Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

angiomyolipoma. Precontrast CT should be used; postcontrast enhancement may increase the density to the level of water. Angiomyolipomas tend to be well marginated but do not have a capsule. Cystic regions are not common. Larger tumors often extend outside the kidney.

With an atypical CT appearance aspiration biopsy or cytology should provide a definitive diagnosis.

A typical sonographic appearance of an angiomyolipoma is that of a well-circum- scribed, homogeneous, hyperechoic tumor.

Shadowing is detected in some, a finding unusual for a carcinoma. Because many small renal cell carcinomas are also hyperechoic, it is necessary to confirm US findings with other imaging. In some instances, however, a homogeneously hyperechoic lesion is accepted without confirmation as a probable angiomyolipoma, especially in patients under the age of 50 and with a tumor smaller than 15mm or possibly 10mm in diameter. In some patients with tuberous sclerosis and a confluence of multiple angiomyolipomas, US simply reveals diffusely increased echogenicity in markedly enlarged kidneys.

Ultrasonography frequency-dependent attenuation is potentially useful in differentiating renal cell carcinomas from angiomyolipomas. Frequency-dependent attenuation values of renal cell carcinomas are significantly lower than those of angiomyolipomas. Some overlap does exist, and the practical application of this technique remains to be established.

The fat component even in small tumors is identified by fat-suppressed T1-weighted MRI. Fat in these tumors tends to be scattered throughout the lesion. Opposed phase MRI alone, being sensitive even to intracytoplasmic lipid found in some renal cell carcinomas, is inappropriate to evaluate the gross fat in angiomyolipomas and differentiate them from carcinomas. Magnetic resonance imaging can reliably identify small (<1.5cm) angiomyolipo-

ADVANCED IMAGING OF THE ABDOMEN

mas using a combination of sequences. Thus in-phase sequences show them to be minimally or moderately hyperintense relative to the renal cortex, and fat-suppressed sequences reveal a loss of signal intensity (84). Some tumors are difficult to identify due to limited signal differences or breathing artifacts.

Complications

Complications include compression of adjacent calyces and intratumoral, subcapsular, perirenal, or pararenal bleeding and hematoma (Fig. 10.30). These tumors undergo cystic degeneration. Spontaneous rupture is not uncommon and an angiomyolipoma should be high in the differential for the patient presenting with spontaneous rupture and bleeding of a renal tumor.

Extrarenal extension into the perirenal fat and lymph nodes is common, and these enlarged pararenal lymph nodes cause additional diagnostic and therapeutic dilemmas. Computed tomography shows these enlarged lymph nodes to be of soft tissue density, although an occasional one contains fat.

Extension into a renal vein and inferior vena cava as a tumor thrombus represents an additional complication. Computed tomography can occasionally detect fat within a thrombus. Renal vein involvement is more common on the right and does not necessarily imply malig-

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nant transformation. Complicating matters, an occasional one is associated with a renal cell carcinoma.

Therapy

Many of these tumors are still treated with a nephrectomy, although a trend is toward a more conservative approach—partial nephrectomy or enucleation is feasible in some patients. Incomplete tumor excision, however, leads to recurrence, often rapidly.

In patients with tuberous sclerosis, embolization is effective in long-term management of hemorrhage; most of these patients require several embolization sessions. If needed, a partial nephrectomy is then performed.

Symptomatic renal angiomyolipomas have been embolized with a mixture of iodized oil and absolute ethanol (85); some tumors decrease in size and others do not change, but most patients remain asymptomatic (except with incomplete embolization). Follow-up after embolization reveals that it is the angiomyogenic components that disappear, while fatty tissue undergoes liquefactive necrosis.

Lymphangiomyoma

Lymphangiomyomas, considered to be hamartomas rather than neoplasms by some authors, are discussed in more detail in Chapter 14. Lymphangiomyomas are related to angiomyolipomas and are occasionally associated with an angiomyolipoma; they are also suspected of being a manifestation of tuberous sclerosis.

One cause of chyluria is renal or retroperitoneal lymphangioleiomyomatosis.

These are cystic, hypervascular lesions, readily confused with renal cell carcinomas.

calcifications. Computed tomography and US reveal a heterogeneous and hypoechoic tumor.

Imaging shows most sarcomas as large, invasive, necrotic tumors. They tend to mimic a large renal cell carcinoma. Poorly defined calcifications are seen in a minority. Metastases at initial presentation are common.

Angiomatous Neoplasms

A primary renal angiosarcoma is very rare, with only case reports described. They are also known as a malignant hemangioendothelioma, hemangioendothelial sarcoma, and vascular sarcoma and appear as single or multiple tumors. They arise from blood vessel endothelial cells. Hemorrhage is a common presentation. Some contain fat cells suggesting an angiomyolipoma.

An epithelioid angiosarcoma is a very aggressive tumor mimicking a primary epithelial tumor.

Clear Cell Sarcoma

Clear cell sarcomas are rare tumors found in younger children. Initially thought to be a variant of Wilms’ tumor, this tumor has distinct immunohistochemical and flow cytometric findings differing from Wilms’ tumors. Its relationship to mesoblastic nephroma is not clear. Clear cell sarcomas exhibit p53 protein nuclear immunopositivity, suggesting alterations in the p53 tumor-suppressor gene.

The imaging findings of clear cell sarcomas are similar to those of a Wilms’ tumor, with differentiation made by histology.

This is an aggressive tumor having a poor prognosis and a predilection for bone metastasis.

Leiomyomatous Neoplasms

Leiomyosarcomas account for half or more of the rare renal sarcomas. Even after resection, differentiation between a leiomyoma and its malignant counterpart is difficult. These tumors originate mostly from blood vessels, although some of the reported renal leiomyomas probably represent atypical angiomyolipomas.

Most benign leiomyomatous tumors are solid and well marginated, but degeneration leads to a cystic appearance. An occasional one contains

Ossifying Renal Tumor of Infancy

The few reported ossifying renal tumors of infancy occurred in infants under the age of 14 months. They appear to be benign. These tumors contain varying proportions of osteoid, osteoblastic cells, and spindle cells. Whether the osteoid component represents a form of urothelial transformation is speculation. The proportion of osteoid and the degree of osseous maturation increases with age.

Gross hematuria is the most common presentation. An occasional infant presents with a palpable abdominal mass.

Typically these tumors are attached to a renal papilla and present as a pelvicalyceal system tumor. Some resemble a staghorn calculus.

Osteogenic Sarcoma

A primary renal osteogenic sarcoma is very rare and should be distinguished from a sarcomatoid variant of renal cell carcinoma with osteoid formation. Whether these tumors represent a sarcomatous version of an angiomyolipoma is speculation.

Patients with a primary renal osteogenic sarcoma have elevated serum alkaline phosphatase levels but no underlying bone disease.

These tumors contain calcifications. A “sunburst” type of calcification, if present, should suggest the diagnosis. The differential diagnosis for a calcified renal tumor includes a calcified inflammatory tumor, renal cell carcinoma, and a metastatic or invading osteogenic sarcoma.

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tension, which can be severe, often associated with hypokalemia and hyperaldosteronism. An occasional incidentally discovered reninoma is nonfunctioning. As its name implies, these tumors originate from juxtaglomerular cells. Whether they are hamartomas or neoplasms is not established. They are benign. Histologically, some of these tumors mimic a hemangiopericytoma, with the latter tumor occasionally also resulting in hypertension.

These are solid, hypovascular tumors on CT and hyperechoic on US. Most reninomas are smaller than 2 to 3cm in diameter. Even if a reninoma is too small to be detected with imaging, with strong clinical suspicion renal vein catheterization and renin-level sampling should suggest a tumor site. It should be kept in mind, however, that some renal cell carcinomas and other renal tumors on occasion also produce renin.

Resection leads to a marked drop in blood pressure in most but not all patients, with the latter presumably having a residual angiopathy.

Histiocytoma

A majority of renal histiocytomas are malignant. A renal capsule origin is common. A review of 44 malignant renal fibrous histiocytomas found an average patient age of 58 years, no sex predilection, and two thirds of tumors involving the left kidney (86). Symptoms tend to be few, and a late presentation is common. Sudden flank pain suggests spontaneous tumor rupture.

Imaging reveals a solid tumor tending to be hypodense on CT, hypoechoic on US, and hypovascular on angiography (86). A correct preoperative diagnosis is not common, with renal cell carcinoma or oncocytoma often being suspected.

These tumors have a poor prognosis. Metastases and local recurrence after resection are common.

Reninoma (Juxtaglomerular Tumor)

These rare renin-secreting tumors, known as reninomas and juxtaglomerular cell tumors, occur both in children and adults and are more common in females. Their primary importance is their renin production and resultant hyper-

Hemangiopericytoma

Hemangiopericytomas are rare in the kidneys. An occasional one leads to hypertension. These are well-marginated, solid, hypovascular renal sinus tumors involving the renal hilum to the point of suggesting a renal pelvis transitional cell carcinoma. Some are large at the initial presentation, compressing the renal pelvis and causing hydronephrosis and parenchymal atrophy.

Needle aspiration cytology may suggest the diagnosis, although the appearance is similar to that seen with other spindle cell tumors. Immunohistochemistry is helpful.

Renal Capsule Neoplasms

Neoplasms originating from the renal capsule are not common. They represent a spectrum of mesenchymal origin, with some earlier authors simply referring to them as capsulomas. They include a histiocytoma, solitary fibrous tumor, leiomyoma, lipoma, and even an angiomyolipoma, the latter a rare site for this tumor. Differentiating capsular from renal and extraperitoneal origin tumors is often difficult, especially with larger ones. Identifying a capsular artery by angiography as a primary tumor

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feeding artery is helpful. Depending on the type of tumor, the adjacent renal parenchyma is either compressed or invaded. Often a renal cell carcinoma or oncocytoma is suspected.

Multiple renal capsular leiomyomas have been reported.

Other Tumors

Although a renomedullary interstitial tumor (previously called medullary fibroma) is detected in 26% to 41% of autopsies (87), the majority are only several millimeters in diameter and not detected by imaging. One did not enhance by CT, was hypointense both on T1and T2-weighted images, and arteriography revealed no neovascularity (87).

A normal kidney does not contain fat, and only limited case reports of renal lipomas are published. They predominate in women.

Metastases

Renal

Metastases to the kidneys are common, with multiple foci and bilateral involvement typical. Problems arise when there is a single renal tumor in a patient with a history of a malignancy. Is this an isolated metastasis or a second primary, such as a renal cell carcinoma? Occasionally even an abscess mimics a neoplasm. The distinction influences subsequent patient management, and in such a scenario a renal biopsy is often necessary.

Most renal malignant melanomas represent metastases, but a rare melanoma believed to be primary to the kidney has been reported (88).

Symptoms related to a primary tumor often overshadow those due to renal involvement. A patient occasionally presents with acute renal failure due to diffuse metastases, but more common are silent metastases. Hematuria and proteinuria develop eventually but renal function tends to be preserved until a marked tumor load develops. In a number of these patients a search for renal metastases is more an academic exercise than a practical endeavor.

Many renal metastases have a CT, US, and MRI appearance similar to that of renal cell carcinoma or lymphoma, although metastases tend to be small, multiple, and not deform the renal

outline. A renal tumor in a patient with widespread metastases is most likely metastatic. Computed tomography often reveals smaller and more numerous metastases than US; most are hypodense and inhomogeneous on precontrast CT, while postcontrast CT reveals most to have inhomogeneous enhancement. Most enhance less than normal renal parenchyma. Metastatic thyroid carcinomas, on the other hand, tend to be hyperdense and have variable contrast enhancement. Renal vein invasion is rare with metastatic disease.

The sonographic appearance of metastases varies; they range considerably in their homogeneity. Most are hypoechoic, although some, such as thyroid carcinomas, are hyperechoic or even isoechoic.

Metastasis from a hepatocellular carcinoma tends to mimic a renal cell carcinoma with both imaging and even needle biopsy.

A metastatic ovarian or appendiceal cystadenocarcinoma presents as a cystic tumor.

Ureteral

Metastases to the ureter can generally be divided into those from a pelvic cancer and those extrapelvic in origin.

A carcinoma metastatic to the periureteral region obstructs the ureter and the patient presents with flank pain and urinary tract infection. One conservative approach in patients with metastatic disease is US to monitor for hydronephrosis.

Drainage of ureteral obstruction due to pelvic or extrapelvic cancer with double-J catheters has a relatively high success rate; if necessary, a percutaneous nephrostomy is performed.

Self-expandable Wallstents have had limited acceptance in bypassing malignant ureteral obstructions. Patient survival is generally related to the primary tumor and not to stent placement.

Rhabdoid Tumor

Malignant rhabdoid tumors were initially thought to be a variant of Wilms’ tumor but currently are believed to represent a distinct clinicopathologic entity. An occasional rhabdoid tumor develops at an extrarenal site. In the kidney it is a highly malignant neoplasm occurring most often in infants, less often in young