Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

tumor, washings tend to be nondiagnostic. Also, positive cytology does not identify the site of origin.

Imaging

Transitional cell carcinomas range from polypoid, to nodular, to flat. Some polypoid ones are on a stalk; these tend to be low grade and are often associated with adjacent ureter dilation, but differentiation from a stone is difficult in some patients, even with retrograde pyelography. The flat type, often containing squamous cell carcinoma components, tends to infiltrate, has little if any intraluminal component, and usually is of a higher grade malignancy; as expected, this type is more difficult to detect. If extensive, a more proximal infiltrating one will obstruct (“amputate”) a calyx. Calcifications are rare.

When large, these tumors infiltrate extensively and mimic a renal cell carcinoma, although, being centrally located, they are less prone to producing a renal contour abnormality. Some intrarenal transitional cell carcinomas are highly invasive, become necrotic, and involve a large portion if not the entire kidney. Only a rare transitional cell carcinoma extends into the vena cava as a tumor thrombus.

Being urothelial in origin, pyelography (IV, retrograde or antegrade) should have a high tumor detection rate, even higher than with CT and gray-scale US. A major limitation of IV pyelography is the lack of sufficient contrast opacification.

Computed tomography identifies a transitional cell carcinoma as a soft tissue density tumor adjacent to water density urine. It is isodense to renal parenchyma. Its density is lower than that of blood clots. Computed tomography identifies a cystic component in a minority. These tumors enhance slightly after contrast, although enhancement is less than that of renal parenchyma.At times a lucent stone is in the differential diagnosis, but CT should differentiate even a uric acid stone, which is denser than a transitional cell carcinoma.

Computed tomography is used in staging transitional cell carcinomas, although the type of staging criteria used and the presence of hydronephrosis influence staging accuracy. While CT detects invasion of adjacent structures and distant metastases, it is limited in

differentiating a superficial tumor from one invading muscle layers or renal pelvis. Proximal hydronephrosis tends to result in overstaging; CT sensitivity and specificity is considerably greater in assessing renal parenchymal invasion than in detecting ureteral or perirenal fat invasion. Staging accuracy is improved by decreasing CT thicknesses through the tumor.

If sufficiently large, US shows these tumors to be hyperechoic to renal parenchyma.

Endoluminal US holds promise; tumor location, size, and staging can be studied with this technique.

These tumors are hypointense on T2weighted MRI. Some of these hypovascular carcinomas enhance considerably with contrast MRI.

Therapy/Survival

Upper tract carcinoma in situ is occasionally treated by bacillus Calmette-Guérin instillation; although normalization of urinary cytology is reported, only limited studies are available.

Ureteroscopic biopsy and cytology are helpful in defining and grading these tumors. Tumor stage and grade are interrelated and have prognostic significance; most tumors with a low or moderate grade are at a low stage and those with a high grade are at a stage T2 or T3.

Among Japanese patients with renal pelvic or ureteral cancer who underwent lymph node dissection, no lymph node involvement was found in about two thirds (71). The 5-year survival rate was 79% for pN0, decreasing to 12% for pN1, 20% for pN2, and 0% for pN3. These are somewhat biased statistics because only patients selected for lymph node dissection are included.

Because of these tumors synchronous and metachronous potential, a nephroureterectomy is generally performed. A search for associated bladder cancers is also warranted. Close followup is necessary for metachronous tumors, especially during the first several years after initial surgery.

Nephrogenic Metaplasia/

Adenoma/Adenocarcinoma

Nephrogenic metaplasia, or nephrogenic adenoma, is a rare, benign urothelial condition histologically consisting of glandular structures. This condition is more common in the

bladder but occasionally is detected in the ureters or renal collecting systems. Most metaplasias are associated with chronic infection, and the pathogenesis presumably includes metaplasia in response to a prior inflammation or trauma. Some tumor cells show severe nuclear atypia and staining for anti-p53, suggesting that p53 has a role in adenoma-to-ade- nocarcinoma transformation of these tumors. A rare adenoma is cystic.

Although nephrogenic metaplasia is considered to be a benign condition, a rare primary nephrogenic (urothelial) adenocarcinoma is reported, at times consisting of nephrolithiasis, nuclear atypia suggesting a malignancy, a tubulovillous adenoma, and intestinal metaplasia in adjacent epithelium. A typical scenario consists of urography in a patient with pain and hematuria revealing a ureteral tumor, a biopsy identifies nephrogenic metaplasia, and this segment is resected.

Although resection should be curative, recurrence is not uncommon. Whether recurrence is due to incomplete resection or a new focus from diseased urothelium is often unknown.

Squamous Cell Carcinoma

Squamous metaplasia (also known as keratinizing and desquamating malpighian metaplasia) and subsequent squamous cell carcinoma develop in a setting of chronic urolithiasis or infection. Upper tract infection with Schistosoma hematobium is associated with this tumor.

No distinguishing imaging characteristics identify squamous cell metaplasia. At best, it appears as a polyp, often in the renal pelvis. It is diagnosed either from a biopsy specimen or on histologic study after nephroureterectomy.

These carcinomas range from flat, ulcerated plaques to large, bulky renal pelvis tumors. They have a tendency for intramural growth. Associated stones and xanthogranulomatous pyelonephritis make detection difficult.At times their imaging appearance mimics a transitional cell carcinoma. Both local spread and metastasis are common at the initial presentation. As an example, a man with a destroyed and painful kidney containing a staghorn calculus and believed to contain pyonephrosis underwent nephrectomy (72); a renal pelvis squamous cell carcinoma had invaded the kidney and psoas muscle.

ADVANCED IMAGING OF THE ABDOMEN

Sarcomatoid Carcinoma

Sarcomatoid carcinomas contain elements of adenocarcinoma, transitional cell carcinoma, and sarcoma. Some of these carcinomas are related to prior radiation therapy to other organs. About 5% of renal cancers are histologically sarcomatoid in nature. Survival of patients with these cancers is measured in months.

Imaging findings of most sarcomatoid carcinomas are similar to the more common renal adenocarcinomas.

Medullary Carcinoma

Renal medullary carcinoma is a rare collecting duct neoplasm affecting teenagers and young adults with sickle cell trait or hemoglobin SC disease. Presumably an unidentified genetic component is a factor in its pathogenesis. The tumor has a distinctive microscopic appearance consisting of a diffuse and glandular growth pattern, an inflammatory infiltrate, and the presence of rhabdoid/plasmacytoid cells or even sarcomatoid cells.

This entity should be considered in a young patient with sickle cell trait or hemoglobin SC disease who develops hematuria. Distant metastasis at the initial presentation is not uncommon. These tumors are centrally located and infiltrate the renal parenchyma and renal sinus; occasionally a necrotic tumor communicates with the collecting system. They show heterogeneous contrast enhancement, presumably due to necrosis. Venous invasion and nodal metastases are common.

This is an aggressive malignant neoplasm having a poor prognosis. The interval from diagnosis to death averages only several months; no objective response is seen to chemoand immunotherapies.

Other Renal Epithelial Tumors

A number of mostly cystic renal neoplasms cause considerable confusion and defy ready classification. The traditional method is to divide them into benign (metanephric adenoma, multilocular cystic nephroma, nephronogenic nephroma, and congenital mesoblastic nephroma) and malignant (Wilms’ tumor, atypical mesoblastic nephroma, and cystic partially differentiated nephroblastoma) neo-

KIDNEYS AND URETERS

plasms, although some defy discrete classification. From a radiologic perspective it appears more appropriate to classify them into solid and cystic neoplasms and then subdivide by patient age (Table 10.7). In addition to the neoplasms listed in the table, a multicystic dysplastic kidney and segmental multicystic dysplasia in a duplicated renal collecting system are also in the differential diagnosis.

An association of fetal nephrogenic rests and multicystic dysplasia has already been discussed. Nephrogenic tissue is also associated with some neoplasms; benign ones include multilocular cystic nephroma and nephrogenic nephroma, and the malignant ones are Wilms’ tumor and mesoblastic nephroma. And then there is cystic partially differentiated nephroblastoma (discussed below); pathologists usually classify it as a separate entity, but strong evidence for this is lacking.

Wilms’ Tumor (Nephroblastoma)

Nephroblastomatosis

Persistence of renal blastema tissue in a neonate is known as nephroblastomatosis. While immature metanephric tissue (nephrogenic rests) found in nephroblastomatosis is not intrinsically neoplastic, it is believed to be able to undergo neoplastic change, and nephroblastomatosis is probably a precursor of Wilms’ tumors, or at least some of them. If one resected

kidney contains metanephric tissue, the contralateral kidney is at increased risk of containing a Wilms’ tumor. Persistent metanephric tissue is found in a number of syndromes, including Beckwith-Wiedemann syndrome (discussed earlier; see Congenital Abnormalities, above), Denys-Drash syndrome (gonadal dysgenesis, nephropathy, and renal failure), Perlman syndrome, hemihypertrophy, neurofibromatosis, and sporadic aniridia. Early and often bilateral Wilms’ tumors develop in patients with these syndromes.

Denys-Drash syndrome is caused by a mutation in the WT1 gene on chromosome 11p13, a tumor-suppressor gene. It is highly expressed during genital and renal development. Wilms’ tumors in this syndrome are derived from cells homozygous for this mutation.

A kidney diffusely affected with nephroblastomatosis is enlarged and has a plaque-like or lobular outline. Imaging shows a subcapsular tumor or tumors. Data from the German Nephroblastoma Study Group identified nephrogenic rests as multinodular, peripheral cortical lesions, with diffuse extension being less common (73); the rests were homogeneous and hypodense, hypoechoic, and hypointense on both T1and T2-weighted images and were best seen with postcontrast CT and T1-weighted MRI. They enhance less post–CT contrast than do renal parenchyma. Lesions <1cm were rarely identified by US. Overall, homogeneity was the most reliable criterion in differentiating nephrogenic rests from Wilms’ tumors.

Children

Clinical

A Wilms’ tumor, or nephroblastoma, is the most common solid abdominal malignancy in childhood. The vast majority originate in the kidneys, with only an occasional one developing in other extraperitoneal structures, presumably from retained mesonephric tissue. Some are bilateral, with most of these being synchronous rather than metachronous. An occasional Wilms’ tumor first manifests as pulmonary metastases. It is rare in the neonate; nevertheless, synchronous bilateral Wilms’ tumors in neonates are described. A majority are detected in children below the age of 5 years.

An occasional Wilms’ tumor is familial. Children with hemihypertrophy are at an increased risk of having Wilms’ tumor; the risk is about 5%, and these children should undergo serial surveys up to about the age of 8 years. The prevalence of Wilms’ tumors is considerably higher in children with aniridia, who should also undergo serial surveillance. An occasional Wilms’ tumor develops in a setting of cryptorchidism. A number of chromosome abnormalities are evident in some of these patients, including a Wilms’ tumor-suppressor gene at 11p13.

A minority of these tumors contain bone, cartilage, fat, or other atypical tissue. Some investigators label these as “teratoid Wilms’” tumors. Some such teratoid Wilms’ tumors contain a cystic component.

A palpable abdominal tumor is a common presentation. Bleeding develops in some tumors and they enlarge suddenly due to hemorrhage. A hematoma or urinoma is not an uncommon association. Renin is produced by a minority and these children develop hypertension.

These tumors spread hematogenously to lungs, liver, bone, and other organs. The overall prognosis (long-term cure) in children with Wilms’ tumors is approximately 80%.

Ultrasonography-guided needle biopsies of pediatric renal tumors, including Wilms’ tumors, have been safely performed, although the technique is controversial.

Imaging

Typically, when first detected these are large, round, soft tissue tumors infiltrating and

Figure 10.25. Anaplastic Wilms’ tumor in a 6-year-old with a palpable abdominal tumor. Computed tomography identifies a left renal tumor deviating the kidney anterior and medial. A claw sign suggests the diagnosis. Adenopathy is also present. Other studies detected pulmonary metastases. (Courtesy of Luann Teschmacher, M.D., University of Rochester.)

distorting adjacent renal parenchyma (Fig. 10.25). Some are mostly exophytic. Necrosis is common. An occasional Wilms’ tumor contains fat or calcifications, although in general calcifications in a juxtarenal tumor in a child should suggest a neuroblastoma.

Postcontrast CT and nonenhanced MRI appear equally accurate in establishing size and tumor origin, although both are inaccurate in staging. Ultrasonography can assess tumor size but not origin. Ultrasonography most often identifies a large, well-marginated tumor of heterogeneous, increased echogenicity due to necrosis and hemorrhage. Possible renal vein and inferior vena caval invasion is evaluated with Doppler US.

Magnetic resonance typically reveals these tumor to be hypointense on T1and hyperintense on T2-weighted images. Larger ones often contain blood and are heterogeneous in appearance. Their postcontrast enhancement is variable.

Renal vein invasion is common, and some extend into the inferior vena cava; and an occasional one even grows into the right atrium (Fig. 10.26). A tumor thrombus extending into the left renal vein can obstruct the gonadal vein and result in a varicocele. Ultrasonography should identify most such tumor extensions. Aortic encasement is uncommon. These tumors also grow into the calyces and adjacent ureter.