Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

A rare one even extends intraluminally in the ureter.

A rare Wilms’ tumor is multicystic and mimics a multilocular cystic nephroma or benign renal cystic disease. Likewise, differentiation from a neuroblastoma can be difficult, although a neuroblastoma tends to be ill-defined and more heterogeneous in appearance. Also, a neuroblastoma tends to metastasize early and extensively. Superficially, a Wilms’ tumor can be confused with xanthogranulomatous pyelonephritis.

Therapy and Outcome

Bilateral Wilms’ tumors present a therapeutic dilemma. Conservative surgical therapy with initial biopsy followed by chemotherapy and delayed tumor resection in children with bilateral Wilms’ tumors in the United Kingdom Children’s Cancer Study Group resulted in a mean preserved renal mass of 45%, but initial surgical resection followed by chemotherapy left a renal mass of 35% (74); children in both groups had similar survival and similar renal function.

Imaging after chemotherapy usually identifies necrosis, hemorrhage and calcifications. In one child CT after chemotherapy showed an increase in tumor size and extensive fatty infiltration (75); resection revealed no residual blastema.

Follow-up of the remaining kidney in children after unilateral nephrectomy for Wilms’ tumor reveals that only half develop renal hypertrophy.

Adults

A Wilms’ tumor is rare in adults. Histologically, these tumors are similar to the ones seen in children, and the therapy likewise is similar, but these tumors grow rapidly and prognosis is worse than in children, possibly due to their more advanced presentation. Even extrarenal Wilms’ tumors have developed in adults (76).

A typical Wilms’ tumor in an adult consists of a complex mass often containing a cystic component. These tend to be large, hypovascular and multilocular tumors containing a limited solid component. The differential includes a cystic carcinoma and a multilocular cystic nephroma.

Multilocular Cystic Nephroma/Cystic

Partially Differentiated Nephroblastoma

Clinical

These tumors consist of a mixture of epithelial and mesenchymal components. In the past, a multilocular cystic nephroma, also called multicystic nephroma, cystic lymphangioma, and

multilocular cyst, was considered a hamartoma, although the current thinking leans toward a neoplastic origin. Some authors classify multilocular cystic nephroma and cystic partially differentiated nephroblastoma in young children as one entity and believe that they represent cystic Wilms’ tumors having little potential for invasion or metastasis (77); in adults, however, these authors believe that multilocular cystic nephroma is a separate benign entity independent of Wilms’ tumor, although a rare one does undergo malignant transformation and develops into a carcinoma or sarcoma. In one adult, a cystic partially differentiated nephroblastoma exhibited cell maturation intermediate between a multilocular cystic nephroma and a Wilms’ tumor (78).

A similar condition is found in some infants with other gastrointestinal and skeletal abnormalities,including visceromegaly (Perlman syndrome), an autosomal-recessive condition.

In children (mostly young boys) a multilocular cystic nephroma is radiographically and grossly indistinguishable from a cystic partially differentiated nephroblastoma. Both are cystic thin-walled tumors containing multiple septations surrounded by a fibrous capsule. These tumors differ histologically in that a multilocular cystic nephroma is composed of mature differentiated tissue and does not contain blastemal elements, while septa in a cystic partially differentiated nephroblastoma do contain embryonal cells. Calcifications are uncommon in children.

Imaging

Imaging detects both of these cystic tumors but cannot differentiate between them. Their appearance is similar to other cystic tumors. They occur unilaterally. In children, the numerous small cysts can appear solid, and imaging findings resemble a Wilms’ tumor. In adults (mostly women), imaging and even aspiration biopsy findings mimic a cystic renal cell carcinoma, although these tumors are not related.

Computed tomography reveals multilocular cystic nephromas to be homogeneous, multicystic tumors containing thin septa without a solid components; for some reason a lower pole location predominates. The cystic component does not enhance with contrast. Their hallmark is that septa tend to be their only prominent

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solid component, and the presence of a solid component should suggest either malignant transformation or another diagnosis.

Magnetic resonance imaging of a multilocular cystic nephroma reveals a complex solitary renal cystic tumor often protruding into the renal collecting system and containing thin septa, with the cystic component varying from hypoto hyperintense on T1-weighted images. Any solid component and septa enhance postcontrast in all cases.

Nephrectomy in a patient with a multilocular cystic nephroma should be curative. With a cystic partially differentiated nephroblastoma, however, some tumors recur locally.

Metanephric (Embryonal) Adenoma

A curious tumor composed of small tubular, papillary, and glomeruloid structures and consisting of proliferating primitive epithelial cells mimicking an embryonal nephron was first described in 1980 and called a nephronogenic nephroma (79). Later, a similar benign mixed epithelial and mesenchymal renal tumor was termed an embryonal adenoma. Since then, a number of reports have used the term metanephric adenoma. Whether this tumor represents a benign counterpart of Wilms’ tumor is conjecture. Metanephric adenomas and metanephric adenofibromas (discussed below) are closely related neoplasms. In either case, a renal metanephric adenoma is a distinctive renal cortical neoplasm usually classified among nephroblastic tumors. Its histologic appearance is similar to that of metanephric hamartomas found in nephroblastomatosis. In spite of a morphologic similarity to Wilms’ tumor, genetics suggest a relationship to papillary adenomas and papillary renal cell carcinomas.

These tumors consist of differentiated cells containing few or absent mitoses; tubules and glomeruli are often identified. Its metanephric (embryonic) architectural and cytologic appearance is characteristic, although pathologic study of some tumors suggests a malignancy. Needle biopsy and aspiration cytology specimens have been misdiagnosed as Wilms’ tumor. The reverse is also true: a renal cell carcinoma and a metastasis from a poorly differentiated thyroid carcinoma can be misdiagnosed as metanephric adenomas. Most authors

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consider these tumors to be benign, although a metanephric adenoma in a 7-year-old girl metastasized to lymph nodes (80).

There are two age peaks; half occur in children and the other half mostly in women in their 50s. Most tumors are discovered incidentally, with only a minority of patients having pain, hematuria, or a palpable tumor. Polycythemia and acquired von Willebrand’s disease develop in some patients. An association of a metanephric adenoma with embryonal hyperplasia of Bowman’s capsular epithelium was reported in a 9-year-old boy on peritoneal dialysis (81).

Metanephric adenomas are well-circum- scribed, unencapsulated tumors varying in size. Some contain fibrosis and calcifications. Computed tomography, US, and MRI usually reveal a solid tumor with occasional necrosis and cystic degeneration, suggestive of a hypovascular renal cell carcinoma. They are isoto hyperdense on CT. They range from hypoto hyperechoic on US.

Simple resection is curative in almost all patients.

Metanephric (Nephrogenic) Adenofibroma

The histology of metanephric adenofibromas (previously also called nephrogenic adenofibromas) reveals a proliferation of spindle cells, similar to what is found in congenital mesoblastic nephroma, encasing nodules of embryonal epithelium similar to the nephroblastomatosis associated with Wilms’ tumor; it is thus not surprising that these tumors have been misdiagnosed as Wilms’ tumors and mesoblastic nephromas.

These are childhood tumors. A not uncommon presentation is hematuria, hypertension, or polycythemia.

Metanephric Stromal Tumor

A metanephric stromal tumor is a renal medullary spindle cell (fibrous) tumor containing smooth-walled cysts that entrap native kidney tissue. Although such a description suggests that it should be classified under mesenchymal tumors, it is histologically identical to the stromal component of metanephric adenofibroma and is discussed with the latter entity (see above).

Histology reveals onion-skin cuffing around entrapped renal tubules, heterologous glial or cartilage differentiation, and vascular changes consisting of angiodysplasia of entrapped arterioles and juxtaglomerular cell hyperplasia of entrapped glomeruli (82).

The mean age of 31 patients with a metanephric stromal tumors was 2 years; most tumors presented as an abdominal tumor (82); they did not metastasize. The differential diagnosis includes renal clear cell sarcoma.

Mesoblastic Nephroma (Bolande’s Tumor)

Infants

Congenital mesoblastic nephroma, at times called congenital Wilms’ tumor, comprises about half of renal tumors encountered during the neonatal period. Most occur within the first month of life, and their prevalence decreases during childhood. They present as a large abdominal tumor. The prognosis depends on the infant’s age, being best for tumors discovered early in life.

Earlier publications considered mesoblastic nephromas to be a benign neoplasm, possibly even hamartomatous in origin. In fact, the favorable prognosis of Wilms’ tumors in neonates reported previously is suspect because many of these were probably mesoblastic nephromas rather than Wilms’ tumors.

The morphologic and cytogenetic evidence suggests a relationship between congenital mesoblastic nephromas and infantile fibrosarcomas. Not only are these tumors similar histologically, but also both are associated with similar chromosome polysomies and they may represent a single entity. Congenital mesoblastic nephromas and Wilms’ tumors also appear to share common genetic pathways and abnormalities.

Mesoblastic nephromas are mostly solid tumors which usually follow a benign course but tend to invade adjacent structures. Some authors subdivide these tumors into congenital and atypical, with the latter including aggressive and potentially malignant variants. The aggressive variety tends to recur or metastasize. A rare aggressive mesoblastic nephroma ruptures spontaneously.

Imaging reveals a solid, homogeneous, usually large tumor. Hemorrhage and necrosis

develop in the aggressive and malignant variants, although cystic degeneration is not common. Their imaging appearance mimics a Wilms’ tumor.

Some mesoblastic nephromas show uptake of Tc-99m-DMSA.

Adults

The rare similar adult renal tumor consisting of epithelial and stromal tissue is probably a different entity than the pediatric variety. Immunohistochemistry and electron microscopy aid in identifying this tumor. Some mesoblastic nephromas in adults may represent cystic hamartomas. An adult mesoblastic nephroma can grow either intraor predominantly extrarenal, at times even mimicking an adrenal tumor.

Lymphoma/Leukemia

Clinical

True lymphatics are not present in the kidneys and primary renal lymphoma is rare. Secondary involvement is either via contiguous spread from perirenal disease or from hematogenous dissemination. Involvement with nonHodgkin’s lymphoma is considerably more common than Hodgkin’s lymphoma. Kidneys of immune-deficient patients are more prone to involvement by lymphoma. Infiltration is usually bilateral.

Lymphoma involvement in other locations tends to dominate the clinical findings. Most renal involvement does not lead to symptoms until late in the course. A rare presentation is acute renal failure.

A granulocytic sarcoma (also called a chloroma) develops in a minority of patients after therapy for some leukemias. These solid renal tumors are hypointense on T1and hyperintense on T2-weighted images. They show little contrast enhancement.

Imaging

Imaging detects only a minority of lymphomatous renal involvement. No typical pattern predominates, and renal lymphoma ranges from

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solitary to multiple tumors, diffuse infiltration, or invasion from adjacent structures. Even perirenal involvement varies considerably from focal tumors to plaque-like infiltration. Retroperitoneal lymphoma generally invades the kidneys through renal sinuses, and thus the renal medulla is more often involved than the cortex.

Necrosis is uncommon in renal lymphomas, even when large. Likewise, lymphoma rarely results in a vascular tumor thrombus.

Computed tomography findings in the rare primary non-Hodgkin’s renal lymphoma range from a single nodule, multiple nodules, and diffuse infiltration, to a lymphomatous mass encasing the kidney. Lymphomatous foci are homogeneous and mostly slightly hyperdense to isodense on precontrast CT and do not enhance as much as normal renal parenchyma postcontrast. Focal tumors maintain a discrete margin. Diffuse infiltration results in an overall renal structure disorganization. A not unusual appearance is renal invasion by retroperitoneal lymphoma, but the lack of adjacent adenopathy is unreliable in excluding lymphoma. While CT allows evaluation of the site and size of involvement, it cannot establish a diagnosis; similar findings are seen with a renal cell carcinoma, transitional cell carcinoma, metastatic disease, or even some chronic inflammatory conditions.

Two sonographic patterns are evident: either a bilateral nodular infiltrate or diffuse renal enlargement with loss of differentiation between renal sinus echoes and parenchyma. The typical infiltration is hypoechoic, but at times homogeneous involvement results in an anechoic pattern mimicking a cyst.

Magnetic resonance imaging shows isoor slight hypointense tumors on T1and hypointense tumors on T2-weighted images. Postcontrast lymphomas show minimal tumor enhancement, in distinction to renal cell carcinomas where early enhancement is common. Whether CT or MR is superior in identifying and defining lymphomatous involvement is not clear. Ureteral involvement consists either of encasement or displacement by an adjacent lymphomatous focus.

Leukemic involvement results in symmetrically enlarged kidneys and compressed collecting systems.

Pathologists describe a rare epithelioid angiomyolipoma as a separate entity. It tends to mimic renal cell carcinoma; it contains regions of hemorrhage and necrosis. Fatty tissue is not identified.

The rare liposarcoma is indistinguishable with imaging from an angiomyolipoma. This problem is especially pertinent when an incidental renal angiomyolipoma is detected in the work-up of another carcinoma and a metastatic liposarcoma is suspected.

Imaging

Angiomyolipomas are diagnosed with CT, US, or MR (Figs. 10.28 and 10.29); most contain sufficient focal fat to be identified with CT, appear as homogeneous and hyperechoic tumors containing smooth margins with US, and have hyperintense signals on T1and hypointense signals on T2-weighted MRI.

Focal fat is also found in lipomas and some liposarcomas, but these tumors are rare in the kidneys. Clear cell renal cell carcinomas contain intracellular lipid and glycogen, which is detectable with chemical shift MR, but this fat is diffuse. Focal regions of fat are rare in renal cell carcinomas.

Calcifications are not a feature of angiomyolipomas except secondary to hemorrhage.

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Even when associated with fat, calcifications should suggest a carcinoma.

Diagnostic difficulty occurs when a tumor is small or contains very little fat. Conventional CT may not detect fat in tumors smaller than several centimeters in size. Multislice helical CT is more sensitive and is preferred for small lesions. These low-fat tumors are homogeneously hyperdense on unenhanced CT, show homogeneous postcontrast CT enhancement, and are isoechoic on US. Recent hemorrhage tends to mask any fatty component. Occasionally in such tumors a small amount of fat is detected with chemical shift MRI but not CT; in these, a renal cell carcinoma is in the differential diagnosis.

The diagnosis appears more difficult in children. Some of these tumors in children contain little fat and imaging simply identifies a large renal tumor. Even a fine needle biopsy may be misleading and a Wilms’ tumor initially suspected. The reverse is also true; US detection of a hyperechoic tumor in a child with von HippelLindau disease suggests an angiomyolipoma, yet an occasional such tumor is a renal cell carcinoma.

A CT diagnosis is straightforward with most angiomyolipomas. Fat intermixed with other tissue detected on thin-section nonenhanced CT is strong presumptive evidence for an