Most renal lesions can be characterized accurately as benign or malignant on computed tomography (CT). Nevertheless, a small percentage of lesions, either cystic or solid, do not fall precisely into either of these categories and are labeled as “indeterminate.” The authors review the CT features of these lesions and propose a strategy for the management of these patients.
is an MRI Fellow and Professor and
is a Professor of Diagnostic Imaging in the Radiology Department
at St. Bartholomew's Hospital, West Smithfield, London, UK.
The vast majority of renal lesions can be characterized
correctly on computed tomography (CT) into simple cysts, cystic
neoplasms, or solid renal cell carcinoma (RCC). However, in about
7% of cases,
CT will fail to establish whether the lesion represents a simple
cyst or a cystic neoplasm, and this lesion will then be classified
as indeterminate. The vast majority of solid space-occupying
lesions of the kidney will prove to be RCC. However, on occasion,
some features within this solid mass lesion suggest that the solid
mass lesion represents some pathology other than RCC.
Increasingly, small lesions (<1.5 cm) are detected
incidentally in patients undergoing routine abdominal CT.
Characterization and management of these small lesions is
difficult, and these space-occupying lesions can often be regarded
Thus, three categories of indeterminate lesions will be
considered: indeterminate cystic lesions; solid lesions with a
feature that suggests pathology other than RCC; and small lesions
that are difficult to characterize.
Precontrast CT is essential to identify calculi, calcification,
and fat within the lesion. The pitch, kV, and mAs should remain
constant between pre- and postcontrast scans to allow accurate and
reproducible measurement of enhancement of the space-occupying
lesion. Although the kidney can be scanned biphasically, scanning
both in the corticomedullary and nephrographic phase, lesion
detection and characterization is best done in the nephrographic
In the case of small lesions, an excretory phase may be helpful in
surgical planning for nephron-sparing surgery to delineate the
relationship of the mass to the renal pelvis.
In these circumstances too, CT angiography (performed at 20 to 25
seconds following injection of contrast) may also be useful in
These two additional phases are important only in planning local
resection and not in characterization.
Measurement of the attenuation value
The measurement of attenuation values on spiral CT is slightly
less accurate than on conventional axial scans. This has been
attributed to partial volume effect secondary to flattening and
broadening of the section sensitivity profile, inherent in
continuous acquisition techniques. In addition, the decrease in mAs
necessary to reduce the heat generated in helical CT results in
noisier images, which may affect the reliability of the measurement
of the attenuation value.
Therefore, the attenuation value should be measured on thin
collimation scans to reduce partial volume effect, and this
measurement should be done in the late nephrographic phase rather
than the initial corticomedullary phase (Figure 1).
Indeterminate cystic lesion
Cystic lesions can be indeterminate in three circumstances: 1)
the criterion for a benign cyst has not been fulfilled; 2) the cyst
occurs in a condition commonly associated with cystic renal cell
carcinomas; and 3) the cystic lesion is multiloculated and
difficult to evaluate.
Criteria for a simple cyst not fulfilled
Computed to-mography attenuation of a simple cyst is that of water.
This value varies with different scanners and is influenced by the
number of pixels in a region of interest, pixel size, matrix size,
and beam-hardening artefacts.
A reading of up to 20 Hounsfield units (HU) is quite consistent
with a benign cyst. A hyperdense cyst may have values between 50
and 90 HU (Figure 2). This is usually seen in hemorrhagic cysts or
cysts containing proteinaceous contents. An ultrasound may
demonstrate appearances of a simple cyst in about 50% of these
cases and can easily differentiate cystic from solid lesions.
However, if these lesions are larger than 3 cm and ultrasound does
not resolve their nature, surgical exploration may become
Simple cysts should not enhance on postcontrast CT. There is
controversy regarding the amount of enhancement that can be
accepted as benign and the values beyond which the alarm bells
begin to sound. Although the values vary in the literature, the
policy at our institute is that a change of <10 HU between pre-
and postcontrast scan is regarded as insignificant, a change >20
HU is definite enhancement, and a change between 10 HU and 20 HU is
probable enhancement. It is important to remember that enhancement
depends not only on the intrinsic nature of the tissue but also on
the level of contrast material in the bloodstream and tissue at the
time of scanning. This, in turn, will depend on the amount of
contrast medium injected, the speed of injection and scanning
Simple cysts do not usually have septa. If these are present and
are few in number, smooth and without nodularity, the cyst is still
considered benign. Ultrasound is better than CT in detection of
these septa. Features such as septa thicker than 1 mm, associated
solid elements, or thickening close to its attachment to the cyst
wall makes a lesion very suspicious (Figure 3).
Fine curvilinear calcification in a cyst is considered benign.
About 1% to 3% of benign cysts calcify, while 15% of cases of RCC
Renal cell carcinomas have a different pattern of calcification,
which is thick, irregular, and nonperipheral (Figure 4).
The walls of a simple cyst are typically imperceptible. Any
thickening, especially if it is irregular or focal, is considered
suspicious (Figure 5). Such thickening may be seen in cystic
carcinomas, or infected or hemorrhagic cysts.
Cystic disease associated with renal cancer
In some conditions, renal cystic disease is so frequently
associated with RCC that it is difficult to characterize these
lesions with certainty. Such conditions include von Hipple-Lindau
disease (VHL) and acquired cystic disease of dialysis.
Von Hipple-Lindau disease--
Renal cell carcinomas occur in 38% to 55% of patients with VHL, but
benign cysts also occur in 85% of patients.
It is essential to note that even cysts that appear benign on CT
may contain malignant cells. Therefore, under no circumstances can
cysts, irrespective of their appearance, be entirely dismissed as
benign (Figure 6). CT is the modality of choice for follow-up.
Acquired cystic disease of dialysis (ACKD)--
Renal cell carcinomas occur in about 17% of patients (Figure 7) and
here, too, care is essential in evaluating these lesions.
Multiloculated appearance of a cyst is seen in multiloculated
cystic nephroma (MCN) and also in localized cystic disease.
Multiloculated cystic nephroma --
Multiloculated cystic nephroma is a rare benign neoplasm that is
usually unilateral, solitary, and sharply demarcated, with a
tendency to herniate into the renal pelvis.
It is not possible to definitely distinguish MCN from RCC on
radiological grounds, but extensive calcification, hemorrhagic
cystic content, solid regions, and hypervascularity favor a
diagnosis of RCC over MCN. On CT, MCN appears as multiple,
fluid-filled cysts separated by thick septa (Figure 8). Peripheral
or central calcification is present in 10% to 15% of cases, in
which the calcification may have a circular, stellate, flocculent,
or granular pattern.
Localized cystic disease--
Localized cystic disease is an entity where multiple benign cysts
are localized to one part of the kidney. This is not a forme fruste
of adult polycystic disease of the kidney, as was previously
considered to be the case.
Cystic neoplasms can be confused with localized cystic disease when
this cluster of cysts appears poorly defined from the adjacent
In practice, there is a spectrum of appearances for cystic
lesions ranging from those typical of a simple cyst to that of a
cystic neoplasm. Bosniak
has classified this spectrum into four groups, as delineated
This class includes simple renal cysts with no unusual features,
and the attenuation value of the cyst's contents is <20 HU.
These cysts do not require any further evaluation or follow-up.
These are benign lesions that are minimally complicated in that
they have some imaging finding of concern. Such findings include:
internal septa, which are thin (¾ 1 mm), smooth, and attached to
the wall without any associated thickening; fine calcification that
lies peripherally in the wall of the cyst or within a septum; a
cluster of cysts; and "hyperdense cysts" that have an attenuation
value of 60 to100 HU prior to administration of contrast medium.
The increased density of hyperdense cysts is due to a high protein
content or breakdown products of blood.
In order to evaluate these Class II cystic lesions completely,
at least 25% of their circumference should project outside the
renal contour. No follow-up is required.
This class was later added to the original Bosniak classification.
These are minimally complicated cysts that nevertheless have
"somewhat suspicious" features that do not require surgical
exploration but do require follow-up to establish stability of the
lesion. Bosniak recommends follow-up at 3-, 6-, and 12-month
intervals. This class includes cystic lesions with more calcium in
the wall or slightly more complicated appearances than can be
accepted for Class II.
These are more complicated cysts that require surgery because it is
not possible to characterize these lesions with certainty by
imaging findings alone. These lesions usually have irregular
margins, thick irregular calcification, thick or enhancing septa,
multiloculated appearance, uniform wall thickening, or small
nonenhancing areas of nodularity. Some of these lesions are benign,
such as hemorrhagic cysts, complex septated cysts, multiloculated
cysts, multilocular cystic nephroma, and densely calcified cysts.
Approximately 50% of these cysts will prove to be malignant. A cyst
that protrudes from the surface of the kidney can be encircled on
its periphery by an effaced wedge of renal parenchyma. This is the
so-called "beak" or "claw" sign and must be differentiated from the
thick wall of a cystic neoplasm.
These are clearly cystic/ necrotic malignant neoplasms and have
nonhomogenous contents, nonuniform wall thickening/enhancement, and
may also contain solid enhancing nodules. Class IV lesions need to
be staged and treated accordingly.
Although the Bosniak classification has proved to be quite
robust, there are problems with the system, and small indeterminate
lesions are difficult to categorize into one class or another. This
is particularly the case between the differentiation of Class II
(nonsurgical) and Class III (surgical) lesions. Personal experience
of the observer may play a role at this stage.
It has been shown that the larger the number of combined
noncystic features, the more likely a renal lesion will be
Indeterminate solid mass
Approximately 85% of solid masses detected on CT will prove to
These typically enhance following intravenous injection of
contrast, although approximately 20% are hypovascular.
Nevertheless, on occasion, certain features may suggest that the
solid lesion represents something other than RCC. These features
include the presence of fat, multiplicity, the pattern of
enhancement, and location.
Presence of fat
Detection of fat in a solid mass is of great importance, as its
presence can reliably make a diagnosis of angiomyo-lipoma. Fat is
not usually seen in RCC. However, there have been a few reports in
literature in which large RCC lesions have engulfed surrounding
perinephric fat or in which cases of RCC have shown osseous
These are isolated case reports and for all practical purposes,
presence of fat in a solid mass virtually excludes the diagnosis of
renal cell carcinoma. Region-of-interest measurements <20 HU in
a renal mass are diagnostic of an angiomyolipoma in nearly every
When the presence of fat is equivocal, thinner collimation and
pixel mapping of a region of interest can demonstrate a small focus
of fat in a renal tumor. Three contiguous measurements <20 HU
after pixel mapping is diagnostic of fat and thus an
Approximately 5% of angiomyolipomas contain no demonstrable fat on
CT or MRI and are then indistinguishable from small RCC.
Multiple lesions point toward an aetiology other than an RCC, as
only 5% of RCCs are multiple.
Lymphoma and metastases are more likely considerations in cases of
multiple lesions. Non-Hodgkin's lymphoma is manifested by small,
multifocal intrarenal hypo-attenuating solid masses with or without
nodal disease. Similarly, in a patient with a known history of
malignancy, metastasis to the kidneys are more common than primary
RCC. Percutaneous biopsy may be useful in these circumstances.
Pattern of enhancement
Almost all solid renal masses enhance with administration of
contrast. Enhancement of RCC and other solid masses is usually
>20 HU. Differentiation between RCC and other tumors is not
possible on the basis of enhancement characteristics.
Typically, RCC tumors are peripheral within the parenchyma,
cause an irregularity of the renal outline, and enhance
inhomogeneously following intravenous contrast medium injection.
Conversely, transitional cell carcinomas lie centrally within the
kidney, the renal outline is maintained, and the carcinomas are
either avascular or enhance only slightly after contrast
administration. These features should alert the radiologist to the
possibility that the solid mass lesion does not represent an
Indeterminate small lesions
Lesions larger than 1.5 cm are usually easily characterized by
imaging. In contrast, lesions smaller than 1.5 cm pose a challenge.
The majority of these smaller lesions represent benign cysts. If
the lesion is <1.5 cm and has an attenuation value of <20 HU
on routine CT performed for a nonrenal indication, it is considered
benign and no further imaging is required. If the lesion has an
attenuation value of <20 HU, it is considered an angiomyolipoma
and no more imaging is done. Lesions that measure >20 HUs are
considered potential neoplasms. An ultrasound is performed first to
determine whether it represents a simple benign cyst. If this is
the case, no further imaging is required. If the ultrasound is not
confirmatory, a dedicated thin-section CT with and without contrast
may be performed in an attempt to diminish partial volume effect.
If there is no definite enhancement (<10 HU) on postcontrast
scans, the lesion is considered a hyperdense cyst.
If there is enhancement, a follow-up CT at 3-, 6-, and 12-month
intervals is initially recommended. If the lesion is stable, yearly
follow-up is advised. If, on the other hand, the lesion increases
to 2 cm in diameter within 1 year, surgery is indicated.
Role of percutaneous biopsy/cytology
Percutaneous fine-needle aspiration biopsy and core biopsy are
less accurate than imaging in diagnosis of an indeterminate renal
space-occupying lesion. Percutaneous renal biopsy of a focal lesion
has been shown to have a sensitivity of 62% to 100% and specificity
ranging from 0% to 100%.
Treatment decision-making should not be based on needle biopsy
results because of significant rate of both false-negative and
The role of biopsy is limited to diagnosis of a relatively large
solitary renal mass in patients with a known primary malignancy
elsewhere and also in cases of suspected lymphoma.
As CT is used more widely, the incidental space-occupying
lesions of the kidney are detected with increasing frequency.
To categorically place these lesions into a definite benign or
malignant group is not always possible. It is essential to pay
careful attention to imaging techniques in order to evaluate each
lesion meticulously, as failure to do so may result in the
dismissal of a potentially curable malignant lesion as benign.
Conversely, excessive investigation of benign lesions will lead to
inappropriate and unnecessary discomfort for the patient.