Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

medulla, while the nephrogram phase reveals accumulation of contrast within pyramids.

Inflammatory Pseudotumor

Inflammatory pseudotumors, of controversial pathogenesis, are rare in the kidneys or ureters (this condition is discussed in Chapter 14). Imaging identifies a soft tissue tumor.

Rosai-Dorfman Disease (Sinus Histiocytosis)

A rare benign entity first described in 1969, Rosai-Dorfman disease consists of sinus histiocytosis with massive lymphadenopathy, although several patients with bilateral renal infiltration by inflammatory cells, including histiocytes, and without adenopathy, have been described.

Ultrasonography in one patient revealed large, heterogeneous hypoechoic renal tumors containing calcifications (39); postcontrast CT showed less homogeneous tumor enhancement than renal parenchyma and no invasion,but collecting system distortion was evident.

Hamartoma

A cystic hamartoma of the renal pelvis is a rare tumor consisting of fibroblasts, smooth muscle, and epithelial tissue. Some mesoblastic nephromas reported in the literature may represent cystic hamartomas. These cystic tumors develop adjacent to the renal pelvis and are detected by most imaging.

Angiomyolipomas are discussed later in this chapter (see Mesenchymal Neoplasms).

Fibroepithelial Polyp

The benign fibroepithelial polyp contains epithelium and stromal tissue. Whether these polyps represent a hamartoma or are part of a reactive postinflammatory process is debatable. Most occur in the renal pelvis. Some develop a thick stalk covered by normal overlying transitional epithelium. Anecdotal reports describe multiple ureteral fibroepithelial polyps. They should be differentiated from papillomas, which are neoplastic in origin. They are rare in children.

Clinically, these polyps bleed or the patient presents with pain due to obstruction.

Intravenous urography and CT identify an intraluminal polyp. Some of these tumors are rather large and fill almost the entire renal pelvis. These polyps are a cause of ureter obstruction and the resultant hydronephrosis. An occasional one is cystic and contains septa.

Ureteral fibroepithelial polyps tend to mimic transitional cell carcinoma. The radiographic and ureteroscopic appearance of a ureteral rhabdomyosarcoma suggested a benign fibroepithelial polyp (40).

Vascular Tumors

Arteriovenous Malformation

Arteriovenous malformations develop in both the kidneys and the ureters. They are a cause of hematuria, which at times is massive.

Imaging findings of some renal arteriovenous malformations are similar to those of renal cell carcinomas. Some renal arteriovenous malformations are misdiagnosed with CT as a solid tumor, suggesting a malignancy.

Hematuria due to an arteriovenous malformation can generally be controlled by transarterial embolization and ablation; hematuria can recur and these malformations are amenable to retreatment; complications include varying degrees of renal infarction, pulmonary embolism, and renin-dependent hypertension.

Hemangioma

Most renal hemangiomas occur as part of Sturge-Weber syndrome or Klippel-Trénaunay syndrome. They are not common. Both capillary and cavernous varieties occur, but cavernous ones are more common. Most renal hemangiomas are discovered incidentally, with an occasional one manifesting by bleeding.

Imaging usually suggests a solid renal tumor similar to a renal cell carcinoma. An occasional one occurs at the renal hilum or even renal capsule.

Papillary Endothelial Hyperplasia

Precontrast CT of renal intravascular papillary endothelial hyperplasia (Masson’s tumor) revealed a 3-cm perirenal hypodense tumor, while dynamic CT identified peripheral enhancement during an early phase and homoge-

neous enhancement during delayed phase (41); the appearance mimicked a hemangioma.

Cystic Nonneoplastic Conditions

Classification

Some of the renal tumors having a cystic component detected with imaging are listed in Table 10.4. Discussed in this section are the more common cystic renal conditions. Cystic hamartomas have already been mentioned. Various cystic neoplasms are covered later in their respective sections.

The Bosniak system of classifying cystic renal masses is based on their CT appearance (Table

Table 10.4. Renal cystic tumors

Simple cyst

Complex cyst

Hemorrhagic cyst

Infected cyst

Dialysis acquired cysts

Congenital cystic disorders Multicystic dysplasia

Autosomal dominant polycystic disease Autosomal recessive polycystic disease Glomerulocystic disease

von Hippel-Lindau disease Tuberous sclerosis Ehlers-Danlos syndrome Congenital megacalyces

Focal hydronephrosis

Calyceal diverticulum

Pyelogenic cyst

Urinoma

Infection/inflammation

Papillary necrosis

Cystic renal pelvis hamartoma

Cystic neoplasms

Multilocular cystic nephroma

Cystic carcinoma

Vascular causes

Aneurysm

Renal vein varices

Arteriovenous malformations

Hematoma

Hemangioma

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Table 10.5. Bosniak system of classifying cystic renal masses

Category II Mildly complicated but clearly benign Thin septa, few calcifications

Category III More complicated cyst—indeterminate, need histological diagnosis

Thicker septa, multilocular, increased density, more calcifications

Category IV Cystic neoplasm

Thick walls, enhancing soft-tissue component

Source: Adapted from Bosniak (42).

10.5). It is of practical use. Cysts in category II contain one to two thin septa; if in doubt, a category III cyst should be considered, which has a more complex gradation, is indeterminate, and many need a histologic diagnosis. About half or so of category III cysts are malignant. Solid nodules in a cyst (category IV cyst) enhancing with contrast imply a malignancy. Aspiration is indicated of cysts in category IV, and if the aspirate is bloody a neoplasm is likely. A chronic renal abscess is in the differential for a category IV cyst. A study of patients with cystic renal tumors from two teaching institutions found that all resected Bosniak category I and II tumors were benign and category IV tumors were malignant (43); on the other hand, 59% of category III tumors were malignant.

The Bosniak classification does have limitations; some minimally complex cystic renal tumors contain malignant cells; some malignant tumors in categories II and III have contrast enhancement of less than 10 HU. In general, category II cysts are better evaluated with CT than US. Magnetic resonance imaging and renal angiography have a role in defining Bosniak category II and III cysts, but even here diagnostic confidence is at times difficult, especially with smaller cystic tumors.

The Bosniak classification has been applied using MR imaging criteria. In general, CT and MR imaging findings are similar, but in some cysts MR depicts additional septa, thicker cyst wall and septa, or different enhancement patterns, resulting in upgrading of the Bosniak classification (44).

A primary imaging function both for cysts and solid tumors is to differentiate intrarenal

KIDNEYS AND URETERS

from extrarenal ones, a task not always possible. Capsular tumors are one example, yet at times even a retroperitoneal lesion adjacent to the kidney mimics an intrarenal neoplasm. A rare mucinous cystadenoma originating in retroperitoneal tissues can suggest a renal cyst.

Simple Cyst

Clinical

Simple renal cysts are present in about 10% of the population. They are not common in children but increase in size and number with age. Their pathogenesis is unknown. Most simple cysts are solitary, unilocular, and located in the cortex, with the more central ones known as parapelvic cysts. Perinephric cysts are those located beneath the renal capsule. The terms parapelvic and perinephric cyst are useful to describe a cyst’s location. Exoparenchymal cysts are larger and have a greater tendency to increase in size than endoparenchymal cysts.

Most simple cysts are detected incidentally. Some larger ones are associated with hematuria or flank pain.

Imaging

A simple cyst should have a sharp, well-defined margin at its interface with normal renal parenchyma. Simple cysts do not have thick walls or prominent septa. Likewise, no associated soft tissue mass should be present. Many of these cysts are defined by US, but any suggestion of an associated solid component or wall thickening requires CT, or, in some instances a biopsy. Some benign cysts have thin, linear calcifications corresponding to the cyst wall or septa. Any mottled or amorphous calcifications should be viewed with suspicion. Septa in a benign cyst should be uniformly thin.

Computed tomography attenuation of a simple cyst should be that of water. A density greater than 10HU suggests a complex cyst. Higher densities are seen with hemorrhage into a simple cyst. Infection results in a thickened wall. Compared to an unenhanced density, the attenuation coefficient of a cyst should increase by no more than 10HU during the corticomedullary and parenchymal phases (45). Any greater enhancement or nodularity should raise suspicion for a malignancy. However, a caveat is

in order: a renal cyst attenuation increase up to about 10HU postcontrast is often due to an artifact from surrounding parenchymal enhancement (46).

With gray-scale US a simple cyst should show an anechoic lumen, well-defined walls, acoustic enhancement posterior to the cyst, and no associated nodularity or wall thickening. Thin septa may be identified, findings often not seen with CT.

The differential diagnosis for a “simple cyst” detected by gray-scale US includes an obstructed calyx, calyceal diverticulum, papillary necrosis, an aneurysm, and a vascular tumor. A pseudoaneurysm mimics a cyst on gray-scale US, and Doppler US is necessary to differentiate between them; Doppler US shows no flow, thus excluding a vascular lesion (except a thrombosed one).

Similar to other nonflowing fluid of water density, a simple cyst is hypointense or almost has a signal void on T1and is hyperintense on T2-weighted MRI. It does not enhance postcontrast. The cyst wall should be imperceptible; otherwise a more complex cyst etiology should be suggested.

Therapy

A high recurrence rate is common after aspiration of a simple cyst. Typical therapy consists of percutaneous injection of a sclerosing agent, with repeat injection if a cyst recurs. Use of 95% to 98% alcohol is effective and leads to complete regression of most cysts, except those >10cm in diameter. One technique consists of catheter insertion under US control, cyst drainage and sampling for bacteriologic and cytologic study, cyst opacification with contrast, instillation of roughly half the cyst volume of ethanol, and clamping for 20 minutes. Some authors prefer two sequential sclerosing agent injections over a 48-hour period. Local infiltration or intravascular injection results in lysis of cell membrane, protein denaturation, local vascular occlusion, and cell death.

Laparoscopic decortication of simple renal cysts is effective therapy for pain relief. Such therapy must be limited to Bosniak categories I and II cysts with their low risk for cancer.

In an occasional hypertensive patient, drainage of a simple cyst corrects hypertension; rather than ascribe hypertension directly to the

cyst, it is more likely that the cyst had compressed and narrowed an adjacent renal artery.

Parapelvic Cyst

Radiologists describe a cyst located in the renal sinus region, adjacent to renal pelvis, as a renal sinus cyst or parapelvic cyst. Most are simple cysts. Many of these cysts presumably form by prior urine extravasation or are embryologic lymphocysts. Most are detected as an incidental finding, but an occasional one compresses the adjacent renal pelvis sufficiently to produce some degree of obstruction. They vary in size, and range from solitary to multiple and unilateral to bilateral.

These cysts mimic a portion of the renal collecting system on precontrast imaging and do not enhance on postcontrast CT. Delayed images postcontrast identify the lack of communication with the urinary collecting system. At times US confuses a parapelvic cyst with hydronephrosis. An occasional one suggests a cystic neoplasm.

Complex Cyst

While a complex cyst is a pathologic entity, radiologists use the term in a descriptive sense for a cyst containing blood, septa, a thick wall, or calcifications. Complex cysts have thicker walls than simple cysts, better defined septations, a suggestion of nodularity, and calcifications may develop, or they do not contain clear fluid. Occasionally a fluid–fluid level develops in a cyst after a recent hemorrhage. Differentiation from a simple cyst is one of gradation. Thus bleeding or infection in a simple cyst changes cyst fluid composition and leads to wall thickening. From an imaging viewpoint, the major importance of a complex cyst is its mimicry of a malignant cyst.

The risk of a complex cyst being malignant increases with imaging findings of more complex structures; thus discrete cyst wall thickening, irregular septations, or any nodularity points toward a cystic malignancy, and aspiration of such a cyst serves little purpose; even if a benign aspirate is obtained, such a cyst should be assumed to contain a malignant component.

Increased fluid echogenicity is generally secondary to blood or infection.

ADVANCED IMAGING OF THE ABDOMEN

Delayed postcontrast CT is occasionally helpful in differentiating a homogeneous complex renal cyst from a neoplasm; those showing no change in attenuation between initial postcontrast and delayed CT are most consistent with a cyst, but cysts showing an attenuation decrease suggest presence of vascularity (47).

Preand postcontrast MR is helpful in evaluating complex cysts, although the MR appearance of benign and malignant complex cysts overlaps. Cyst content varies considerably in intensity on precontrast T1and T2-weighted images due to different protein concentrations, but cyst content should not enhance postcontrast. In particular, a complex cysts containing a heterogeneous MR fluid signal intensity should be viewed with suspicion for a malignancy. Cysts with an irregular, thick cyst wall or mural nodules tend to be malignant.

An occasional cyst is more dense than renal parenchyma on precontrast CT. These hyperdense cysts contain old blood, milk of calcium, or a similar component. In fact, a cyst may not be suspected because of the hyperdense CT appearance; US should reveal their cystic nature. These cysts may contain internal echoes. While some of these cysts may be followed serially, an occasional cystic renal carcinoma also has a hyperdense CT appearance.

Infected Cyst

An infected cyst often develops a discrete, thickened wall. Gas or debris is part of the cyst content in some. Septations develop in both infected and hemorrhagic cysts.

Most suspected infected cysts are drained both for therapy and to exclude an underlying malignancy.

Hemorrhagic Cyst

In adults, hemorrhage into a benign renal cyst is relatively common. It is rare in children. Nevertheless, caution is required in ascribing hemorrhage to an underlying benign cyst; hemorrhage also occurs into renal cell carcinomas, especially in patients with renal failure.

Computed tomography of a hemorrhagic cyst reveals a hyperdense cyst. Cyst content does not enhance postcontrast. Gray-scale US

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shows increased internal echoes. If followed serially, findings should regress with time. In an occasional hemorrhagic cyst both CT and US suggest a solid tumor, and these cysts are generally resected.

T2-weighted MRI within days of hemorrhage into a cyst reveals a hypointense signal. Later, hemorrhage results in a hyperintense signal on both T1and T2-weighted images. Some hemorrhagic cysts contain a fluid–fluid level. A benign cyst should have a very thin wall.

Lymphangiectasia

Renal lymphangiectasia (also called lymphangiomatosis and lymphangiomas when cystic) is considered to be a lymphatic developmental malformation, although some authors believe inflammation and lymphatic blockage are inciting factors while still others classify it as a mesenchymal neoplasm. A rare familial association is described. These cysts are lined by endothelial cells. They are rare in the kidneys. Most affected patients are asymptomatic, but pain or hematuria is an occasional presentation. A rare association is with renal vein thrombosis.

Detected mostly in children and young adults, renal lymphangiectasia is one of the causes of an enlarged kidney. Lymphangiomas

appear as multicystic tumors, peripelvic or perinephric in location. Rather than being cystic, at times diffuse perinephric fluid is identified. This fluid is close to water density, except if tainted by hemorrhage. Aside for an often more peripheral location and perinephric fluid, imaging findings tend to mimic multilocular cysts found in more common disorders.

In some, lymphangiectasia is associated with a preserved rim of renal cortex and decreased medullary attenuation. Ultrasonography reveals a hyperechoic kidney. Computed tomography and US findings suggest the diagnosis (Fig. 10.9). Aspiration of chyle from perinephric fluid should be diagnostic.

Localized Cystic Disease

Localized cystic disease of the kidney is a descriptive term for a condition that cannot be readily classified. These patients had unilateral multiple cysts varying in size, separated by normal renal parenchyma, and contained in a larger nonneoplastic tumor (48). No family history of autosomal-dominant polycystic kidney disease is found. Hematuria or flank pain is a usual presenting symptom. Imaging reveals one or several cysts in a larger tumor, at times suggesting a cystic neoplasm.

Renal Cell Adenoma and

Adenocarcinoma

Throughout this book most benign and malignant neoplasms are discussed as separate entities, but such differentiation is impractical with renal cell adenomas and carcinomas. Whether a renal adenoma is a separate and distinct entity or whether it is a premalignant version of a renal cell carcinoma is debatable. Indeed, pathologic criteria defining a renal cell adenoma are not clear and while clinically a distinction is obviously desirable, both pathologically and radiologically the boundaries between renal cell adenomas and adenocarcinomas are blurred. Tumors smaller than about 3cm tend not to metastasize, and some investigators believe these represent mostly adenomas. From an imaging viewpoint it appears best to discuss these tumors as part of a spectrum.

The term hypernephroma is often used synonymously for a renal adenocarcinoma. An older term, Grawitz tumor, is rarely encountered.

Classification

One practical classification of renal cell adenomas and carcinomas is based on their primary morphological features (49):

1.Clear cell type carcinomas

2.Chromophil (papillary) adenomas and carcinomas

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metastases at initial presentation. Classification of these carcinomas is incomplete. Renal medullary carcinomas associated with sickle cell trait (discussed later) are also collecting duct neoplasms.

One limitation of a morphologic classification as outlined above is the heterogeneity of some of these tumors. To overcome this, the Heidelberg Classification of Renal Cell Tumors was developed in 1996, based on tumor genetic properties correlated with their histologic findings (50); this classification subdivides renal cell tumors into benign and malignant parenchymal neoplasms. Benign tumors are further classified into metanephric adenomas and adenofibromas, papillary renal cell adenomas, and oncocytomas. Malignant tumors are classified into conventional renal cell adenocarcinoma, papillary (chromophil) renal cell carcinoma, chromophobe renal cell carcinoma, and collecting duct carcinomas. Some of these tumors cannot be readily classified. Others represent separate entities.

At times helical CT findings can suggest a particular subtype. Clear cell carcinomas have greater contrast enhancement than chromophil (papillary), chromophobe and duct of Bellini carcinomas (51); chromophobe carcinomas tend to have homogeneous contrast enhancement. Calcifications are more common in chromophil and chromophobe carcinomas than in other types. The latter tend not to invade venous structures.

3.Chromophobe adenomas and carcinomas

4.Duct of Bellini carcinomas

5.Oncocytic adenomas (oncocytoma)

6.Metanephroid adenomas

7.Transitional cell carcinomas

8.Neuroendocrine carcinomas

Tumors originating from proximal tubules (clear cell and chromophilic cell types) make up approximately 85% of renal cancers (49); tumors from connecting tubules and ducts (chromophobic, ducts of Bellini, and oncocytic) make up about 11%. Others are rare.

Collecting duct carcinomas occur in a wide age range and are more common in males. They are centered in the renal medulla but infiltrate extensively, and many patients already have

Clinical Aspects

General

Discussed in the next few sections are the clinical, imaging, and therapeutic aspects of the most common type of renal cell carcinoma, namely clear cell adenocarcinomas. Cystic carcinomas, chromophil carcinomas, and others are covered in later sections.

No specific serum markers are available for renal cell carcinomas. Renal cell carcinomas do not produce a-fetoprotein, with an occasional exception. Although of limited sensitivity and specificity, a radioimmunoassay of tumorassociated trypsin inhibitor appears promising.

Renal cell carcinomas are not common in children; their prevalence increases with age. When detected in childhood, these tumors tend