral lesions occur in about 40%.103 Aneurysm formation is also common, often poststenotic in location (Fig. 17.49).
measure pressure gradients, as well as evaluate the aorta (Figs. 17.48 and 17.49). CBDSA has superior spatial resolution compared to other imaging modalities, detects accessory renal artery stenoses, and allows for simultaneous endovascular therapy of certain lesions.102 Renal artery stenoses are frequently intrarenal in children,102 and bilateral lesions occur in about 40%.103 Aneurysm formation is also common, often poststenotic in location (Fig. 17.49).
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
