Ultrasound is the first imaging modality used for the evaluation of a renal mass in a child. It also can evaluate extension of malignant tumors into the renal vein, inferior vena cava, and right atrium. Due to the large size of some malignant renal tumors, adequate evaluation of the renal veins and IVC may be difficult because of marked compression and displacement by the tumors. CT or MRI are required for adequate evaluation when this is the case.

Conditions including hydronephrosis, ureteropelvic junction obstruction, obstructed duplicated system, multicystic dysplastic kidney, and polycystic kidney disease can be diagnosed with ultrasonography. Additionally, abdominal CT is valuable in the evaluation of the site of origin, extent, and character of renal neoplasms at the time of diagnosis and follow-up. It identifies vascular and nodal involvement and hepatic metastases, as well as images the contralateral kidney. Color coded 3-D surface rendering of renal tumors and their surrounding structures may be applied to contrast-enhanced helical CT data sets to improve surgical planning. ² Three-dimensional CT imaging may be useful in patients that will benefit from renal sparing surgery, including patients with a solitary kidney, bilateral renal tumors, von Hippel-Lindau syndrome, and compromised contralateral renal function.

MRI is also considered an excellent imaging modality for the evaluation of renal masses in children due to its multiplanar capability and excellent inherent contrast. However, as it is more readily available than MRI in most hospital settings, CT is more commonly used in this setting. Following resection of renal tumor, the tumor bed frequently contains bowel loops, which makes US assessment difficult. Abdominal CT following oral and IV contrast administration provides adequate evaluation of the tumor bed for evaluation of residual tumor and recurrence, as well as contralateral tumor development.

Approximately 87% of solid renal neoplasms in children are Wilms' tumors; other renal tumors include clear cell sarcomas (6%), mesoblastic nephroma (2%), rhabdoid tumors (2%), lymphoma (<0.5%) and renal cell carcinoma (<0.5%). ³ This paper describes the clinical and CT features of a spectrum of benign and malignant renal masses in children, including Wilms' tumor, renal cell carcinoma, clear cell sarcoma, rhabdoid tumor, renal lymphoma, metastatic renal mass, nephroblastomatosis, mesoblastic nephroma, and multilocular cystic nephroma.

Wilms' tumor is the most common primary pediatric malignant abdominal neoplasm; it is the third most common malignancy in children, after leukemia and brain tumors. It also is the third most common of all renal masses in childhood, after hydronephrosis and multicystic dysplastic kidney. Wilms' tumor occurs equally in males and females.

Wilms' tumor occurs bilaterally in 5 to 10% of patients and is multifocal in 10% of patients. Approximately 15% of children with Wilms' tumor have other clinical problems or associated syndromes. ⁴ About 33% of patients with sporadic aniridia develop Wilms' tumor (figure 3), ⁵ while 10 to 20% of patients with Beckwith-Wiedemann syndrome (macroglossia, omphalocele, and visceromegaly) develop these lesions. ⁶ There also are strong associations with hemihypertrophy (figure 3) and Drash syndrome (pseudohermaphroditism and glomerulonephritis), ⁵ and an increased incidence of Wilms' tumor in children with congenital genitourinary tract anomalies, including cryptorchidism, horseshoe kidney, ambiguous genitalia, and hypospadias. ⁵

The current screening recommendations for patients at risk for Wilms' tumor include sonography every 4 months for a year, followed by repeat imaging every 8 months for 2 years, and then every 12 months until the child is 10 years of age; and CT scanning every 6 months for a year, followed by every 12 months for 4 years and again at 10 years of age. ⁷

Wilms' tumor arises from undifferentiated metanephric blastema within the renal cortex. Ninety percent of Wilms' tumors have a favorable histology and contain differentiated renal tissue which forms primitive glomerular and tubular structures within a spindle cell stroma. The remaining 10% of Wilms' tumors, those with an unfavorable histology, contain anaplastic elements. ⁸

Hemorrhage and necrosis occur commonly in Wilms' tumor. Ninety percent of patients present with an asymptomatic abdominal mass. ⁴ Microscopic and gross hematuria occurs in approximately 20% and 8% of patients, respectively. Additionally, hypertension may occur due to renin production by the tumor or vascular compression. ⁹ Twenty percent of Wilms' tumor patients may have low-grade fever, ⁴ and 25% of patients have abdominal pain. Other less common presentations include tumor hemorrhage after minor trauma, ascites due to venous obstruction, and varicocele from left-sided tumor. ⁴

CT may demonstrate invasion of vessels (figure 2), rather than displacement or encasement as seen in neuroblastoma. Five to 10% of Wilms' tumors demonstrate vascular extension into the renal vein, with variable extension into the inferior vena cava and right atrium. ⁹ There may be invasion of adjacent organs, lymphadenopathy, contralateral disease, and hematogenous metastatic disease mainly to the liver and lungs. In evaluation of a possible Wilms' tumor, chest CT also should be performed, as approximately 20% of patients develop pulmonary metastases at the time of diagnosis.

Renal cell carcinoma accounts for less than 1% of pediatric renal tumors; ¹ its mean age of presentation is 9 years. In studies, patients with this tumor have been found to present with palpable mass (60%), abdominal pain (50%), and hematuria (30 to 60%). ¹⁰ Clinical findings include hypertension, polycy-
themia, and bone resorption, which reflect the production of renin, erythropoietin, and parathyroid hormone, respectively. ⁹ Patients with von Hippel-Lindau disease (cerebellar hemangioblastoma, retinal angioma, pancreatic cysts and tumors, pheochromocytoma, and renal cysts and tumors) are at increased risk for renal cell carcinoma.

Imaging studies cannot distinguish renal cell carcinoma from Wilms' tumor. However, an older presenting age may suggest a diagnosis of renal cell carcinoma. Such tumors tend to be smaller than Wilms' tumor at presentation. Twenty-five percent of renal cell carcinomas have calcification that tends to be more dense, central, and homogenous than the calcification typically found in Wilms' tumor. ^l0 Contrast-enhanced CT can demonstrate the tumor, as it enhances less than normal renal parenchyma. Additionally, there may be areas of low attenuation due to hemorrhage and necrosis (figure 4). Twenty-five percent of renal cell carcinomas have intravascular extension.

Clear cell sarcoma constitutes 6% of pediatric renal tumors. ³ This lesion generally presents between 1 and 6 years of age and occurs equally in males and females. Pathologically, the clear cell sarcoma is characterized by polygonal or stellate cells with clear cytoplasm, ovoid to round nuclei, and a tendency toward cyst formation separated by septa. ⁸ Clinical features include abdominal mass, lethargy, weight loss, and hematuria. It is a highly malignant tumor with a worse prognosis than Wilms' tumor, and has a propensity for skeletal metastases. Clear cell sarcoma cannot be distinguished from Wilms' tumor by imaging studies alone. ¹¹ CT demonstrates a heterogenous solid mass with low attenuation areas due to necrosis, and water density areas reflecting the cystic component (figure 5). ¹² Amorphous and linear calcifications occur in 25% of tumors. ¹² Skeletal scintigraphy is useful for staging and follow-up evaluation.

Rhabdoid tumors constitute 2% of pediatric renal tumors. ³ They commonly present in early childhood, at a mean age of 17 months, and have a poor prognosis, particularly in the presence of metastatic disease at presentation. ^l3 Rhabdoid tumors usually originate in the renal sinus, unlike Wilms' tumors, which arise from renal cortex. It commonly metastasizes to the lungs, liver, and brain. Rhabdoid tumor is associated with primary brain tumors of neuroectodermal origin, including medulloblastoma, ependymoma, glioma, and primitive neuroectodermal tumor. ^l3 As with the other tumors described here, these lesions may not be distinguished from Wilms' by imaging studies alone, though a centrally located renal mass with subcapsular fluid collection and associated posterior fossa mass will suggest the diagnosis. CT scans will demonstrate a centrally located heterogenous renal mass (figure 6). There may be thickening of the renal capsule, as well as subcapsular fluid collection. ¹⁴ A peripheral subcapsular fluid collection adjacent to solid tumor lobules is present in 70% of malignant rhabdoid tumor. ^l5 In studies, this collection has represented subcapsular hematoma in 47% of cases and necrotic cavity in 53%. ¹⁵ Head MRI also is useful for staging and follow up.

Primary renal lymphoma is uncommon, as the kidney lacks lymphoid tissue. The kidneys may be involved by direct extension from adjacent pararenal lymphomatous disease or hematogenous dissemination. Non-Hodgkin's lymphoma more commonly involves the kidney than does Hodgkin's lymphoma. ⁹ Renal involvement in non-Hodgkin's lymphoma usually occurs late in the disease and often is clinically silent. Occasionally there may be flank pain, palpable mass, hematuria, hypertension, and renal failure due to urinary tract obstruction, renal vein compression, and diffuse infiltration. Bilateral renal involvement is more common than unilateral disease. ⁹ CT findings include hypodense renal mass(es), which enhance less than normal renal parenchyma following IV contrast administration (figure 7), and diffuse nephromegaly due to diffuse infiltration, occurring in 5% of patients. ^l6 The CT appearance is non specific; however, the diagnosis should be suggested in the presence of associated splenomegaly and lymphadenopathy.

Pediatric renal metastases are uncommon. ⁹ The most common form of metastatic disease to the kidney in this population is local extension of a neuro-blastoma (figure 8). ⁹ Hematogenous metastases may arise from a sarcoma, which also results in diffuse metastatic involvement of other organs and lymph nodes. The clinical presentation usually reflects the primary tumor or metastatic site. There may be hematuria due to tumor hemorrhage. When there is local extension of neuroblastoma to the kidney, CT findings will include a mass with the epicenter in the adrenal gland which invades the ipsilateral kidney (figure 8). The CT features of neuroblastoma include an extrarenal epicenter which may be suprarenal or paravertebral and of irregular shape and inhomogeneity due to tumor necrosis, as well as encasement and displacement of vessels (figure 8). CT demonstrates calcification in about 85% of cases, which may be dense, stippled, punctate, or amorphous. Adrenal mass may cross the mid-line, and retroperitoneal adenopathy and contiguous extension are common. In hematogenous metastasis to the kidney, CT usually demonstrates multiple renal masses. The clinical history, lesion's multiplicity, and the presence of other metastatic disease will suggest the diagnosis. ¹⁷

Nephroblastomatosis is a dysontogenetic process with persistence of fetal metanephric blastema within the renal cortex after 36 weeks of gestation. The fetal metanephric blastema is a precursor of Wilms' tumor. ^l8 Ninety to 100% of kidneys with bilateral Wilms' tumor (figure 3) and up to 40% of kidneys with unilateral Wilms' tumor have foci of metanephric blastema present on pathologic examination. ¹⁸ Nephroblastomatosis is associated with hemihypertrophy, sporadic aniridia, Beckwith-Wiedemann syndrome, and Drash syndrome.

Most cases of nephroblastomatosis are seen in patients under 2 years of age. Nephroblastomatosis may be diffuse or multifocal. In the diffuse form, there is circumferential subcortical tissue associated with bilateral nephromegaly and distortion of the collecting system and renal parenchyma. On unenhanced CT the subcortical rind is seen which is hypodense to normal renal parenchyma; following intravenous administration of contrast it enhances less than normal renal tissue. ¹⁹ In the multifocal form, small rests of metanephric blastema are scattered throughout the renal parenchyma. Those rests large enough to be demonstrated on CT may be indistinguishable from small Wilms' tumors. ²⁰ Due to the malignant potential of the persistent nephrogenic blastema, patients with nephroblastomatosis should be followed at close intervals with sonography and CT scanning to exclude the development of Wilms' tumor. ¹⁹

Mesoblastic nephroma is the most common renal neoplasm in infancy. It typically presents as a flank mass; less commonly it may present with hematuria, hypertension, and anemia. Pathologically, it is solid, unencapsulated, and grows by infiltrating through the renal parenchyma. ²⁰ Mesoblastic nephroma is considered a benign lesion; there are, however, several reports in the literature of metastatic disease and death. ^21,22 Contrast-enhanced CT scans of a mesoblastic nephroma demonstrate an intrarenal mass with uniform enhancement but less than normal renal parenchyma. In large lesions there may be areas of low attenuation due to hemorrhage or necrosis (figure 9). Tumor calcification is rare, and it does not invade vessels or the collecting system. ⁹ Nephrectomy is usually curative for this type of lesion. If there is no concern for residual disease after nephrectomy and histology does not appear aggressive, no imaging follow up is necessary. However, if there is extensive necrosis, extrarenal extension, hypercellular or immature histology, or the patient is greater than 3 months of age, clinical surveillance and follow-up CT scanning is recommended to exclude recurrence or metastasis. ⁹

MLCN is a benign cystic tumor with biphasic age and sex distribution. It occurs in children aged 2 months to 4 years with a 75% male predilection, and in adults over 40 years of age with a 95% female predilection. ⁴ MLCN usually presents as an asymptomatic mass, though there may be hematuria due to prolapse of the mass into the renal pelvis. Pathologically, the lesion is well encapsulated and consists of multiple well-circumscribed, noncommunicating locules; there also may be fibrous stroma containing septations. ²³

There are two distinct histologic entities of MLCN: cystic nephroma without blastema elements within the septa, and cystic, partially differentiated nephroblastoma, which has blastema elements in the septa. These two entities are indistinguishable on the basis of imaging characteristics. The relationship between cystic nephroma, cystic partially differentiated nephroblastoma, and Wilms' tumor is controversial. ^23,24 Contrast-enhanced CT scans demonstrate a well defined intrarenal, multi-locular mass which compresses or displaces the adjacent renal parenchyma, forming a rim or beak adjacent to the mass. The septations enhance, but the cysts do not (figure 10). The attenuation of an individual cyst depends on the protein content and the presence or absence of hemorrhage. ⁹ There may be linear calcification within the septations or capsules.

Benign and malignant renal neoplasms comprise a large proportion of childhood tumors. Wilms' tumor is the most common primary malignant abdominal neoplasm in children, and mesoblastic nephroma is the most common renal neoplasm in infancy. Other important causes of renal masses in childhood include renal cell carcinoma, clear cell sarcoma, rhabdoid tumor, renal lymphoma, metastatic neuroblastoma, nephroblastomatosis and multilocular cystic nephroma. Although the CT features of most pediatric renal masses are not specific, it is, however, a useful modality in the evaluation of renal masses and postoperative/therapy follow up. AR