Baert Renal Tumors

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MODERN IMAGING OF RENAL TUMORS

Albert L. Baert, M.D.

TUMORS OF THE RENAL PELVIS

Transitional cell carcinoma is the most common epithelial tumor of the renal

pelvis.

Transitional cell tumors are primarily detected on excretory urography and

present as a "filling defect". A very small tumor may be invisible to all types of

imaging. As the tumor grows, the renal sinus fat is invaded and eventually the

renal parenchyma becomes invaded. At this stage, soft tissue mass is seen

on CT and ultrasound. Especially on ultrasound, the tumor mass might be

indistinguishable from renal adenocarcinoma.

RENAL TUMORS

Ultrasound

The advent of ultrasound was a major advance for the evaluation of renal

masses in a non-invasive way. The major advantage of ultrasound is its

capability to identify simple cysts with a high degree of accuracy and to

demonstrate which cysts are complex and require further investigation.

Simple CystsThe ultrasonic diagnosis of a simple cyst is based on identifying a renal fluid

collection with no internal echoes, with a thin and sharply defined distal wall

and presence of acoustic enhancement. Any deviation from those criteria

should raise the possibility of the presence of renal carcinoma and call for a

more definite study, such as CT.

Angiomyolipoma

Angiomyolipoma contains fat, muscle and blood vessels, and presents as

highly reflective nodular mass on ultrasound. Although the finding of a

reflective mass suggests the diagnosis of renal angiomyolipoma, it should be


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remembered that some renal adenocarcinomas also present as a reflective

mass.


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Multilocular cystic nephroma

Multilocular cystic nephroma contains multiple cystic loculi with intervening

septae and is thus identified as a "complex renal cyst".

Renal oncocytoma / renal adenoma

Renal oncocytoma and renal adenoma present as solid renal masses and are

difficult to differentiate from renal adenocarcinoma. The presence of

calcification and central stellate scar might, however, suggest the diagnosis

of renal oncocytoma.

Renal adenocarcinoma

Renal adenocarcinoma is the most common malignant renal tumor and

mostly presents as a solid mass with inhomogeneous texture. The reflectivity

of the mass is usually similar to that of the adjacent parenchyma. Rarely,

adenocarcinoma may have a reflectivity similar to that of the renal sinus.

Computed Tomography

Whereas large renal tumors are easily recognized, small renal tumors may be

difficult to detect, because they have usually a distinct surface with the kidney

with no signs of extrarenal spread, and are usually homogeneous and sharply

marginated. The localization of these small renal tumors is improved by spiral

CT, since spiral CT eliminates respiratory misregistration. It has been shown

that nephrographic phase (NP) images are superior to corticomedullary

phase (CMP) images in detecting renal masses. The difference is most

marked in the renal medulla. Therefore, spiral CT is initiated 70-80 sec after

the injection of contrast material. A total amount of 120-150 ml of contrast

material at a rate of 2-3 ml/min is administered. A collimation of 8 mm and a

pitch (ratio of table speed to collimation) of one is used. This technique

permits scanning of the kidney during nephrographic phase, as well as the

evaluation of the liver parenchyma prior to the equilibrium phase. Once a

renal mass is detected, the major task for the radiologist is differentiating

simple renal cyst from benign or malignant renal tumor.

Renal cysts

Detection of tumor vascularization is an important clue in differentiating renal

cysts from small renal tumors. Therefore, it remains necessary to obtain non-enhanced images prior to the administration of IV contrast material.


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Angiomyolipoma

Besides the detection of (small) renal tumors, the differentiation between

benign and malignant tumors remains a major challenge to radiologists.

Although there are no characteristics useful in distinguishing between benign

and malignant renal tumors, an exception must be made of angiomyolipomas,

that can be reasonable detected on the basis of the presence of fat. Thin

section CT is necessary, as the detection of fat is difficult if the tumors are

small, or if there is a small amount of fat. When, besides a small amount of

fat, intratumoral calcifications are identified on CT, renal cell carcinoma

should be considered in the differential diagnosis.

Multilocular cystic nephroma

Multilocular cystic nephroma presents as a densely encapsulated solitary

intrarenal mass, composed of multiple non-communicating cysts of varying

size.

Renal oncocytoma

The typical CT appearance of renal oncocytoma is a homogeneous solid

renal mass that may be either hypodense or only slightly less dense than the

renal parenchyma after injection of contrast medium. A central star shaped

scar is highly suggestive of renal oncocytoma. Although renal cell carcinomas

do not have a central scar, a similar pattern may be mimicked by a renal cell

carcinoma.

Besides detection of renal cell carcinoma, CT remains the most common

technique for staging. Tumoral invasion of the inferior vena cava or main

renal vein can be diagnosed with a 78 to 93 percent accuracy. Tumor

extension produces hazy tumor margins, thickens the renal capsule, and

obliterates the perinephric fat. False negative interpretation occurs in the

presence of microscopic invasions, and false positive diagnosis is caused by

necrosis of perirenal fat of perinephric hematoms.

Lymph nodes can be detected with a high degree of sensitivity. However,

since lymph node enlargement not always correlates with the presence or

absence of tumoral involvement, the use criteria are neither 100% sensitive

nor specific.


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Overall sensitivity and specificity rates for stage I, III and IV disease vary

between 90 and 100%. Stage II disease remains difficult to differentiate with

reported sensitivity and specificity of respectively 44 and 91 percent.

Magnetic Resonance Imaging

Magnetic resonance imaging has also been advocated as an accurate

technique to diagnose, characterize and stage renal mass lesions.

Magnetic resonance imaging in patients with renal tumors should include

standard T1- and T2-weighted imaging sequences.

In patients with renal adenocarcinoma, the venous system is better evaluated

with sagittal and coronal images.

Fast spin-echo sequences improve spatial resolution and decrease motion-

induced artifacts.

Possible advantages of MR imaging over CT include the ability to

discriminate between tumor thrombus and a bland thrombus in the inferior

caval vein and the ability to detect vein wall invasion.

A major disadvantage of MRI is that small renal tumors are frequently

isointense with the renal parenchyma, and might remain unidentified. The

advent of contrast-enhanced MRI, however, may improve the accuracy of

MRI in diagnosing renal cell carcinoma.

SUGGESTED READING

1. Cohan RH, Sherman LS, Korobkin M, et al.

Renal masses: Assessment of Corticomedullary-Phase and

Nephrographic-Phase CT Scans.

Radiology 1995; 196:445-451

2. Kalender WA, Seissler W, Klotz E, Vock P

Spiral volumetric CT with single-breath-hold technique, continuous

transport, and continuous scanner rotation.


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Radiology 1990; 176:181-183

3. Pollack HM, Banner MP, Arger PH, et al.The accuracy of grey-scale renal ultrasonography in differentiating

cystic neoplasms from benign cysts.

Radiology 1982; 143:741-751

4. Zagoria RJ, Bechtold RE, Dyer RB

Staging of renal adenocarcinoma: role of varying imaging procedures.

AJR 1995; 164:363-370

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Baert Renal Tumors