(Radiographics. 2000;20:1585-1603.)
© RSNA, 2000
Pediatric Renal Masses: Wilms Tumor and Beyond1
Lisa H. Lowe, MD,
Bernardo H. Isuani, MD,
Richard M. Heller, MD,
Sharon M. Stein, MD,
Joyce E. Johnson, MD,
Oscar M. Navarro, MD and
Marta Hernanz-Schulman, MD
1 From the Departments of Radiology and Radiological Sciences (L.H.L., B.H.I., R.M.H., S.M.S., M.H.S.) and Pathology (J.E.J.), Vanderbilt University Children's Hospital and Medical Center, D-1120 Medical Center North, 1211 22nd Ave S, Nashville, TN 37232; and the Department of Diagnostic Imaging, Hospital for Sick Children, Toronto, Ontario, Canada (O.M.N.). Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received March 1, 2000; revision requested March 28 and received May 15; accepted May 16. Address correspondence to M.H.S. (e-mail: marta.schulman@mcmail.vanderbilt.edu).
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Abstract
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A variety of pediatric renal masses may be differentiated from Wilms tumor on the basis of their clinical and imaging features. Wilms tumor is distinguished by vascular invasion and displacement of structures and is bilateral in approximately 10% of cases. Nephroblastomatosis occurs most often in neonates and is characterized by multiple bilateral subcapsular masses, often associated with Wilms tumors. Renal cell carcinoma is unusual in children except in association with von Hippel–Lindau syndrome and typically occurs in the 2nd decade. Mesoblastic nephroma is the primary consideration in a neonate with a solid renal mass. Multilocular cystic renal tumor is suggested by a large mass with multiple cysts and little solid tissue. Clear cell sarcoma is distinguished by frequent skeletal metastases, and rhabdoid tumor is distinguished by its association with brain neoplasms. Angiomyolipoma frequently contains fat and is associated with tuberous sclerosis. Renal medullary carcinoma occurs in patients with sickle cell trait or hemoglobin SC disease and manifests as an infiltrative mass with metastases. Ossifying renal tumor of infancy is differentiated from mesoblastic nephroma by the presence of ossified elements. Metanephric adenoma lacks specific features but is always well defined. Renal lymphoma is characterized by multiple homogeneous masses, often with associated adenopathy.
Index Terms: Kidney neoplasms, diagnosis, 81.30 • Kidney neoplasms, in infants and children, 81.30 • Kidney, nephroblastomatosis
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LEARNING OBJECTIVES FOR TEST 4
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After reading this article and taking the test, the reader will be able to:
- Describe a variety of pediatric renal masses.
- Recognize the features of these masses at evaluation with various imaging modalities.
- Correlate the anatomic, radiologic, and histopathologic findings in pediatric renal masses.
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Introduction
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Many pediatric renal tumors were previously lumped together and categorized as Wilms tumor. However, in recent years several specific tumors have been recognized as distinct pathologic entities. The diagnosis of these newly described lesions can be suggested by their unique clinical history, such as age at presentation (Table 1) and distinctive imaging features (Table 2). Knowledge of these lesions can help suggest a specific diagnosis, which in turn has implications for preoperative planning and prognosis (Table 3). However, even when the most sophisticated imaging techniques are used, renal neoplasms cannot always be diagnosed with preoperative imaging.
This article reviews solid renal masses in children, with an emphasis on clinical information and imaging characteristics that are helpful in distinguishing between lesions. The specific lesions discussed are Wilms tumor, nephroblastomatosis, renal cell carcinoma, mesoblastic nephroma, multilocular cystic renal tumor, clear cell sarcoma, rhabdoid tumor, angiomyolipoma, renal medullary carcinoma, ossifying renal tumor of infancy, metanephric adenoma, and lymphoma.
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Wilms Tumor
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Wilms tumor (nephroblastoma) accounts for 87% of pediatric renal masses and occurs in approximately 1:10,000 persons (1,2). Its peak incidence is at 3–4 years of age (3), and 80% of patients present before 5 years of age (4). It is rare in neonates, with less than 0.16% of cases manifesting in this age group (3). Wilms tumor is bilateral in 4%–13% of children (4) and may be associated with congenital anomalies such as cryptorchidism (2.8% of cases), hemihypertrophy (2.5%), hypospadias (1.8%), and sporadic aniridia (5).
Two loci on chromosome 11 have been implicated in the genesis of a minority of Wilms tumors. Locus 11p13 is known as the WT1 gene, and locus 11p15 is known as the WT2 gene. An abnormal WT1 gene is present in patients with WAGR syndrome (Wilms tumor, aniridia, genitourinary abnormalities, mental retardation) or Drash syndrome (male pseudohermaphroditism, progressive glomerulonephritis); an abnormal WT2 gene is present in patients with Beckwith-Wiedemann syndrome or hemihypertrophy. However, the genetics of Wilms tumor appear to be multifactorial, and abnormalities at other sites, including chromosomes 1, 12, and 8, are also recognized. Familial Wilms tumor is rare, occurring in approximately 1% of cases, and is not associated with mutations in chromosome 11 (3). Screening for Wilms tumor in patients with associated syndromes should begin at 6 months of age with initial computed tomography (CT) followed by serial ultrasonography (US) every 3 months up to 7 years of age. After the age of 7 years, screening can be discontinued because the risk of developing Wilms tumor decreases significantly (4,6).
Discovery of Wilms tumor most commonly follows detection of a palpable mass, but it is discovered after coincidental trauma in up to 10% of cases (4). Hematuria and pain are infrequent clinical findings. Constitutional symptoms are usually lacking, but hypertension may be present in up to 25% of cases due to renin production by the tumor (4).
Wilms tumor arises from mesodermal precursors of the renal parenchyma (metanephros) and occasionally is found to arise in the extrarenal retroperitoneum, presumably within mesonephric remnants. At histologic analysis, Wilms tumor is composed of variable amounts of blastema, stroma, and epithelium (Fig 1). The term "teratoid Wilms tumor" may be applied if there is differentiation along tissue lines not normally found in the kidney, such as bone, cartilage, and muscle. Anaplasia, consisting of atypical mitoses or hyperchromatic cells with large nuclei, correlates directly with a negative prognosis and resistance to chemotherapy (3). The vast majority of Wilms tumors demonstrate favorable rather than unfavorable histopathologic findings.
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Figure 1a. Wilms tumor. (a) Photograph of a gross specimen shows a lesion that extends into the renal pelvis. The lesion has a pseudocapsule, and its surface is divided by septa. (b) Photomicrograph (original magnification, x62.5; hematoxylin-eosin stain) shows the triphasic pattern of stromal (curved arrow), blastemal (straight arrow), and tubular (arrowhead) elements. (Courtesy of Maria M. Rodriguez, MD, Miami, Fla; reprinted, with permission, from reference 7.)
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Figure 1b. Wilms tumor. (a) Photograph of a gross specimen shows a lesion that extends into the renal pelvis. The lesion has a pseudocapsule, and its surface is divided by septa. (b) Photomicrograph (original magnification, x62.5; hematoxylin-eosin stain) shows the triphasic pattern of stromal (curved arrow), blastemal (straight arrow), and tubular (arrowhead) elements. (Courtesy of Maria M. Rodriguez, MD, Miami, Fla; reprinted, with permission, from reference 7.)
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Wilms tumor manifests as a solid intrarenal mass with a pseudocapsule and distortion of the renal parenchyma and collecting system. The tumor typically spreads by direct extension and displaces adjacent structures but does not typically encase or elevate the aorta; such encasement or elevation is a distinguishing characteristic of neuroblastoma. There may be vascular invasion of the renal vein and inferior vena cava with occasional extension into the right atrium. Metastases are most commonly found in the lungs (85% of cases), liver, and regional lymph nodes, and metastatic disease may also produce vascular invasion (4).
At US, the mass has heterogeneous echogenicity, which represents hemorrhage, fat, necrosis, or calcification (5). Examination of the inferior vena cava is critical to detect tumor extension, which could necessitate modification of the surgical approach. CT demonstrates the heterogeneous mass and nodal metastases (Fig 2), as well as areas of calcification and fat. Intravenous administration of contrast material is mandatory to detect nodal or hepatic metastases, tumor extension into the renal vein or inferior vena cava, contralateral synchronous tumor, and associated nephrogenic rests. At magnetic resonance (MR) imaging, Wilms tumor demonstrates low signal intensity on T1-weighted images (Fig 3) and high signal intensity on T2-weighted images. MR imaging also permits assessment of caval patency and multifocal disease. Wilms tumors are often very large at presentation and can cause severe distortion of adjacent organs, including the inferior vena cava. Determining whether there is direct invasion of the inferior vena cava or adjacent structures may be extremely difficult. MR imaging has been reported to be the most sensitive modality for determination of caval patency, but it requires sedation. In many children, the appropriate information for surgical planning may be obtained from meticulous US and contrast material–enhanced CT. Wilms tumor is occasionally largely cystic, in which case it may not be differentiated from cystic partially differentiated nephroblastoma, a subtype of multilocular cystic nephroma.
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Figure 2a. Wilms tumor in a 4-year-old boy with an abdominal mass. (a) CT scan shows a left renal mass with heterogeneous enhancement (thick arrow) and multiple hepatic metastases (thin arrows). (b) CT scan obtained at a higher level again shows multiple hepatic metastases in addition to tumor thrombus within the portal veins (arrows).
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Figure 2b. Wilms tumor in a 4-year-old boy with an abdominal mass. (a) CT scan shows a left renal mass with heterogeneous enhancement (thick arrow) and multiple hepatic metastases (thin arrows). (b) CT scan obtained at a higher level again shows multiple hepatic metastases in addition to tumor thrombus within the portal veins (arrows).
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Figure 3. Wilms tumor in a 3-year-old boy with an abdominal mass. Gadolinium-enhanced coronal fat-suppressed T1-weighted MR image shows a large, well-defined mass in the right kidney (arrows) that enhances less than adjacent renal parenchyma and contains multiple hypointense hemorrhagic foci. The hypointense area in the medial spleen is due to partial volume artifact.
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Unilateral Wilms tumor is generally treated with nephrectomy followed by adjuvant chemotherapy. Presurgical treatment with chemotherapy may be used to promote shrinkage of the tumor and improve outcome. Local radiation therapy of the tumor bed is advocated in some cases, and complete abdominal irradiation can be used when there is gross tumor spillage at surgery or peritoneal tumor implantation. In children with bilateral Wilms tumor, preoperative chemotherapy is especially important because each kidney is staged separately, and complete resolution of disease in one kidney may allow surgery on the contralateral kidney with eventual cure. In patients with bilateral Wilms tumor, the current approach is tumor resection with sparing of normal parenchyma. Cure rates are based on histologic findings and disease stage (Table 4) and have improved from 10% in the 1920s to over 90% today (5).
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Nephroblastomatosis
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Nephroblastomatosis consists of diffuse or multifocal involvement of the kidneys with nephrogenic rests. Nephrogenic rests are foci of metanephric blastema that persist beyond 36 weeks gestation and have the potential for malignant transformation into Wilms tumor. They are found incidentally in 1% of infants (4). It is currently believed that nephrogenic rests give rise to approximately 30%–40% of Wilms tumors (4), and they are found in up to 99% of bilateral Wilms tumors (5).
Nephrogenic rests are classified histologically as dormant, sclerosing, hyperplastic, or neoplastic. Dormant and sclerosing rests are usually microscopic and are not considered to have malignant potential. Hyperplastic and neoplastic rests are grossly visible as small tan nodules surrounded by normal parenchyma (Fig 4).
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Figure 4. Hyperplastic perilobar nephroblastomatosis. Photograph of a gross specimen shows diffuse overgrowth of peripheral mesoblastic tissue, which causes loss of corticomedullary differentiation and nephromegaly with preservation of reniform shape. (Courtesy of Frederic B. Askin, MD, St Louis, Mo; reprinted, with permission, from reference 7.)
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Nephrogenic rests can also be classified into perilobar and intralobar on the basis of location and the syndromes with which they are associated. Although this classification system may be more useful to the imager because it is anatomically based, the location and type of lesion may be definitely determined only with histopathologic analysis. Perilobar rests lie in the peripheral cortex or columns of Bertin. They are associated with Beckwith-Wiedemann syndrome and hemihypertrophy, Perlman syndrome (visceromegaly, gigantism, cryptorchidism, polyhydramnios, characteristic facies), and trisomy 18. Malignant degeneration into Wilms tumor is most common in patients with Beckwith-Wiedemann syndrome and hemihypertrophy, occurring in 3% of cases (4). Intralobar nephrogenic rests are considerably less common than the perilobar type but have a higher association with Wilms tumor development. These rests are found in 78% of patients with Drash syndrome and nearly 100% of patients with sporadic aniridia and are also seen in patients with WAGR syndrome.
At CT, macroscopic nephrogenic rests appear as low-attenuation peripheral nodules with poor enhancement relative to that of adjacent normal renal parenchyma (Fig 5). At MR imaging, the nodules demonstrate low-signal-intensity foci on both T1-weighted and T2-weighted images. US may demonstrate hypoechoic nodules but is less sensitive than MR imaging and CT. Diffuse nephroblastomatosis is usually seen as reniform enlargement with a thick peripheral rind of tissue that may show striated enhancement (4). At US, the enlarged kidney may have diffusely decreased echogenicity. Lymphoma can mimic either appearance of nephroblastomatosis but is unusual in infants and young children.
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Figure 5a. Nephroblastomatosis in a 16-month-old boy with an abdominal mass. (a) Plain radiograph shows displacement of bowel loops into the midabdomen by bilateral flank masses. (b) Contrast-enhanced CT scan shows large, homogeneous Wilms tumors (W) superimposed on a background of multiple peripheral nephrogenic rests (arrowheads).
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Figure 5b. Nephroblastomatosis in a 16-month-old boy with an abdominal mass. (a) Plain radiograph shows displacement of bowel loops into the midabdomen by bilateral flank masses. (b) Contrast-enhanced CT scan shows large, homogeneous Wilms tumors (W) superimposed on a background of multiple peripheral nephrogenic rests (arrowheads).
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Treatment for nephrogenic rests is controversial. Some investigators recommend chemotherapy, whereas others maintain that close serial radiologic evaluation of enlarging masses is sufficient. Screening for Wilms tumor in patients with syndromes associated with nephrogenic rests should be performed as described in the section on Wilms tumor (4).
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Renal Cell Carcinoma
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Renal cell carcinoma has been reported in patients less than 6 months of age. However, the tumor is rare in children, accounting for less than 7% of all primary renal tumors manifesting in the first 2 decades of life. Less than 2% of all cases of renal cell carcinoma occur in pediatric patients, with a peak incidence in the 6th decade of life. Wilms tumor outnumbers renal cell carcinoma in childhood by a ratio of 30:1 (8). However, their incidence is nearly equal in the 2nd decade of life (9). Renal cell carcinoma is associated with von Hippel–Lindau syndrome, in which the tumors tend to be multiple and manifest at a younger age. This syndrome must be ruled out in pediatric patients diagnosed with renal cell carcinoma, especially when the tumor is bilateral (9).
Clinical manifestations are similar to those in adults. Gross painless hematuria, flank pain, and a palpable mass are the most common presenting symptoms. Hematuria is more frequent in patients with renal cell carcinoma than in patients with Wilms tumor (8).
Renal cell carcinoma is thought to represent an adenocarcinoma with renal tubular differentiation, although the origin remains unclear (Fig 6). Although renal cell carcinoma tends to be smaller than Wilms tumor, its gross morphology is similar, and the two can be indistinguishable preoperatively. The tumor forms an infiltrative solid mass with variable necrosis, hemorrhage, calcification, and cystic degeneration. There is distortion of the normal renal architecture and formation of a pseudocapsule. The tumor invades locally with spread to adjacent retroperitoneal lymph nodes. Metastases to the lungs, bones, liver, or brain are found in 20% of patients at diagnosis. Compared with Wilms tumor, renal cell carcinoma is more likely to manifest bilaterally and more likely to metastasize to bone (10).
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Figure 6a. Renal cell carcinoma. (a) Photograph of a gross specimen of the clear cell type of renal cell carcinoma shows a golden color due to cytoplasmic lipids. (b) Photomicrograph (magnification reduced from x330; hematoxylin-eosin stain) shows an acinar arrangement and clear cytoplasm. (Reprinted, with permission, from reference 7.)
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Figure 6b. Renal cell carcinoma. (a) Photograph of a gross specimen of the clear cell type of renal cell carcinoma shows a golden color due to cytoplasmic lipids. (b) Photomicrograph (magnification reduced from x330; hematoxylin-eosin stain) shows an acinar arrangement and clear cytoplasm. (Reprinted, with permission, from reference 7.)
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Since renal cell carcinoma is usually significantly smaller than Wilms tumor at presentation, the mass may be subtle at intravenous urography and US and is most easily identified with CT or MR imaging as a nonspecific solid intrarenal mass with little enhancement (Fig 7). Heterogeneous areas of hemorrhage and necrosis may be present. There is a higher frequency of calcification in renal cell carcinoma (25%) than in Wilms tumor (9%) (4). The prognosis is influenced most by the stage at presentation (Table 5), with an overall survival rate of approximately 64% (11). The best outcomes have resulted from radical nephrectomy and regional lymphadenectomy. The tumor is extremely resistant to chemotherapy, rendering metastatic disease difficult to treat.
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Figure 7a. Renal cell carcinoma in a teenage boy. (a) Coronal T1-weighted MR image shows a large right renal mass (arrows) that is isointense to renal parenchyma except for a central area of high signal intensity. (b) Gadolinium-enhanced coronal T1-weighted MR image shows enhancement of the lesion (arrows). (Courtesy of Michael Ambrosino, MD, New York University Medical Center.)
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Figure 7b. Renal cell carcinoma in a teenage boy. (a) Coronal T1-weighted MR image shows a large right renal mass (arrows) that is isointense to renal parenchyma except for a central area of high signal intensity. (b) Gadolinium-enhanced coronal T1-weighted MR image shows enhancement of the lesion (arrows). (Courtesy of Michael Ambrosino, MD, New York University Medical Center.)
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Mesoblastic Nephroma
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Mesoblastic nephroma is the most common solid renal tumor in the neonate. Originally thought to represent congenital Wilms tumor, mesoblastic nephroma has been recognized as a distinct entity, often referred to as fetal renal hamartoma or leiomyomatous hamartoma. It is usually identified within the first 3 months of life, with 90% of cases discovered within the 1st year of life. There is a slight male predominance (10).
The most common clinical presentation is a palpable abdominal mass, with hematuria less frequent. Some cases are detected at prenatal US and may be associated with polyhydramnios, hydrops, premature delivery, and increased renin levels.
Mesoblastic nephroma is believed to consist of proliferation of early nephrogenic mesenchyme. At gross analysis, the tumor has a rubbery appearance, similar to a uterine leiomyoma. It tends to be a large infiltrative mass with ill-defined margins and no capsule (Fig 8). At histologic analysis, the tumor is monomorphic, with fingerlike, infiltrating projections of spindle-shaped mesenchymal cells and embryonal metaplasia of entrapped renal tissue (3,10).
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Figure 8. Mesoblastic nephroma. Photograph of the cut surface of a gross specimen shows a whorled, myomatous appearance with prominent medial extension (arrow) and an ill-defined margin. Very little normal kidney (K) remains visible. (Courtesy of Jane Chatten, MD, Philadelphia, Pa; reprinted, with permission, from reference 7.)
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Imaging studies demonstrate a large solid intrarenal mass that typically involves the renal sinus. The mass replaces a large portion of renal parenchyma (Fig 9) and may contain cystic, hemorrhagic, and necrotic regions. Local infiltration of the perinephric tissues is common.
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Figure 9a. Mesoblastic nephroma in a 5-week-old male infant with an in utero renal mass. (a) Longitudinal US scan shows a mass of mixed echotexture replacing the lower pole of the right kidney (arrows). (b) CT scan shows moderate enhancement (arrow).
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Figure 9b. Mesoblastic nephroma in a 5-week-old male infant with an in utero renal mass. (a) Longitudinal US scan shows a mass of mixed echotexture replacing the lower pole of the right kidney (arrows). (b) CT scan shows moderate enhancement (arrow).
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Mesoblastic nephroma generally exhibits benign behavior and is successfully treated with nephrectomy alone (1). However, a wide surgical margin is necessary due to the infiltrative nature of the lesion. Rarely, the lesion may recur locally if incompletely resected or metastasize to the lungs, brain, or bones. Histologic characteristics are not reliable for predicting the biologic behavior of the tumor. Therefore, it is currently recommended that patients be closely followed up for 1 year after surgical resection (3,12). The prognosis is best if the tumor is diagnosed and resected before 6 months of age.
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Multilocular Cystic Renal Tumor
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Multilocular cystic renal tumor encompasses a spectrum ranging from a purely cystic lesion lined by epithelium and fibrous septa with mature tubules (cystic nephroma) to a lesion in which the septa contain foci of blastemal cells (cystic partially differentiated nephroblastoma) (13,14). Septa are the only solid components of these tumors, and cystic partially differentiated nephroblastoma is distinguished from cystic Wilms tumor by the absence of expansile solid masses of nephroblastomatous tissue. Cystic nephroma and cystic partially differentiated nephroblastoma are uncommon, benign lesions that cannot be differentiated by means of their gross or radiographic appearance (3) (Fig 10).
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Figure 10. Cystic partially differentiated nephroblastoma. Photograph of a gross specimen shows a cystic mass with well-defined, smooth margins between the lesion and normal kidney. (Courtesy of Esperanza M. Tiamson, MD, Baltimore, Md; reprinted, with permission, from reference 7.)
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Multilocular cystic renal tumors tend to manifest at two age peaks: in children aged 3 months to 4 years (predominantly boys with cystic partially differentiated nephroblastoma) and in adults (predominantly women with cystic nephroma) (4). Patients frequently present with a painless abdominal mass, and systemic symptoms are rare.
Imaging studies demonstrate a well-circumscribed, encapsulated mass consisting of multiple cysts from several millimeters to 4 cm in diameter, with variably enhancing septa and no excretion of contrast material into the loculi (Fig 11). When the cystic spaces are small, the tumor may appear solid. US is best at demonstrating its multicystic structure. The tumor may or may not extend beyond the renal capsule.
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Figure 11a. Cystic partially differentiated nephroblastoma in a 12-year-old girl with an abdominal mass. (a) CT scan shows a right renal mass with heterogeneous enhancement and cystic foci (arrow). (b) CT scan obtained at a lower level shows numerous septa and a rind of enhancing solid tissue.
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Figure 11b. Cystic partially differentiated nephroblastoma in a 12-year-old girl with an abdominal mass. (a) CT scan shows a right renal mass with heterogeneous enhancement and cystic foci (arrow). (b) CT scan obtained at a lower level shows numerous septa and a rind of enhancing solid tissue.
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Treatment is surgical, and the prognosis is excellent with complete excision. The rare cases in which there is tumor recurrence are successfully treated with local radiation therapy or chemotherapy. Metastatic disease has not been documented, to our knowledge.
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Clear Cell Sarcoma
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Clear cell sarcoma of the kidney (bone metastasizing renal tumor of childhood), once thought to be a variant of Wilms tumor, accounts for 4%–5% of primary renal tumors in childhood (3,10). The peak incidence is at 1–4 years of age, and a male predominance has been reported (3). To our knowledge, all documented cases have been unilateral. Clear cell sarcoma has a nonspecific presentation, most often manifesting as an abdominal mass.
At gross analysis, the tumor is soft and well circumscribed. At histologic analysis, small cells with inconspicuous nucleoli and ill-defined cell membranes and a prominent capillary network commonly characterize this tumor (Fig 12). However, there is a spectrum of appearances, in which only 20% have clear cells.
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Figure 12. Clear cell sarcoma. Photomicrograph (original magnification, x62.5; hematoxylin-eosin stain) shows characteristic histologic findings, with numerous cytoplasmic vesicles. The cells contain extensive clear cytoplasm with displaced nuclei. (Reprinted, with permission, from reference 7.)
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Imaging studies do not allow differentiation of clear cell sarcoma from Wilms tumor. A sharply demarcated solid intrarenal mass without intravascular extension is most often demonstrated (Fig 13).
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Figure 13a. Clear cell sarcoma in a 13-month-old girl with an abdominal mass. (a) CT scan shows extensive central necrosis within a mass arising from the right kidney. (b) CT scan obtained at a higher level shows heterogeneous enhancement (arrows) with mass effect and hydronephrosis of adjacent calices (arrowheads). (Courtesy of Tim Booth, MD, Children's Medical Center, Dallas, Tex.)
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Figure 13b. Clear cell sarcoma in a 13-month-old girl with an abdominal mass. (a) CT scan shows extensive central necrosis within a mass arising from the right kidney. (b) CT scan obtained at a higher level shows heterogeneous enhancement (arrows) with mass effect and hydronephrosis of adjacent calices (arrowheads). (Courtesy of Tim Booth, MD, Children's Medical Center, Dallas, Tex.)
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The tumor is characterized by its aggressive behavior and is associated with a higher rate of relapse and mortality than Wilms tumor. It may metastasize to the bones, lymph nodes, brain, liver, and lungs, in some cases long after nephrectomy. Treatment consists of nephrectomy and chemotherapy, with current long-term survival rates of 60%–70% (3).
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Rhabdoid Tumor
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Rhabdoid tumor is a rare, highly aggressive malignancy of early childhood. It is not related to Wilms tumor or rhabdomyosarcoma and was recently recognized as a distinct pathologic entity (15). Its name is derived from its histologic appearance, which resembles that of a tumor of skeletal muscle origin, although a myogenic origin has not been proved.
Rhabdoid tumor occurs exclusively in children (10), comprising 2% of pediatric renal malignancies (3). Approximately 80% occur in patients less than 2 years of age and 60% in patients less than 1 year of age, with the majority (25%) diagnosed between 6 and 12 months of age (3). The median age at diagnosis is 11 months, with the lesion reported at up to 9 years of age. Male patients predominate by a ratio of 1.5:1 (15). Rhabdoid tumor may manifest as hematuria, but due to its aggressive nature, symptoms may be referable to metastatic disease.
The association of rhabdoid tumor with synchronous or metachronous primary intracranial masses or brain metastases has been established as a distinctive feature. The brain lesion is usually near the midline and often in the posterior fossa. Primitive neuroectodermal tumor, ependymoma, and cerebellar and brainstem astrocytoma have all been documented (15). Clinically, patients may develop hypercalcemia secondary to elevated parathormone levels. After surgical resection, the serum calcium level tends to normalize (10,15).
At histologic analysis, the tumor is characterized by monomorphic noncohesive cells with prominent eosinophilic nucleoli and characteristic filamentous intracytoplasmic inclusions (3). Some areas of the tumor may superficially resemble the blastemal pattern of Wilms tumor. At gross analysis, the tumor has a nonspecific appearance (Fig 14).
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Figure 14. Rhabdoid tumor. Photograph of a gross specimen shows a round, lobulated mass with a nonspecific appearance. A small amount of normal kidney (K) is noted at the edge of the specimen. (Reprinted, with permission, from reference 7.)
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Imaging demonstrates a large, centrally located, heterogeneous soft-tissue mass involving the renal hilum with indistinct margins (Fig 15). Although the appearance may closely resemble that of Wilms tumor, several features can suggest the diagnosis: subcapsular fluid collections, tumor lobules separated by dark areas of necrosis or hemorrhage, and linear calcifications outlining tumor lobules. Vascular and local invasion is common (10,15,16).
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Figure 15. Rhabdoid tumor in an 8-month-old boy with hematuria. CT scan shows a left renal mass with heterogeneous enhancement (arrow). (Courtesy of Tim Booth, MD, Children's Medical Center, Dallas, Tex.)
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Rhabdoid tumor has the worst prognosis of all renal tumors. It is highly aggressive and metastasizes early, with most patients presenting with advanced disease. Eighty percent develop metastases, most commonly to the lungs and less often to the liver, abdomen, brain, lymph nodes, or skeleton. Survival is poor, with an 18-month survival rate of only 20% (3,16).
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Angiomyolipoma
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Angiomyolipoma is an uncommon tumor that consists of a disordered arrangement of vascular, smooth muscle, and fatty elements. Its histologic composition suggests a hamartoma, but it is currently believed to represent a benign neoplasm (17,18). These tumors most often occur sporadically. However, they may occur in 40%–80% of patients with tuberous sclerosis (19). Angiomyolipoma is also associated with neurofibromatosis and von Hippel–Lindau syndrome. The mean age at presentation is 41 years, with a 4:1 female predominance. In children, angiomyolipomas are rare in the absence of tuberous sclerosis. Eighty percent of children with tuberous sclerosis may be expected to develop lesions by the age of 10 years (17). Angiomyolipomas are more often bilateral, multifocal, and larger than in patients with tuberous sclerosis.
Symptoms are related to intratumoral bleeding from aneurysms that develop due to the abundant abnormal, elastin-poor vascularity of the tumor. Lesions smaller than 4 cm in diameter are typically asymptomatic; those larger than 4 cm in diameter are more likely to spontaneously hemorrhage, leading to flank or abdominal pain, hematuria, or even severe life-threatening hemorrhage (10,18). Severe retroperitoneal hemorrhage has been termed Wunderlich syndrome (20).
The imaging appearance of angiomyolipoma varies considerably based on the amount and type of histologic elements present. CT scans and MR images are diagnostic when fat is found within the mass (Fig 16). US demonstrates highly echogenic nonshadowing foci, which correlate with the fatty elements. Angiography can demonstrate the characteristic dilated tortuous vessels with aneurysm formation, although some lesions are hypovascular. Although fat is occasionally seen in Wilms tumor and renal cell carcinoma, the diagnosis of angiomyolipoma is typically straightforward in the appropriate clinical setting. Rarely, angiomyolipoma may become locally aggressive and invade neighboring structures. Extension into the inferior vena cava and regional lymph nodes has been described.
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Figure 16a. Angiomyolipoma in a 17-year-old girl with tuberous sclerosis. (a) Nonenhanced CT scan shows multiple fat-attenuation foci within the kidneys (arrowheads). (b) Contrast-enhanced CT scan shows heterogeneous enhancement of soft tissue within the lesions. (c) CT scan of the lung bases shows multiple bilateral small cysts.
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Figure 16b. Angiomyolipoma in a 17-year-old girl with tuberous sclerosis. (a) Nonenhanced CT scan shows multiple fat-attenuation foci within the kidneys (arrowheads). (b) Contrast-enhanced CT scan shows heterogeneous enhancement of soft tissue within the lesions. (c) CT scan of the lung bases shows multiple bilateral small cysts.
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Figure 16c. Angiomyolipoma in a 17-year-old girl with tuberous sclerosis. (a) Nonenhanced CT scan shows multiple fat-attenuation foci within the kidneys (arrowheads). (b) Contrast-enhanced CT scan shows heterogeneous enhancement of soft tissue within the lesions. (c) CT scan of the lung bases shows multiple bilateral small cysts.
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The recent literature advocates US evaluation of patients with tuberous sclerosis every 2–3 years before puberty and yearly thereafter to identify growing lesions (17). Angiomyolipomas larger than 4 cm in diameter should be treated with partial nephrectomy or selective catheter embolization to prevent potentially life-threatening hemorrhage (10). However, since massive replacement of the renal parenchyma by angiomyolipomas and cysts in patients with tuberous sclerosis may result in end-stage renal disease, preservation of functional renal tissue is an important consideration when partial nephrectomy is performed.
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Renal Medullary Carcinoma
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Renal medullary carcinoma is a recently described highly aggressive malignant tumor of epithelial origin that occurs almost exclusively in adolescent and young adult blacks with sickle cell trait or hemoglobin SC disease but not with homozygous hemoglobin SS (sickle cell) disease (21,22). This tumor has been termed the seventh sickle cell nephropathy. The age range is 10–39 years with a mean age of 20 years. In patients less than 25 years of age, there is a male-to-female ratio of 3:1, but in those older than 25 years there is no gender predominance. Presenting symptoms include gross hematuria, abdominal or flank pain, and less commonly weight loss, a palpable mass, or fever (23).
Renal medullary carcinoma is thought to arise at the renal pelvic-mucosal interface. The tumor quickly grows to fill the renal pelvis and invade vascular and lymphatic structures. Intraparenchymal satellite nodules are frequently present. At histologic analysis, a variable architectural pattern, drepanocytes (sickle cells), hemorrhagic foci, necrosis, and prominent stromal desmoplasia with inflammation characterize the lesion.
Imaging demonstrates a centrally located infiltrative lesion invading the renal sinus with peripheral caliectasis, reniform enlargement, and smaller peripheral satellite nodules (Fig 17). There is heterogeneous enhancement at CT, and US shows heterogeneous echotexture. The differential diagnosis includes transitional cell carcinoma, which is exceedingly rare and poorly documented in children, and rhabdoid tumor, which typically occurs in children aged 3 years or younger. Black race, sickle cell trait, and hemoglobin SC disease further support the diagnosis.
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Figure 17a. Renal medullary carcinoma in a 10-year-old boy with hematuria and sickle cell trait. (a) Abdominal radiograph obtained during intravenous urography shows lack of excretion from the right kidney. Normal excretion from the left kidney is noted, as is contrast material in the small intestine from a prior CT scan. (b) Longitudinal US scan of the right kidney shows loss of corticomedullary differentiation and hydronephrotic calices (arrows). (c) Contrast-enhanced CT scan shows a heterogeneous mass (curved arrows) infiltrating the right kidney and causing hydronephrosis. Extensive adenopathy is seen surrounding the retroperitoneal vessels (straight arrows). (d) Anterior and posterior images from a bone scan show multiple foci of increased uptake (arrows) and lack of renal excretion, findings consistent with extensiv | |
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Abstract
LEARNING OBJECTIVES FOR TEST...
