AJR 2000; 175:945-955
© American Roentgen Ray Society
Imaging of Small Renal Masses
A Medical Success Story
Ronald J. Zagoria1
1
Department of Radiology, Wake Forest University School of Medicine, Medical
Center Blvd., Winston-Salem, NC 27157-1088.
Received May 1, 2000;
accepted after revision May 24, 2000.
Honoring David R. Bowen, MD and James T. Case, MD
This is the tenth in a series of Centennial Dissertations that the
AJR is publishing this year in honor of the former presidents of the
American Roentgen Ray Society, two of whom are pictured above.
Address correspondence to R. J. Zagoria.
Introduction
Renal cell carcinoma is the most common primary malignancy of the kidney.
This tumor accounts for 2% of all cancer diagnoses in humans
[1]. More than 30,000 new cases
of kidney cancer are discovered and 12,000 deaths are caused by this disease
in the United States each year
[2]. Despite a great deal of
research and many innovations in the treatment of renal cell carcinoma, the
disease remains essentially resistant to radiation therapy and chemotherapy
[3]. Modifiers of host biologic
response, including agents such as interferon and interleukin, have been
researched extensively for the treatment of advanced renal cell carcinoma
[3]. However, therapeutic
techniques have led to little improvement in the prognosis for patients with
metastatic renal cell carcinoma; only 5-10% of these patients will be alive 5
years after diagnosis [1]. In
addition, the incidence of renal cell carcinoma has risen steadily, increasing
by 38% between 1974 and 1990
[3]. However, even though
little progress has been made in the therapy for renal cell carcinoma, the
5-year survival rate for patients with renal cell carcinoma has improved
significantly from 37% for patients whose disease was diagnosed in the early
1960s, to 52% for diagnoses between 1974 and 1976
[3,
4], to 58% for diagnoses
between 1983 and 1989 [3]. Both
of these trendsthe increased incidence and the improved survival
rateappear to be due to improvements in the radiologic diagnosis of
renal cell carcinoma [3] that
allow the diagnosis to be made at an earlier stage of the disease. Low-stage
renal cell carcinoma can be successfully treated with surgery, and the
prognosis for patients with low-stage disease at diagnosis is considerably
more favorable than for those with a more advanced stage at diagnosis
[3]. Lead time and length
biases may play some role in the apparent survival benefits
[5], but the data strongly
suggest that improved outcomes are due to earlier diagnosis.
Many renal tumors are diagnosed incidentally. Approximately 25-40% of renal
cell carcinomas are diagnosed after the incidental detection of a renal mass
[6,7,8].
Before the widespread use of cross-sectional imaging techniques, including
sonography, CT, and MR imaging, only 10% were incidentally detected
[9,
10]. The increase in
incidental detection of renal masses is not surprising given the increasing
use of cross-sectional imaging. In a study that analyzed autopsies performed
between 1958 and 1969, two thirds of cases of renal cell carcinoma occurred in
patients for whom the diagnosis was not clinically suspected
[11]. Several studies have
confirmed that the number of renal cell carcinoma cases detected during
imaging studies performed for nonurologic reasons is increasing
[6,7,8].
Patients with incidentally discovered renal cell carcinoma have a more
favorable prognosis than patients who present with urologic symptoms
attributable to renal cell carcinoma
[8,
10,
12]. The difference in
prognosis reflects the fact that most incidentally detected renal cell
carcinoma is either stage T1 or stage T2, resulting in a much more favorable
outlook than that of advanced disease. In fact, the clinical course and
disease stage in symptomatic patients have not changed significantly during
the last 40 years [8].
Therefore, it appears that progress has been substantial with regard to
survival for patients with renal cell carcinoma. Much of this improvement can
be attributed to early radiologic diagnosis of renal malignancies, which in
turn has resulted in a higher proportion of tumors that can be cured with
surgical resection. For this reason the detection and accurate diagnosis of
renal masses are important tasks for radiologists.
Imaging Techniques
The small renal mass is defined as a geographic renal lesion that measures
3 cm or less in diameter [4,
13,14,15].
Small renal masses are commonly detected during imaging of the kidneys.
Although most of these masses are simple cysts with typical imaging features
[16], lesions must be
evaluated carefully because the opportunity to cure renal cell carcinoma
largely depends on making the diagnosis before the disease spreads to lymph
nodes, or before distant metastases occur
[3]. Metastatic disease is
found to coexist with approximately 30% of all cases of renal cell carcinoma
at diagnosis [17], although
renal cell carcinoma smaller than 3 cm seldom metastasizes
[14]. Therefore,
characterization (i.e., diagnosis and staging) should be considered after
detection of a small renal mass. Previous studies have shown that the
sensitivity for detecting renal masses varies with different imaging
modalities. The sensitivity is 67% for excretory urography, 79% for
sonography, and 94% for conventional nonhelical CT
[18]. Further, excretory
urography lacks sufficient specificity for accurately characterizing any renal
masses as benign [15,
19]. Therefore, every renal
mass detected with or suggested by excretory urography must be imaged with
another technique.
The most cost-effective approach is to go directly to renal sonography
[19]. With this technique 80%
of detected renal masses are characterized as simple cysts
[19], thus ending their
diagnostic evaluation. The remaining 20% of renal masses require further study
with CT or MR imaging. Any mass detected initially on sonography (Fig.
1A,1B)
or evaluated with sonography after detection with another imaging technique
that does not meet the strict sonographic criteria for a simple cyst should be
further evaluated with CT or MR imaging of the kidneys
[15]. For diagnosis of a
simple renal cyst based on sonographic findings, a renal mass must be anechoic
with a sharply defined back wall and enhancement of through sound transmission
[16]. One or two thin
septations may also be visible sonographically in simple renal cysts
[16]. Because these findings
are diagnostic, no further imaging or follow-up is needed in the evaluation of
these lesions. However, other atypical featuressonographically detected
calcifications, more than two septations, septal thickening or nodularity
(Fig.
2A,2B),
and the presence of solid componentsindicate that sonography alone will
not be adequate for complete evaluation (i.e., diagnosis and possible staging)
of these renal masses. The addition of Doppler sonography, color Doppler
sonography, power Doppler sonography
[20,21,22],
and sonographic contrast agents may further improve the detection and
characterization of renal masses. However, none of these techniques precludes
the need for CT or MR imaging of renal masses that do not meet the sonographic
criteria for diagnosis of a simple cyst. Whereas renal masses 0-5 mm in
diameter can be particularly enigmatic on CT or MR imaging, masses of this
size are rarely problematic for sonography because they are almost universally
undetectable with current sonography techniques
[13].

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Fig. 1A. Papillary renal cell carcinoma incidentally detected on sonography
in 57-year-old man examined for epigastric pain. Sagittal sonogram shows 2-cm
hyperechoic renal mass (arrows). CT or MR imaging is mandatory for
further evaluation of this mass.
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Fig. 1B. Papillary renal cell carcinoma incidentally detected on sonography
in 57-year-old man examined for epigastric pain. Contrast-enhanced CT scan
shows minimally enhancing solid right renal mass (arrow). It is
denser than fluid seen in gallbladder and there is no fat in mass.
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Fig. 2A. Bilateral cystic renal cell carcinoma in 60-year-old man with
hematuria. CT scan shows bilateral predominantly cystic renal masses. Mass in
left kidney has enhancing components (arrows) typical of renal cell
carcinoma. Mass in right kidney is bilobed and 2.5 cm in diameter but appears
to be benign.
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Fig. 2B. Bilateral cystic renal cell carcinoma in 60-year-old man with
hematuria. Sagittal sonogram of right kidney shows solid nodule
(arrow) in cyst. Patient underwent left radical nephrectomy and right
partial nephrectomy.
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Sonography is clearly superior to excretory urography for both detection
and characterization of renal masses, and CT is superior to sonography for
these tasks [18]; however, the
role of MR imaging in evaluating renal masses remains uncertain. Most studies
indicate that optimal MR imaging is comparable with optimal CT for detection,
diagnosis, and staging of renal masses
[23,24,25,26,27].
CT requires the injection of IV contrast material, which may be unacceptable
for a small percentage of patients. CT has the advantages of widespread
availability, more rapid examination time in comparison with MR imaging, and
lower cost than MR imaging. MR imaging is particularly useful for patients
with contraindications to receiving IV radiographic contrast media
[24]. For the optimal
diagnosis of renal masses, IV gadolinium is usually required for MR imaging,
but its use is not contraindicated in patients who are at high risk for side
effects from the injection of iodinated radiographic contrast material
[4]. Other possible advantages
of MR imaging traditionally include its vascular imaging capabilities and
multiplanar imaging capacity
[28,29,30]
(Fig.
3A,3B,3C,3D).
However, because helical CT provides the ability to obtain and reconstruct
thinly collimated images for CT angiography and multiplanar viewing
[31,
32] (Fig.
4A,4B,4C),
the advantages of MR imaging in these areas no longer pertain.

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Fig. 3A. One-centimeter oncocytoma in 64-year-old woman with microscopic
hematuria. CT scan of right kidney in corticomedullary phase has normal
findings. Small renal mass is undetectable in this phase.
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Fig. 3C. One-centimeter oncocytoma in 64-year-old woman with microscopic
hematuria. Axial gadolinium-enhanced MR image (fast spoiled gradient
echo;TR/TE, 150/4.2) of right kidney shows enhancement in renal mass
(arrow).
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Fig. 3D. One-centimeter oncocytoma in 64-year-old woman with microscopic
hematuria. Sagittal gadolinium-enhanced spin-echo MR image (450/9) shows mass
(arrow) and its relationship to renal sinus. This mass was resected
with partial nephrectomy.
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Fig. 4A. Bilateral renal cell carcinoma in 45-year-old man with hematuria.
Contrast-enhanced CT scan shows 5-cm left renal cell carcinoma mass and 2-cm
right renal cell carcinoma mass (arrow).
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Fig. 4B. Bilateral renal cell carcinoma in 45-year-old man with hematuria.
Coronal reconstruction image from CT arteriography phase shows segmental and
interlobar arteries (arrows) in area of right kidney harboring
mass.
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CT is the standard imaging technique for the evaluation of small renal
masses. Helical CT has led to further improvements in renal mass imaging, such
as rapid multiphase imaging of the kidneys without misregistration artifacts
attributable to patient breathing and the capability for high-quality
multiplanar (three-dimensional) postprocessing of images
[32]. With helical CT, the
kidneys can be rapidly scanned before injection of contrast material and
during different phases of opacification of the kidneys. Multidetector helical
CT scanners allow rapid scanning of the kidneys with narrow collimation that
can be completed during a single breath-hold. Helical CT may improve not only
the characterization of lesions but also their detection. Two recent studies
analyzed the detection of surgically resected small renal masses and showed
discrepancies between results obtained with helical CT and those obtained with
standard incremental CT. Incremental CT revealed 153 (75%) of 205 renal masses
identified at surgery: 47% of masses less than 5 mm in diameter, 60% of masses
5-10 mm in diameter, 75% of masses 10-15 mm in diameter, and 100% of masses 15
mm and larger [13]. Another
study in which helical CT was used to scan the kidneys during both the
corticomedullary phase and the nephrographic phase of contrast enhancement
reported better detection of small renal masses than with incremental CT; 97
(95%) of 102 renal masses 8-15 mm in diameter were detectable
[33]. All renal masses larger
than 15 mm were also detectable with helical CT
[33]. Lesions smaller than 8
mm were not resected, but 46 of 62 lesions in this small range could be
characterized as cysts with multiphase helical CT
[33].
For the optimal evaluation of a renal mass with helical CT, multiphase
imaging is required
[33,34,35].
Helical CT is also preferable because of its ability to obtain multiphase
imaging and high-quality multiplanar reconstructions
[32]. Use of thin (
5 mm)
collimation is essential for all renal imaging. The other imaging
parameterspitch, kilovoltage, and milliampere settingsshould
remain constant on all sequences for accurate measurements of renal mass
enhancement with injected contrast material
[16]. When a renal mass is
suspected, the unenhanced scan should be obtained to serve as the baseline for
measurements of enhancement on images obtained after contrast material
administration. For the accurate detection of renal masses, the nephrographic
phase (Figs.
3A,3B,3C,3D
and
5A,5B)
obtained approximately 3 min after the initiation of contrast material
injection is optimal [33]. The
corticomedullary phase should also be included not for detection or diagnosis
but for staging information
[34]. Commencing approximately
70 sec after the initiation of contrast material injection, this is the
optimal phase for evaluation of the renal vein and the other solid abdominal
viscera, including the liver, which is a common site of metastatic spread
[34].

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Fig. 5A. Renal cell carcinoma incidentally discovered in 55-year-old woman
involved in motor vehicle collision. Mass (arrow) is barely
discernible on this CT scan during corticomedullary phase of contrast
enhancement.
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Fig. 5B. Renal cell carcinoma incidentally discovered in 55-year-old woman
involved in motor vehicle collision. Mass measuring 1.5 cm (arrow) is
more conspicuous on this CT scan obtained 2 min after A during
nephrogram phase.
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An arterial phase image is useful if CT arteriograms will be constructed.
Imaging for CT arteriography should commence 20-25 sec after the initiation of
contrast material injection
[31,
36]. The contrast material
should be injected at a rate of 4 mL/sec, and collimation for CT angiography
should be 1-2 mm [31,
36]. Therefore, when a
suspected renal mass is being evaluated with CT, 5-mm collimation through the
entirety of the kidneys should be used for the unenhanced scanning. For CT
arteriography, rapid injection of contrast material followed by
thin-collimation scanning from the level of the superior mesenteric artery to
the iliac artery bifurcation is preferable. This should be followed by
scanning of the entire abdomen, commencing 70 sec after initiating the
contrast material injection. Five-millimeter collimation is used through the
section containing the kidneys. Nephrographic images using 5-mm collimation
can be obtained 2 min later, but only the kidneys need to be scanned during
this segment. Finally, excretory urograms of the kidneys may be helpful for
surgical planning when partial nephrectomy is a consideration
[35]. Scanning during this
phase provides information about the relationship of the renal tumor to the
pelvicaliceal system [35]
(Fig.
6A,6B).
After scanning, postprocessing of the CT angiograms (Fig.
4A,4B,4C)
in multiple planes can be performed. Attenuation values of any detected renal
mass should be obtained by using region-of-interest measurements on the
unenhanced image and the nephrographic phase images. If a high-attenuation
(>30 H) renal mass is detected on CT when only a contrast-enhanced study
was performed, mass enhancement cannot be assessed by comparison with an
unenhanced study. In this situation, CT scans of the mass can be obtained in
15 min. If the attenuation of the mass on the delayed images decreases by 10 H
or more, the mass is likely a neoplasm
[37].

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Fig. 6B. Renal cell carcinoma in 68-year-old man with solitary kidney.
Coronal reconstruction of CT scan obtained during excretory phase illustrates
how mass (arrows) abuts pelvicaliceal system. Mass was resected by
partial nephrectomy.
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Diagnosis of Small Renal Masses
Again, excretory urography reveals only 67% of renal masses 3 cm or
smaller. Further, 76% of detectable renal masses can be correctly
characterized as simple cysts or as solid masses with excretory urography.
This level of detection and accuracy is inadequate and indicates that an
excretory urogram with normal findings does not exclude a small renal mass and
that any renal mass detected with excretory urography requires further imaging
for a conclusive diagnosis. Most renal masses detected with excretory
urography are simple cysts and, after sonography to evaluate these renal
masses, the workup can be terminated. All masses that are not simple cysts
require further evaluation with CT or MR imaging. Sonography is less accurate
than CT for revealing small renal masses
[13]. Of 205 renal masses
detected on CT in a recent study, 79 surgically confirmed masses were
undetectable on sonography
[13]. Only one mass was
detectable on sonography but not on CT
[13]. Therefore, normal
findings on renal sonography do not exclude a small renal mass that might be
detectable on CT.
Nearly all renal masses can be detected on CT. However, one consequence of
increased detection of small renal masses is a decrease in the accuracy of
diagnosis of the detected masses. Approximately 15% of all renal masses
detected on CT are benign
[38]. The remaining 85% are
malignant renal tumors, and almost all of these are renal cell carcinoma
[38]. However, the proportion
of benign tumors increases as the detection of small renal masses increases
[33,
39,
40]. In one recent study, 18%
of renal masses 4 cm in diameter or smaller were benign oncocytomas, and,
overall, 22% of these small renal masses were benign
[40]. Most solid benign renal
tumors are oncocytomas. Although benign, these tumors are generally resected
because of their gradual enlargement and the risk of coexisting renal cell
carcinoma [41,
42]. Despite decreased
accuracy for determining whether small renal masses are benign or malignant,
CT is the most accurate diagnostic tool for use before resection. CT is more
accurate than fine-needle aspiration with cytology or histologic evaluation of
core biopsy samples taken from small renal masses.
Small renal masses should be categorized as either solid or cystic.
Attenuation of solid renal masses will be greater than 20 H or less than -20 H
on unenhanced CT scans [43].
Attenuation of small renal masses should be measured on thin-collimation
scans, for which partial-volume averaging is minimal or nonexistent. In
general, collimation should be no greater than half the diameter of the
detected renal mass [15].
Thinner collimation can help in the diagnosis of a small renal mass when
results with standard collimation are equivocal
[15].
Solid Masses
With solid small renal masses, CT or MR imaging can be diagnostic if the
renal mass contains fat. Detectable fat in a small renal mass is diagnostic of
angiomyolipoma [44,
45] (Fig.
7A,7B).
Because angiomyolipoma is a benign mass with a low risk of hemorrhage when
smaller than 4 cm, it is generally not treated surgically
[46,
47]. Therefore, the diagnosis
of this lesion is of great significance. Although the imaging appearance of
angiomyolipomas on sonography overlaps substantially with that of some small
renal cell carcinomas [48],
the CT features are definitive in more than 90% of angiomyolipomas
[44,
45]. Region-of-interest
measurements less than -20 H in a renal mass are diagnostic of angiomyolipoma
in nearly every case [44,
45].

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Fig. 7A. Pixel mapping of angiomyolipoma containing fat adjacent to
angiomyolipoma without detectable fat in 48-year-old woman with abdominal
pain. Two solid masses are visible on this CT scan of left kidney.
Five-millimeter mass surrounded by square cursor is fat density.
Three-centimeter mass (arrow) is solid without detectable fat.
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Fig. 7B. Pixel mapping of angiomyolipoma containing fat adjacent to
angiomyolipoma without detectable fat in 48-year-old woman with abdominal
pain. Portion of pixel map (showing Hounsfield unit values) of smaller mass
from A confirms fat in this mass. Both masses were resected and found
to be angiomyolipomas.
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When a mass appears to contain fat or is equivocal but the attenuation
measurement fails to confirm fat, further analysis is warranted. Thinner
collimation and pixel mapping of a region of interest can help show a small
focus of fat in a renal tumor
[45,
49]; otherwise, values of the
fat would be averaged with those of surrounding nonfat solid components. With
pixel mapping (Fig.
7A,7B),
three contiguous measurements of fat density less than -20 H are diagnostic of
fat and thus of an angiomyolipoma
[49]. Approximately 5% of
angiomyolipomas contain no demonstrable fat on CT or MR imaging
[16] (Fig.
7A,7B).
These masses are therefore indistinguishable from small renal cell
carcinomas.
Further imaging will not contribute to distinguishing these two tumors.
With other solid renal masses, features are rarely diagnostic. Although some
imaging features suggest benign renal masses, these are rarely evident with
small renal masses [50]. A
central stellate scar can be seen in oncocytomas, and renal pelvis herniation
of a multilocular cystic mass strongly suggests multilocular cystic nephroma
[16]. However, these signs are
usually seen with larger renal masses, and even then are not sufficient to
exclude renal cell carcinoma
[16].
Other features may be helpful in the diagnosis of solid renal masses. Renal
cell carcinoma originates in the renal cortex and therefore usually occurs at
the periphery or near the corticomedullary junction of the kidney.
Transitional cell carcinoma and other tumors arising from the urothelium
spread into the kidney from the renal pelvicaliceal system. These are located
more centrally in the kidney and usually displace surrounding renal sinus fat
[51,52,53]
(Fig. 8).

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Fig. 8. Transitional cell carcinoma in 68-year-old man with gross hematuria.
Contrast-enhanced CT scan shows solid 3-cm-diameter mass (arrow)
centered in left renal sinus. Mass has displaced renal sinus fat and has
invaded kidney parenchyma.
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Collecting duct carcinoma and renal medullary carcinoma arise from
Bellini's ducts or from the distal collecting ducts of the kidney
[53]. Therefore, the location
of these two types of carcinoma is more central than that of renal cell
carcinoma, and they grow in a more infiltrative pattern
[53]. Because of their
aggressive nature, these masses are usually quite large when they are
discovered [53].
All solid renal masses enhance with contrast material infusion. From the
baseline level measured on the unenhanced CT scan, enhancement of renal cell
carcinoma and other solid renal masses is usually greater than 20 H after the
injection of contrast medium as measured on the nephrographic phase CT scans
[39]. Some less vascular renal
cell carcinoma masses enhance 10-20 H
[39], and this level of
enhancement is more commonly seen with cystic renal cell carcinoma. Although
enhancement in the range of 10-20 H can be seen with some benign lesions such
as complicated cysts [39],
enhancement to this extent is sufficient to make the presumptive diagnosis of
renal cell carcinoma [14,
16].
Renal cell carcinoma, oncocytoma, and angiomyolipoma that does not contain
detectable fat all have similar characteristics on CT (Fig.
7A,7B).
They appear as solid round renal masses that enhance after contrast material
infusion [33,
54,
55]. Calcification is unusual
in small renal masses, and its presence in a solid renal mass is nonspecific
[54].
Other solid renal masses, including transitional cell carcinoma, lymphoma
(Fig. 9), and inflammatory
lesions, tend to have slightly different imaging characteristics. These masses
are not usually contour-deforming, and they have an infiltrative growth
pattern with irregular ill-defined margins and an indistinct interface with
the normal kidney [51]
(Fig. 8). Inflammatory tumors
include focal pyelonephritis, tuberculoma, and areas of malocoplakia. The
central location and origin in the renal pelvicaliceal system strongly suggest
the diagnosis of transitional cell carcinoma or other urothelial tumor. In
these situations, an excretory urogram or retrograde pyelogram can be helpful
to evaluate the pelvicaliceal system for the presence of transitional cell
carcinoma. Imaging may be augmented with endoscopic biopsy or urine cytology
to confirm the diagnosis of a urothelial neoplasm. Inflammatory tumors are
often associated with symptomatic infections or histories of recurrent urinary
tract infections. These lesions often are multifocal, suggesting a diagnosis
other than renal cell carcinoma
[51]. Again, urinalysis is
sometimes helpful to confirm the diagnosis of an inflammatory mass.

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Fig. 9. Bilateral renal lymphoma in 47-year-old man. Contrast-enhanced CT
scan shows non-contour-deforming solid renal masses in both kidneys in this
patient with non-Hodgkin's lymphoma. Masses are homogeneous, which is typical
imaging feature of renal lymphoma.
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Cystic Masses
Cystic renal masses that do not meet the imaging criteria of simple cysts
are also common. The Bosniak classification can further characterize these
lesions [4,
14,
43] and is useful even with
small renal masses [14]. A
cystic renal mass with one or two septations thinner than 3 mm with thin
peripheral or septal calcifications, a nonenhancing hyperdense cyst, and an
obviously infected renal cyst or abscess are all considered benign; therefore,
no further imaging or followup is recommended
[43]. Any cystic renal mass
containing milk of calcium is benign as well. If findings are equivocal or
suspicious, follow-up in 3-6 months with renal CT is recommended.
Alternatively, when imaging features on CT are equivocal for benign versus
surgical cystic renal masses, sonography may further show the internal
architecture of the cystic renal mass
[43] (Fig.
10A,10B,10C).
A sonographic finding of internal nodularity, or more than two septations,
advances the cystic lesion into the class III (surgical renal mass) category
[43]. Cystic renal masses with
more complex features are categorized as class III or class IV in the Bosniak
system [43]. In these cases,
malignant and benign lesions are indistinguishable, and most of these lesions
are renal cell carcinoma. Therefore, they should be considered surgical renal
masses. Features that advance cystic renal masses into the III and IV Bosniak
classifications include more than two septations, thickening of septations,
dense or irregular calcifications, solid components, and enhancement after
contrast material infusion in areas of the renal mass
[43] (Figs.
10A,10B,10C
and
11A,11B).

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Fig. 10A. CT and sonography of cystic renal cell carcinoma in 45-year-old
woman with hematuria. Contrast-enhanced CT scan shows 2.5-cm cystic mass in
left kidney. Note subtle enhancement in mass.
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Fig. 10B. CT and sonography of cystic renal cell carcinoma in 45-year-old
woman with hematuria. Sonogram shows multilocular cystic mass that is Bosniak
class IV lesion based on enhancement seen on CT.
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Fig. 11B. Cystic renal cell carcinoma in 67-year-old woman who was examined
for pancreatitis. Coronal reconstruction shows intraparenchymal extent of this
tumor (arrows) better than A. On the basis of these findings,
patient was treated with radical nephrectomy.
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Treatment Planning
When a small renal mass is detected and evaluated, staging and treatment
planning should be considered in the imaging algorithm. One advance in the
surgical treatment of small renal masses during the past decade has been
nephron-sparing surgery, or partial nephrectomy, with cure rates comparable
with those of radical nephrectomy
[56,57,58].
This treatment can be used successfully for patients with renal masses to
preserve functioning renal parenchyma. This technique is typically used when a
patient has a solitary functional kidney with a renal mass 4 cm or smaller in
a location amenable to nephron-sparing surgery and no evidence of advanced
disease [56]. The technical
success rate with nephron-sparing surgery and the long-term survival rate are
comparable to those rates obtained with conventional radical nephrectomy
[56,57,58].
After nephron-sparing surgery, the cancer-free survival rate is significantly
better in patients with tumors measuring 4 cm or smaller than in those with
large tumors [56].
Preoperative planning for nephron-sparing surgery often requires
information not routinely obtained on standard CT of the kidneys; this
includes renal arteriography and accurate depiction of the relationship
between the renal mass and the pelvicaliceal system of the kidney
[32]. Ideally, preoperative
imaging should provide information about the tumor location and depth of tumor
extension into the kidney, relationship of the tumor to the renal
pelvicaliceal system, and arterial and venous anatomy of the tumor-containing
kidney. Until recently, obtaining all this imaging information preoperatively
was difficult, if not impossible. However, reconstruction techniques make it
possible to obtain this information routinely on CT (Figs.
4A,4B,4C,
6A,6B,
11A,11B)
and MR imaging (Fig.
3A,3B,3C,3D)
[32]. In many institutions,
including mine, CT arteriography and multiplanar reconstructions have replaced
preoperative angiography and other imaging techniques for preoperative
assessment of small renal masses. Urologic surgeons find this information
helpful for treatment planning
[32]
(Fig. 8). CT arteriography is
highly accurate in the depiction of the main renal arteries, and in many
situations interlobar renal arteries can also be shown accurately
[31,
32,
36] (Fig.
4A,4B,4C).
Multiplanar reconstructions allow depiction of the anatomy in multiple planes
that are easily correlated with the surgical field viewed intraoperatively
[32].
In addition, adrenalectomy is now considered optional by many urologic
surgeons when resecting a renal mass
[59]. If the renal mass is
located well away from the adrenal gland (a common situation with small renal
masses) and the adrenal gland appears normal on a CT scan, this gland may be
left intact at the time of nephrectomy or nephron-sparing surgery
[59].
Both CT and MR imaging are extremely accurate and are the methods of choice
for preoperative staging and follow-up of patients with renal cell carcinoma,
both small and large [15,
25]. Diagnosis of venous
extension of tumor is approximately 95% accurate when thin (5-mm) collimation
is combined with rapid bolus infusion of contrast material
[60,
61]. The diagnosis of lymph
node metastases based on imaging findings is somewhat less accurate. Lymph
nodes that appear larger than 2 cm in longest dimension almost always contain
metastatic disease [62]. Those
1-2 cm in length may be hyperplastic or may contain metastatic disease and are
therefore indeterminate [62,
63]. Lymphadenectomy may be
performed at the time of surgery to establish the presence of metastatic
disease in regional lymph nodes. However, because of the lack of effective
systemic therapy, the presence or absence of lymph node disease is determined
for prognostic information only
[3]. Lymphadenectomy does not
appear to improve survival or lessen the risk of metastatic disease in
patients with renal cell carcinoma
[3]. The spread of tumor into
the venous system, size of lymph nodes, and status of the adrenal gland can be
accurately determined with either contrast-enhanced CT or MR imaging.
After surgery, CT and MR imaging are useful for surveillance and detection
of recurrent disease. Disease recurs or metastasizes in approximately 20-30%
of patients with renal cell carcinoma after surgical excision of the tumor
[64,
65]. Most metastases occur in
the lungs, and the median time until detectable recurrent disease is 15-18
months after nephrectomy [64,
65]. Other common sites of
metastatic disease include the liver and the skeletal system, both of which
are easily evaluated with CT or MR imaging
[66].
Indeterminate Masses
Despite technologic advances and increased knowledge regarding small renal
masses, many masses are detected but are diagnostically indeterminate. In
asymptomatic low-risk patients, lesions 10 mm and smaller are assumed to be
incidental simple renal cysts unless they are clearly solid
(Fig. 12), clearly enhance
with contrast material infusion (Fig.
13A,13B),
or contain coarse calcifications or fat
[14,
16,
67]. When fat is evident, the
diagnosis is angiomyolipoma (Fig.
7A,7B).
However, the other imaging features support a diagnosis of small renal cell
carcinoma.

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Fig. 12. Eight-millimeter renal cell carcinoma mass incidentally detected in
40-year-old woman with abdominal pain. Contrast-enhanced CT scan in patient
with hematuria shows cortical mass (arrowhead) that is solid on this
3-mm collimated scan. Mass was resected by partial nephrectomy.
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|

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Fig. 13B. One-centimeter renal cell carcinoma detected incidentally in
67-year-old man with prostate cancer. After contrast enhancement, small cystic
mass is visible in right kidney. Note clear enhancement (arrowhead)
in mass, which was resected by partial nephrectomy.
|
|
Management of these small tumors should be directed by the urologist.
Alternatives include surgical resection or surveillance imaging with follow-up
renal CT at 3- to 6-month intervals for at least 1 year. If the lesion is
stable, surveillance imaging can be continued at yearly intervals
[68,
69]. Evidence of lesion growth
or development of more aggressive features, such as marginal irregularity,
increases the likelihood that the mass is a small renal cell carcinoma
[69]. Surveillance imaging is
indicated only for either imaging-indeterminant tumors when the mass is likely
a cyst or small solid tumors when the patient is a very-high-risk surgical
candidate
[68,69,70].
Successful percutaneous radiofrequency ablation
[71] and laparoscopic
cryoablation [72] of small
renal masses have been reported recently. These minimally invasive techniques
present new options for treatment of small renal tumors.
This approach to the renal mass that is too small to characterize can be
used with most patients. However, in patients who have a high risk for
development of renal cell carcinoma, even these lesions should be considered
likely neoplasms [67,
68]. In particular, renal cell
carcinoma develops in 40% of patients with von Hippel-Lindau disease
(Fig. 14). Of these 40%, three
fourths have multifocal renal cell carcinoma. Although these lesions may be
solid, many appear to be simple cysts but have cellular metaplasia in the cyst
wall [67]. Some of these will
become renal cell carcinoma tumors if left unresected. Therefore, in patients
with von Hippel-Lindau disease, any renal mass is worrisome and should be
scrutinized carefully, even those that appear to be simple cysts on the basis
of imaging features. Because of the multifocal nature of renal cell carcinoma
in these patients, nephron-sparing surgery is often undergone, sometimes
repeatedly, as lesions develop during surveillance imaging. In these patients,
detection of any new renal mass may prompt surgical resection.

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Fig. 14. Renal and pancreatic masses in 37-year-old man with von
Hipple-Lindau disease. Two solid renal cell carcinoma masses (straight
arrows) are visible in left kidney along with cysts in both kidneys and
pancreas (curved arrow). Cystic masses in patients with von Hippel
Lindau sometimes contain malignant cells.
|
|
Several rare familial forms of reanl masses are characterized in young
patients by autosomal dominant inheritance of multifocal renal cell carcinoma.
Several groups of patients with familial clear cell carcinoma have a
demonstrable translocation abnormality between the short arm of chromosome 3
(3p) and either chromosome 6 or chromosome 8
[73,
74]. There is another
hereditary form of multiple papillary (chromophilic) renal cell carcinoma, but
the chromosomal abnormality has not been identified in that group
[75]. Analysis of renal masses
in these groups of patients and in patients with von Hippel-Lindau disease is
identical, with a high index of suspicion for all renal masses.
Other occurrences of renal cell carcinoma are sporadic, although the
likelihood of renal cell carcinoma increases in association with cigarette
smoking; obesity in female patients; development and treatment of
hypertension; unopposed estrogen therapy; and exposure to petroleum products,
heavy metals, or asbestos [3].
The risk of renal cell carcinoma is greater in patients with acquired cystic
disease of dialysis [76] and
may be slightly increased in patients with tuberous sclerosis
[77]. However, except in the
hereditary patterns of development of renal cell carcinoma, the disease is
sufficiently uncommon, even in those with risk factors, that the standard
imaging classification of small renal masses pertains to patients with risk
factors as well as to the general population.
Biopsy of the Small Renal Mass
Percutaneous fine-needle aspiration biopsy and core biopsy have been used
with great success for diagnosis of many extrarenal masses. The desire for
definitive tissue diagnosis for treatment planning, coupled with the lack of
diagnostic imaging features of these extrarenal masses, has made needle biopsy
a routine procedure. This is not true for renal masses. Studies have shown
repeatedly that the imaging diagnosis of a solitary renal mass is more
accurate than cytologic analysis of fine-needle aspirates or histologic
analysis of core biopsy specimens taken from those renal masses
[78,79,80,81,82].
Several studies have compared the results of analysis of preoperative biopsy
specimens to the results of analysis of resected tumors
[80,81,82].
Results of percutaneous biopsy have had sensitivities of 62-100% and
specificities ranging from 0% to 100%
[40]. A study that compared
histologic analysis of intraoperatively obtained core needle biopsy specimens
with the definitive histologic evaluation of the surgical specimen showed a
sensitivity of 81% and a specificity of only 67% for biopsy
[40]. The biopsy results had a
positive predictive value of 95% and a negative predictive value of 71% for
malignancy [40]. A
false-negative rate of 20% and a false-positive rate of 34% were seen with
histologic evaluation of these core biopsy specimens
[40].
Previous studies have shown that core biopsies are not better than
fine-needle aspirates for evaluation of masses
[81]. Several investigators
have studied fine-needle aspirates of renal masses and found them unreliable
for diagnosis of solid renal masses or complex cystic renal masses
[40,
78,79,80,81,82].
Treatment decision making should not be based on needle biopsy results
because of the significant rate of both false-negative and false-positive
biopsy results for renal masses
[40,
78,79,80,81,82,83].
Malignant renal masses may be inaccurately diagnosed as benign and benign
masses as malignant if needle biopsy results are considered definitive.
Further, several studies have shown that needle-track seeding, although rare,
does occur [78,
83]. Minor bleeding induced by
needle biopsy may compromise the performance of partial nephrectomy
[78]. Pneumothorax and
biopsy-induced hemorrhage are recognized complications of this procedure
[78].
These studies clearly indicate that the imaging features of renal masses
are more reliable for accurate diagnosis than are cytologic or histologic
findings from needle biopsy specimens. Imaging-guided biopsy of a solitary
renal mass is generally accepted only for patients with a known primary
malignancy elsewhere [16,
82]. When the presence of a
metastasis would change the treatment approach to this solitary renal mass,
imaging-guided needle biopsy is recommended. For instance, a renal mass
develops in approximately 5% of patients with non-Hodgkin's lymphoma
[51]. On occasion, a solitary
renal lymphoma may be indistinguishable from a renal cell carcinoma but will
be suspected for metastasis because of the patient's known primary malignancy.
Biopsy of this mass may be useful to divert treatment away from surgery if a
diagnosis of lymphoma can be made on the basis of biopsy analysis. One recent
study showed that a solitary renal mass in a patient with a known nonrenal
malignancy was more likely to be a metastasis than a renal cell carcinoma
[82]. If a definitive
diagnosis can be made with imaging-guided needle biopsy, treatment may be
affected substantially by this minimally invasive technique.
Conclusion
Cross-sectional imaging has had a major positive impact on the survival of
patients with renal cell carcinoma because small renal cell carcinoma is
frequently detected before it becomes symptomatic and before the disease
progresses to an advanced stage. CT and MR imaging are nearly ideal techniques
for the detection, diagnosis, staging, and preoperative evaluation of small
renal masses. CT is generally the preferred technique, but it must be used
according to rigorous guidelines. Three-phase CT is essential and includes
unenhanced thin-collimation CT followed by CT during the corticomedullary
phase for staging information and CT during the nephrographic phase of
contrast enhancement for lesion detection and characterization. These images
can be augmented with CT arteriography and multiplanar reconstructions for
surgical planning.
MR imaging is equivalent to CT for renal mass evaluation, but its drawbacks
are longer examination time, higher charge, inability to scan some patients
with claustrophobia or unusual body habitus, and lack of established objective
enhancement guidelines for diagnosis of malignant renal masses. MR imaging is
useful for patients who cannot tolerate IV radiographic contrast material and,
on occasion, for additional staging information. Sonography is particularly
useful for the evaluation of complex cystic renal masses that are equivocal
for Bosniak class II on the basis of CT criteria. Imaging-guided biopsy of a
solitary renal mass is rarely indicated, and the accuracy of the imaging
diagnosis of renal masses exceeds that of cytology or histology evaluation of
needle biopsy specimens.
The detection and diagnosis of small renal masses can be challenging.
However, the radiologist's interpretation of the findings associated with
small renal masses can have a substantial impact. Tangible advances in patient
care have resulted from the radiologic diagnosis of small renal
carcinomas.
Acknowledgments
I thank Vickie Brinkley and Donna Garrison for their help in preparing this
article.
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