# Radiologic Evaluation of Small Renal Masses (I): Pretreatment Management

Authors: A. Marhuenda; M. I. Martín; C. Deltoro; J. Santos; Jose Rubio Briones
Journal: Advances in Urology
 (2008)

---

## Abstract

When characterizing a small renal mass (SRM), the main question to be answered is whether the mass represents a surgical or nonsurgical lesion or, in some cases, if followup studies are a reasonable option.  Is this a task for a urologist or a radiologist? It is obvious that in the increasing clinical scenario where this decision has to be made, both specialists ought to work together. This  paper  will focus on the principles, indications, and limitations of ultrasound, CT, and MRI to characterize an SRM in 2008 with a detailed review of relevant literature. Special emphasis has been placed on aspects regarding the bidirectional information between radiologists and urologists needed to achieve the best radiological approach to an SRM.


---

## Body

## 1. Introduction

Over the last 3 decades, there has been a rising trend to define small
renal masses (SRMs)
as masses below 4 cm in diameter [1], making it the major reason for the 126%
increase in incidence of renal cell carcinoma (RCC) in the United States. The
reason for that is well known;  the increasing number of imaging examinations
performed for unrelated indications with many renal neoplasms of small size and
early stage incidentally detected. Faced with this situation, urologists do not only suggest surgery as 30
years ago, but also offer different options to deal with the problem.
Most of
their decisions are based on radiological characterization of the SRM, as
biopsies of these masses have not been completely accepted by the international
urological community.So, evidently urologists
have to ask their colleagues in the Radiology Department to improve their
explorations expecting more and more extensive radiological reports analyzing
not just the presence of the mass. The SMR analysis must be carried out by both
a radiologist and an urologist, as bidirectional information is extremely
important to define the most probable nature of the mass.The accurate
diagnosis of a renal mass depends on many factors, including the
clinical history; so there is
some clinical information that urologists have to report to their radiologists:(i)
presence of a
familial syndrome,(ii)
presence of a urological tract infectious disease previous or concomitant
to the diagnoses of the SRM,(iii)
presence of previous stone disease and related treatments,(iv)
presence of previous renal trauma,(v)
presence of kidney disease and renal insufficiency.A
high-quality imaging examination, under the control of a radiologist,
is essential. The most accurate diagnosis of a renal mass is then made
according to the nature of the imaging findings, the experience of
the radiologist, and the quality of the examination, as well as the exclusion
of conditions that can mimic a renal neoplasm. There are some key points that, due to their therapeutic decision-making
importance, radiologists need to provide in their reports:(i)
signs suspecting fat involvement in an SRM,(ii)
metabolic behavior during the different phases of CT and MRI after
contrast administration, allowing to characterize benign SRM,(iii)
the need (or not) to complete studies with different techniques,(iv)
accurate and standard (for followup in case of watchful waiting
policy) measurement of 3 diameters of the SRM,(v)
signs of active tumoral tissue after conservative treatments which do not
remove the SRM,(vi)
differential diagnosis of residual tumour with
complications after partial nephrectomies and foreign bodies used to achieve
haemostasis.Having established the
collaboration between urologist and radiologist for this review paper, the aim
of the two complementary chapters submitted for the SRM diagnoses and
characterization is to give some light on the new challenges which face
radiologists nowadays, extremely important for the SRM management.


## 2. Objectives

Renal cell carcinoma and oncocytoma are
indistinguishable from each other at imaging. Many other renal
lesions must be considered, such as angiomyolipoma (AML), lymphoma, metastatic
disease, renal anomalies, and other pseudotumors that can mimic
renal cell carcinoma. Although it is possible to make this differentiation by using the imaging findings alone, the clinical history can
often be very important in making the correct diagnosis. In fact, before making
a diagnosis of renal cell carcinoma, one should be certain that none
of these possible mimickers of renal cell carcinoma are potentially
present.Staging by TNM system can be considered a prognostic classification, and
there is evidence that the smaller the size, the better the prognosis [2, 3]
. The increasing
incidence of renal mass manifestations of tumours that are confined to the renal capsule and
relatively small in size has
stimulated a growing trend toward nephron-sparing surgical techniques, as
current data show survival rates comparable to those associated with radical
nephrectomy.Imaging findings that can affect the decision
to perform partial nephrectomy included tumor size in three planes:
tumor location within the kidney; presence of a pseudocapsule (a thin band of
fibrous tissue and compressed renal parenchyma surrounding the lesion); tumor invasion
of the renal sinus fat, collecting system, renal vein, or
perinephric fat; presence of lymphadenopathy; morphologic and
physiologic status of the contralateral kidney. All these aspects
are evaluated by means of different imaging techniques.The increased implementation of kidney-sparing surgery for renal cell
carcinoma may create an important role for diagnostic imaging in the discovery
of small synchronous carcinomas. Radiologist should be aware of the possibility
of tumor multifocality or of adrenal metastases from a high-grade small renal
tumor as well as of the association of RCC with lymphoma [4].The challenge is to detect and
delineate all lesions to ensure complete surgical excision while preserving the
maximal amount of functioning parenchyma. For patients who are not surgical
candidates, imaging staging, along with the other factors, can provide
prognostic information.


## 3. Ultrasound

The fact
that renal neoplasms have been detected earlier and with increased frequency is
well documented in the literature [5, 6].
This is probably due to two major factors. One factor is that there has been a
considerable increase in the number of people who undergo kidney imaging in the
general population because of the widespread use of ultrasound (US). The other
reason is that this imaging technique is able to depict lesions of the kidneys
that could be missed with urography [7].
This increased detection of renal neoplasms also results in the increased
detection of benign lesions and nonneoplastic masses, particularly renal cysts.
Therefore, the differentiation between a neoplastic and a nonneoplastic lesion
is a common dilemma.To
differentiate benign from malignant SRM can prove even more problematic because
the findings can also become smaller, hence requiring more detailed and more
sensitive imaging studies.Ultrasound
plays an important role in the detection and evaluation of these SRMs. While this
technique may not be as sensitive as contrast-enhanced CT or MR for revealing SRM, US
has been the initial technique in the discovery of a large number of these
incidentally discovered tumors when the kidney is studied in the course of
abdominal imaging. Sonography is very accurate in distinguishing liquid from
solid tissue. Therefore, its major use in these small lesions is to help
differentiate small cysts (see Figure1) from small solid tumors [8]. Maintaining rigid criteria is necessary to
maintain the high accuracy possible with this technique.Figure 1
Simple cyst as anechoic lesion with a sharply defined back wall
and enhancement of through sound transmission.In the
general population, renal cysts are the most common space-occupying lesions in
the kidney. With this technique, 80% of detected renal masses are characterized
as simple cysts [9] thus ending their diagnostic
evaluation. The remaining 20% of renal masses require further study with CT or
MR imaging [10]. Any mass detected that does not
meet the strict sonographic criteria for a simple cyst should be further
evaluated with CT or MR imaging of the kidneys. However, one or two thin
septations may also be visible sonographically in simple renal cysts [11]. Because these findings are diagnostic, no
further imaging or followup is needed in the evaluation of these lesions.
However, other atypical features sonographically detected calcifications; more
than two septations, septal thickening or nodularity, and the presence of solid
components indicate that sonography alone will not be adequate for complete
evaluation of these renal masses (see Figure 2). The addition of Doppler
sonography, color Doppler sonography, power Doppler sonography [12, 13], and sonographic contrast agents may further
improve the detection and characterization of renal masses. However, none of
these techniques preclude the need for CT or MR imaging of renal masses that do
not meet the sonographic criteria for diagnosis of a simple cyst.Figure 2
A cyst with nodular thickening of the wall and internal
septa.In the study
of solid renal masses, the role for US has been mainly centred on the
differentiation of RCC and AML, which are the most common malignant and benign
solid renal tumors, respectively [14–17]. When a solid mass is diagnosed, RCC or AML should be initially
considered because of the high frequency of their occurrence. At US, most AML
lesions are markedly hyperechoic relative to renal parenchyma. They may appear
less echogenic depending on the relative proportion of fat, smooth muscle,
vascular components, and haemorrhage in the lesion [18, 19]. RCC displays a broad range of echogenicities. Although
often thought of as hypoechoic or isoechoic, recent studies have shown that
most RCC are hyperechoic relative to renal parenchyma and that up to 12% simulate
AML [14–17]. Forman et al. [14] have shown that one third of small RCC are as
echogenic as a “classical” AML. An echotexture equal to that of renal sinus fat
seen in a small renal mass is, therefore, no longer considered adequate to
exclude the diagnosis of malignancy (see Figure 3).Figure 3
Well-defined hyperechoic
small renal mass. Pathologic analysis of the surgical specimen revealed a renal
cell carcinoma.Other
ultrasound signs have been used to differentiate between hyperechoic RCC and
AMLs. The presence of an anechoic rim and/or an intratumoral cyst is only seen
in RCC (see Figure4). The presence of acoustic shadow is specific of AML.
However, the detectability of these findings varies [15, 16], their diagnostic value has not been
established, and the presence of these features is not sufficient to
differentiate RCC from the other solid renal masses that are incidentally
detected on gray-scale US.
On the power Doppler US, the analysis of the vascular distribution has not
increased the diagnostic accuracy for small renal tumors [12]. Contrast-enhanced Doppler US can increase the
detection of intratumoral vascularity compared to color/power Doppler US [20]. However, their signal intensity has not been
found to be sufficiently intense for tumor characterization. Recently, the
development of contrast-enhanced harmonic US imaging has provided a better
assessment of the diagnostic accuracy of RCC as compared with gray-scale US by
allowing better visualization of the intratumoral anechoic areas and the
pseudocapsule than can the gray-scale US [21],
but there still exists an overlapping of signs of RCC and the other solid renal
masses, making it necessary to use CT or MR imaging in the study of small renal
masses.Figure 4
Well-defined hyperechoic
small renal mass with hypoechoic rim and intratumoral cystic area, confirmed
with pathologic analysis as renal cell carcinoma.


## 4. CT

Helical CT  is generally accepted as the critical imaging test for the classification of renal masses. Radiation exposure is the greatest disadvantage of this technique. MRI is comparable to helical CT for detection, diagnosis, and
staging of renal masses. However, CT has the advantages of widespread availability, shorter examination time, and lower cost in comparison with MRI.A detailed analysis of a variety of CT features
is required, including the size, location, appearance on unenhanced scan, the
presence and location of calcifications, the presence and size of a cyst wall
or septations, and the amount and pattern of contrast enhancement [7, 22, 23].


### 4.1. CT Technique

Single detector and especially multidetector
spiral (MDCT) have refined the diagnostic evaluation of renal pathologic conditions.
Compared with single-detector helical CT, MDCT allows the kidneys to be scanned
with a collimation of less than 5 mm during a single breath hold [24]. From a single data set obtained with thin collimation, both thin and
thick sections can be reconstructed and no additional radiation exposure is
required to obtain the thin sections. This dataset is manipulated by using a
workstation to produce volume-rendered and three-dimensional (3D) images when
necessary. The 3DCT images can be viewed in multiple planes and orientations to
define the lesion.A triphasic imaging protocol consists of an unenhanced
phase through the kidneys, an arterial or corticomedullary phase through the
liver and kidneys (between 25 and 70 seconds after the start of injection of
contrast), and a portal venous or nephrographic phase of the entire abdomen
(between 80 and 180 seconds). Excretory phase (>180  seconds) is occasionally
helpful.An initial series of unenhanced scans provides a
baseline from which to measure the enhancement within the lesion.The corticomedullary phase is useful to perform 3D
reconstructions and to depict the renal vasculature. Furthermore, this
phase is considered essential for staging.The nephrographic phase provides greater lesion detection and
improved lesion characterization of renal masses than corticomedullary phase [25, 26]. However, a case of renal cell carcinoma
visible only during the corticomedullary phase has been shown in the literature [27]. The excretory phase is occasionally helpful to
better delineate the relationship of a centrally located mass within the
collecting system. Delayed scanning (15 minutes) can also be used in lieu of
unenhanced scanning to characterize an incidental renal lesion detected on a
routine contrast-enhanced CT scan [28].At present, there is no worldwide agreement
upon the specific number that can be used as definitive and
unequivocal evidence of enhancement within a renal mass, and it has been
proposed by many authors that the previously used threshold of 10 HU should be
increased to 20 HU (a currently accepted criterion) (see Figure5). Some authors
think that a renal mass that enhances 10–20 HU is
indeterminate and needs further evaluation [29].Hemorrhagic cyst. (a) Unenhanced
CT scan shows a hyperattenuating small renal mass (62 H) (arrow).  (b)
Contrast-enhanced CT scan during the nephrographic phase reveals light
enhancing (attenuation value increased 9 H:71 H)  (arrow).
(a)(b)


### 4.2. Imaging of Specific Small Renal Masses

#### 4.2.1. Cysts

Most small renal masses incidentally discovered
on CT are simple cortical cysts that need no further evaluation. The Bosniak
classification system is used to asses the likelihood of malignancy in cystic renal masses on
the basis of lesion complexity in CT imaging [30, 31]. Application of the Bosniak classification can be
difficult in the case of an indeterminate renal lesion, especially if it is
small. Bosniak admits that distinction between category II (not requiring
surgery) and category III (requiring surgery) can be very difficult [32].


#### 4.2.2. Angiomyolipoma

It is typically
a solid lesion that exhibits fat density on CT scans (−10 to –100 HU) (see Figure6). However, in some cases, it may contain very small quantities of fat that
can be overlooked. Angiomyolipomas rarely contain calcification, and,
therefore, a diagnosis of angiomyolipoma should not be made if a lesion
contains fat and calcium [33, 34]. There
have been few case reports of fat from RCC that also contain calcification. In
such cases, a renal cell carcinoma must be considered the most likely diagnosis.Figure 6
Angiomyolipoma. Contrast-enhanced
CT shows a small homogeneous fat-containing mass.


#### 4.2.3. Oncocytoma

It is usually a
hypodense mass, homogeneous, with smooth contours and a tendency to enhance
avidly (see Figure7). Until
now, an oncocytoma was suggested on postcontrast CT by the presence of a
central hypoenhancing scar.
Because of its lack of
specificity, patient management has been unaffected by the presence of this
finding. Renal cell carcinoma
and oncocytoma are indistinguishable from each other at imaging.Figure 7
Oncocytoma. Small mass
isoattenuated to renal parenchyma after contrast.


#### 4.2.4. Renal Cell Carcinoma

The
imaging characteristics of RCC are extremely varied, with masses ranging from
cystic to solid, from homogeneous to heterogeneous and necrotic, from small to
large, and from localized to extensive. The typical CT appearance of small RCC
is a homogeneously isodense/hypodense mass, noncalcified, with an attenuation value of 20 HU or more, that enhance
avidly and early with contrast medium (see Figure8) [23]. The early-stage contrast enhancement is believed to be caused by tumor
angiogenesis. However, a small proportion of RCC are hypovascular, and the
amount of enhancement may be minimal. Small RCC with a predominantly cystic
growth pattern, necrosis, or calcifications (peripheral curvilinear or punctate
central) are uncommon. Areas of fat attenuation can be
present within renal cell carcinomas, but are uncommon in small tumors.Small hyper vascular renal cell carcinoma. (a) Contrast-enhanced CT shows
small renal mass that enhances early in corticomedullary phase. (b) Rapid washout in nephrographic phase.
(a)(b)Furthermore, a small RCC can be
hyperattenuating. If the lesion is depicted only on enhanced CT, delayed
scanning can also be used. Macari and Bosniak [28] have suggested that measurement of
the washout of contrast material from at least 15 minutes allows
differentiation between hyperdense cyst and renal neoplasms. The washout of 15 HU or more indicates that, excluding vascular abnormality, the mass is solid. A
lack of washout indicates that the mass is probably a hyperattenuating cyst.On the other hand, renal cortical tumors are
family neoplasms with distinct cytogenetic and molecular characteristics and
varying malignant potential. In
the 1997 Heidelberg classification, renal cell carcinoma was subdivided in
subtypes [35]. It has been suggested that certain imaging
features may be associated with different subtypes of solid renal cortical
tumors [36, 37]. The most consistent finding in these studies was that the degree of
enhancement was the most valuable parameter for differentiation of RCC
subtypes, as clear cell RCC enhance to a greater degree than other subtypes,
especially papillary RCC. Clear
cell RCC is also strongly associated with a mixed enhancement pattern of both
enhancing soft-tissue components and low-attenuation areas (necrotic or cystic
changes). When
homogeneous or peripheral enhancement is present, clear cell RCC is a less
likely diagnosis, and other cell types should be considered. Notably, a
majority of papillary tumours were either homogeneous or peripheral enhancement
(see Figure 9). The presence to
neovascularity was mildly associated with more aggressive tumor. The clear cell
RCC (and oncocytomas) enhanced avidly during the parenchymal phase; the
chromophobe RCC (and lipid poor angiomyolipoma) enhanced moderately and
papillary RCC enhance mildly.Figure 9
Papillary renal cell carcinoma.  Contrast-enhanced CT shows small homogeneous
mass that is mild and less enhanced than renal parenchyma does.


#### 4.2.5. Non-Hodgkin’s Lymphoma

Lymphoma
can have a variable appearance and may on occasion resemble renal cell
carcinoma. Most frequently, it
manifests as bilateral solid renal masses, and in a patient with systemic
lymphoma, the proper diagnosis is not difficult. Characteristically, lymphoma
often infiltrates into the kidney via the renal sinus or surrounds the kidney. In a patient with known systemic
lymphoma to whom a renal mass with imaging is detected, systemic treatment for
lymphoma should be instituted. If the patient’s systemic disease responds, and
the renal mass does not respond, biopsy of the mass is indicated. However,
lymphoma may rarely manifest as a solitary renal mass or a homogeneous infiltrating
renal mass. In this case, biopsy of the mass is indicated previous to systemic
therapy.


#### 4.2.6. Metastases

Metastatic disease to the kidney
typically manifests as multiple bilateral renal masses, often associated with
metastatic disease to other organs. They are often poorly defined and
infiltrate the renal parenchyma. With the appropriate clinical history, the
diagnosis is straightforward. However, in a patient with a solitary renal mass
(especially an infiltrating mass) and history of previous malignancy,
percutaneous renal biopsy is indicated for a definitive diagnosis.


#### 4.2.7. Benign Mesenchymal Tumors

Included leiomyomas, lipomas,
fibromas, and mixed mesenchymal nodules. They are usually small (<1 cm)
lesions, found in autopsies.


#### 4.2.8. Pseudotumors

They
include congenital anomalies (prominent renal columns of Bertin, renal
dimorphisms, and dromedary humps) and acquired pseudotumors (hypertrophied
normal renal parenchyma). This condition enhances identically to the normal
renal parenchyma. In these situations it proves appropriate to scan during the
corticomedullary and nephrographic phase.


#### 4.2.9. Renal Mass Mimickers

These include inflammatory masses (including focal
pyelonephritis, renal abscess) and hematoma. A careful evaluation with
high-quality CT or MR examination combined with the clinical context of the
case and a familiarity with this group of “lesions” should reveal its true
nature. Most hematomas are perinephric and are surrounded by fat stranding.
They may occasionally have a masslike appearance, and some may not be
discovered until long time after the traumatism. Chronic hematomas can have
calcifications and do not enhance.


### 4.3. Diagnosis and Management of Renal Call Carcinoma with MD-CT

CT remains the most widely available and single most effective modality for staging renal call carcinoma  [38, 39]. 3DCT
combined with CT angiography has the potential to provide all the critical
information needed to plan the surgical procedure.  3DCT images can be viewed in multiples planes
and orientations to define the tumor and its relationship to the renal surface,
the collecting system, and adjacent organs. A 3DCT angiogram can be created to
delineate the renal arterial and venous anatomy.The anatomic extent of the tumor at the time of
diagnosis is the single most important factor in determining prognosis (see Table1)  [38].Table 1
Staging system for renal carcinoma and CT criteria.
Tumour positionRobsonTNMCT findingsConfined withinISoft-tissue mass
enhancesrenal capsuleless than normal renalSmall (<7 cm)T1parenchyma; centralLarge (≥7 cm)T2necrosis in large RCC.IIT3aPerinephric stranding;Spread to
perinephric fatPerinephric collateral
vessels;Soft-tissue mass in
perinephric spaceVenous
thrombusIII ARenal vein onlyT3bLow-attenuation
filling defect vein;IVC infradiaphragmaticT3cDirect continuity of
thrombus with primary mass;IVC supradiaphragmaticT4bEnhanced thrombusRegional
lymphIII BN1-N3Lymph nodes 1 cm in
diameter or largernode metastasesDirect invasion of adjacent organsIV AT4aObliteration of normal
soft-tissues planes between tumor and adjacent organsDistant
metastasesM1Metastases enhance with
IV contrast material;Hepatic metastases
best in arterial phaseIV: intravenous, IVC: inferior vena cava.Most
urological surgeons continue to refer to Robson’s classification, which is
essentially a surgical staging approach. This system includes the important
staging variables that have survived scrutiny over the years. Confinement
within the renal capsule, penetration into the perirenal fat, invasion into the
renal vein, and lymph node metastases are all important in determining the
prognosis.Under and overstating
of perinephric invasion are the most common staging errors at CT  [40]. The most specific finding of stage T3a, the presence of an enhanced
nodule in the perinephric space, is highly specific but also low sensitive. The
differentiation between stage T2 and T3a tumors is very problematic.If tumoral spread
within the IVC is identified, precise delineation of the superior extent of the
thrombus is essential for the surgeon to plan the optimal surgical strategy for
thrombectomy and minimize the risk of embolism. The level of involvement of the
IVC dictates the surgical approach. Involvement of the IVC is best shown during
corticomedullary phase. Because of its multiplanar capability, magnetic
resonance imaging is the preferred modality to image. However, the
three-dimensional CT with sagittal and coronal reconstructions is also
effective in depicting the superior extent of inferior vena cava thrombus (see Figure10), with the advantages of widespread availability, shorter
examination time, and lower cost in comparison with MR  [40].Renal cell carcinoma. (a) Three-dimensional
CT with coronal reconstruction, (b) sagittal oblique reconstruction.
(a)(b)


## 4.1. CT Technique

Single detector and especially multidetector
spiral (MDCT) have refined the diagnostic evaluation of renal pathologic conditions.
Compared with single-detector helical CT, MDCT allows the kidneys to be scanned
with a collimation of less than 5 mm during a single breath hold [24]. From a single data set obtained with thin collimation, both thin and
thick sections can be reconstructed and no additional radiation exposure is
required to obtain the thin sections. This dataset is manipulated by using a
workstation to produce volume-rendered and three-dimensional (3D) images when
necessary. The 3DCT images can be viewed in multiple planes and orientations to
define the lesion.A triphasic imaging protocol consists of an unenhanced
phase through the kidneys, an arterial or corticomedullary phase through the
liver and kidneys (between 25 and 70 seconds after the start of injection of
contrast), and a portal venous or nephrographic phase of the entire abdomen
(between 80 and 180 seconds). Excretory phase (>180  seconds) is occasionally
helpful.An initial series of unenhanced scans provides a
baseline from which to measure the enhancement within the lesion.The corticomedullary phase is useful to perform 3D
reconstructions and to depict the renal vasculature. Furthermore, this
phase is considered essential for staging.The nephrographic phase provides greater lesion detection and
improved lesion characterization of renal masses than corticomedullary phase [25, 26]. However, a case of renal cell carcinoma
visible only during the corticomedullary phase has been shown in the literature [27]. The excretory phase is occasionally helpful to
better delineate the relationship of a centrally located mass within the
collecting system. Delayed scanning (15 minutes) can also be used in lieu of
unenhanced scanning to characterize an incidental renal lesion detected on a
routine contrast-enhanced CT scan [28].At present, there is no worldwide agreement
upon the specific number that can be used as definitive and
unequivocal evidence of enhancement within a renal mass, and it has been
proposed by many authors that the previously used threshold of 10 HU should be
increased to 20 HU (a currently accepted criterion) (see Figure5). Some authors
think that a renal mass that enhances 10–20 HU is
indeterminate and needs further evaluation [29].Hemorrhagic cyst. (a) Unenhanced
CT scan shows a hyperattenuating small renal mass (62 H) (arrow).  (b)
Contrast-enhanced CT scan during the nephrographic phase reveals light
enhancing (attenuation value increased 9 H:71 H)  (arrow).
(a)(b)


## 4.2. Imaging of Specific Small Renal Masses

### 4.2.1. Cysts

Most small renal masses incidentally discovered
on CT are simple cortical cysts that need no further evaluation. The Bosniak
classification system is used to asses the likelihood of malignancy in cystic renal masses on
the basis of lesion complexity in CT imaging [30, 31]. Application of the Bosniak classification can be
difficult in the case of an indeterminate renal lesion, especially if it is
small. Bosniak admits that distinction between category II (not requiring
surgery) and category III (requiring surgery) can be very difficult [32].


### 4.2.2. Angiomyolipoma

It is typically
a solid lesion that exhibits fat density on CT scans (−10 to –100 HU) (see Figure6). However, in some cases, it may contain very small quantities of fat that
can be overlooked. Angiomyolipomas rarely contain calcification, and,
therefore, a diagnosis of angiomyolipoma should not be made if a lesion
contains fat and calcium [33, 34]. There
have been few case reports of fat from RCC that also contain calcification. In
such cases, a renal cell carcinoma must be considered the most likely diagnosis.Figure 6
Angiomyolipoma. Contrast-enhanced
CT shows a small homogeneous fat-containing mass.


### 4.2.3. Oncocytoma

It is usually a
hypodense mass, homogeneous, with smooth contours and a tendency to enhance
avidly (see Figure7). Until
now, an oncocytoma was suggested on postcontrast CT by the presence of a
central hypoenhancing scar.
Because of its lack of
specificity, patient management has been unaffected by the presence of this
finding. Renal cell carcinoma
and oncocytoma are indistinguishable from each other at imaging.Figure 7
Oncocytoma. Small mass
isoattenuated to renal parenchyma after contrast.


### 4.2.4. Renal Cell Carcinoma

The
imaging characteristics of RCC are extremely varied, with masses ranging from
cystic to solid, from homogeneous to heterogeneous and necrotic, from small to
large, and from localized to extensive. The typical CT appearance of small RCC
is a homogeneously isodense/hypodense mass, noncalcified, with an attenuation value of 20 HU or more, that enhance
avidly and early with contrast medium (see Figure8) [23]. The early-stage contrast enhancement is believed to be caused by tumor
angiogenesis. However, a small proportion of RCC are hypovascular, and the
amount of enhancement may be minimal. Small RCC with a predominantly cystic
growth pattern, necrosis, or calcifications (peripheral curvilinear or punctate
central) are uncommon. Areas of fat attenuation can be
present within renal cell carcinomas, but are uncommon in small tumors.Small hyper vascular renal cell carcinoma. (a) Contrast-enhanced CT shows
small renal mass that enhances early in corticomedullary phase. (b) Rapid washout in nephrographic phase.
(a)(b)Furthermore, a small RCC can be
hyperattenuating. If the lesion is depicted only on enhanced CT, delayed
scanning can also be used. Macari and Bosniak [28] have suggested that measurement of
the washout of contrast material from at least 15 minutes allows
differentiation between hyperdense cyst and renal neoplasms. The washout of 15 HU or more indicates that, excluding vascular abnormality, the mass is solid. A
lack of washout indicates that the mass is probably a hyperattenuating cyst.On the other hand, renal cortical tumors are
family neoplasms with distinct cytogenetic and molecular characteristics and
varying malignant potential. In
the 1997 Heidelberg classification, renal cell carcinoma was subdivided in
subtypes [35]. It has been suggested that certain imaging
features may be associated with different subtypes of solid renal cortical
tumors [36, 37]. The most consistent finding in these studies was that the degree of
enhancement was the most valuable parameter for differentiation of RCC
subtypes, as clear cell RCC enhance to a greater degree than other subtypes,
especially papillary RCC. Clear
cell RCC is also strongly associated with a mixed enhancement pattern of both
enhancing soft-tissue components and low-attenuation areas (necrotic or cystic
changes). When
homogeneous or peripheral enhancement is present, clear cell RCC is a less
likely diagnosis, and other cell types should be considered. Notably, a
majority of papillary tumours were either homogeneous or peripheral enhancement
(see Figure 9). The presence to
neovascularity was mildly associated with more aggressive tumor. The clear cell
RCC (and oncocytomas) enhanced avidly during the parenchymal phase; the
chromophobe RCC (and lipid poor angiomyolipoma) enhanced moderately and
papillary RCC enhance mildly.Figure 9
Papillary renal cell carcinoma.  Contrast-enhanced CT shows small homogeneous
mass that is mild and less enhanced than renal parenchyma does.


### 4.2.5. Non-Hodgkin’s Lymphoma

Lymphoma
can have a variable appearance and may on occasion resemble renal cell
carcinoma. Most frequently, it
manifests as bilateral solid renal masses, and in a patient with systemic
lymphoma, the proper diagnosis is not difficult. Characteristically, lymphoma
often infiltrates into the kidney via the renal sinus or surrounds the kidney. In a patient with known systemic
lymphoma to whom a renal mass with imaging is detected, systemic treatment for
lymphoma should be instituted. If the patient’s systemic disease responds, and
the renal mass does not respond, biopsy of the mass is indicated. However,
lymphoma may rarely manifest as a solitary renal mass or a homogeneous infiltrating
renal mass. In this case, biopsy of the mass is indicated previous to systemic
therapy.


### 4.2.6. Metastases

Metastatic disease to the kidney
typically manifests as multiple bilateral renal masses, often associated with
metastatic disease to other organs. They are often poorly defined and
infiltrate the renal parenchyma. With the appropriate clinical history, the
diagnosis is straightforward. However, in a patient with a solitary renal mass
(especially an infiltrating mass) and history of previous malignancy,
percutaneous renal biopsy is indicated for a definitive diagnosis.


### 4.2.7. Benign Mesenchymal Tumors

Included leiomyomas, lipomas,
fibromas, and mixed mesenchymal nodules. They are usually small (<1 cm)
lesions, found in autopsies.


### 4.2.8. Pseudotumors

They
include congenital anomalies (prominent renal columns of Bertin, renal
dimorphisms, and dromedary humps) and acquired pseudotumors (hypertrophied
normal renal parenchyma). This condition enhances identically to the normal
renal parenchyma. In these situations it proves appropriate to scan during the
corticomedullary and nephrographic phase.


### 4.2.9. Renal Mass Mimickers

These include inflammatory masses (including focal
pyelonephritis, renal abscess) and hematoma. A careful evaluation with
high-quality CT or MR examination combined with the clinical context of the
case and a familiarity with this group of “lesions” should reveal its true
nature. Most hematomas are perinephric and are surrounded by fat stranding.
They may occasionally have a masslike appearance, and some may not be
discovered until long time after the traumatism. Chronic hematomas can have
calcifications and do not enhance.


## 4.2.1. Cysts

Most small renal masses incidentally discovered
on CT are simple cortical cysts that need no further evaluation. The Bosniak
classification system is used to asses the likelihood of malignancy in cystic renal masses on
the basis of lesion complexity in CT imaging [30, 31]. Application of the Bosniak classification can be
difficult in the case of an indeterminate renal lesion, especially if it is
small. Bosniak admits that distinction between category II (not requiring
surgery) and category III (requiring surgery) can be very difficult [32].


## 4.2.2. Angiomyolipoma

It is typically
a solid lesion that exhibits fat density on CT scans (−10 to –100 HU) (see Figure6). However, in some cases, it may contain very small quantities of fat that
can be overlooked. Angiomyolipomas rarely contain calcification, and,
therefore, a diagnosis of angiomyolipoma should not be made if a lesion
contains fat and calcium [33, 34]. There
have been few case reports of fat from RCC that also contain calcification. In
such cases, a renal cell carcinoma must be considered the most likely diagnosis.Figure 6
Angiomyolipoma. Contrast-enhanced
CT shows a small homogeneous fat-containing mass.


## 4.2.3. Oncocytoma

It is usually a
hypodense mass, homogeneous, with smooth contours and a tendency to enhance
avidly (see Figure7). Until
now, an oncocytoma was suggested on postcontrast CT by the presence of a
central hypoenhancing scar.
Because of its lack of
specificity, patient management has been unaffected by the presence of this
finding. Renal cell carcinoma
and oncocytoma are indistinguishable from each other at imaging.Figure 7
Oncocytoma. Small mass
isoattenuated to renal parenchyma after contrast.


## 4.2.4. Renal Cell Carcinoma

The
imaging characteristics of RCC are extremely varied, with masses ranging from
cystic to solid, from homogeneous to heterogeneous and necrotic, from small to
large, and from localized to extensive. The typical CT appearance of small RCC
is a homogeneously isodense/hypodense mass, noncalcified, with an attenuation value of 20 HU or more, that enhance
avidly and early with contrast medium (see Figure8) [23]. The early-stage contrast enhancement is believed to be caused by tumor
angiogenesis. However, a small proportion of RCC are hypovascular, and the
amount of enhancement may be minimal. Small RCC with a predominantly cystic
growth pattern, necrosis, or calcifications (peripheral curvilinear or punctate
central) are uncommon. Areas of fat attenuation can be
present within renal cell carcinomas, but are uncommon in small tumors.Small hyper vascular renal cell carcinoma. (a) Contrast-enhanced CT shows
small renal mass that enhances early in corticomedullary phase. (b) Rapid washout in nephrographic phase.
(a)(b)Furthermore, a small RCC can be
hyperattenuating. If the lesion is depicted only on enhanced CT, delayed
scanning can also be used. Macari and Bosniak [28] have suggested that measurement of
the washout of contrast material from at least 15 minutes allows
differentiation between hyperdense cyst and renal neoplasms. The washout of 15 HU or more indicates that, excluding vascular abnormality, the mass is solid. A
lack of washout indicates that the mass is probably a hyperattenuating cyst.On the other hand, renal cortical tumors are
family neoplasms with distinct cytogenetic and molecular characteristics and
varying malignant potential. In
the 1997 Heidelberg classification, renal cell carcinoma was subdivided in
subtypes [35]. It has been suggested that certain imaging
features may be associated with different subtypes of solid renal cortical
tumors [36, 37]. The most consistent finding in these studies was that the degree of
enhancement was the most valuable parameter for differentiation of RCC
subtypes, as clear cell RCC enhance to a greater degree than other subtypes,
especially papillary RCC. Clear
cell RCC is also strongly associated with a mixed enhancement pattern of both
enhancing soft-tissue components and low-attenuation areas (necrotic or cystic
changes). When
homogeneous or peripheral enhancement is present, clear cell RCC is a less
likely diagnosis, and other cell types should be considered. Notably, a
majority of papillary tumours were either homogeneous or peripheral enhancement
(see Figure 9). The presence to
neovascularity was mildly associated with more aggressive tumor. The clear cell
RCC (and oncocytomas) enhanced avidly during the parenchymal phase; the
chromophobe RCC (and lipid poor angiomyolipoma) enhanced moderately and
papillary RCC enhance mildly.Figure 9
Papillary renal cell carcinoma.  Contrast-enhanced CT shows small homogeneous
mass that is mild and less enhanced than renal parenchyma does.


## 4.2.5. Non-Hodgkin’s Lymphoma

Lymphoma
can have a variable appearance and may on occasion resemble renal cell
carcinoma. Most frequently, it
manifests as bilateral solid renal masses, and in a patient with systemic
lymphoma, the proper diagnosis is not difficult. Characteristically, lymphoma
often infiltrates into the kidney via the renal sinus or surrounds the kidney. In a patient with known systemic
lymphoma to whom a renal mass with imaging is detected, systemic treatment for
lymphoma should be instituted. If the patient’s systemic disease responds, and
the renal mass does not respond, biopsy of the mass is indicated. However,
lymphoma may rarely manifest as a solitary renal mass or a homogeneous infiltrating
renal mass. In this case, biopsy of the mass is indicated previous to systemic
therapy.


## 4.2.6. Metastases

Metastatic disease to the kidney
typically manifests as multiple bilateral renal masses, often associated with
metastatic disease to other organs. They are often poorly defined and
infiltrate the renal parenchyma. With the appropriate clinical history, the
diagnosis is straightforward. However, in a patient with a solitary renal mass
(especially an infiltrating mass) and history of previous malignancy,
percutaneous renal biopsy is indicated for a definitive diagnosis.


## 4.2.7. Benign Mesenchymal Tumors

Included leiomyomas, lipomas,
fibromas, and mixed mesenchymal nodules. They are usually small (<1 cm)
lesions, found in autopsies.


## 4.2.8. Pseudotumors

They
include congenital anomalies (prominent renal columns of Bertin, renal
dimorphisms, and dromedary humps) and acquired pseudotumors (hypertrophied
normal renal parenchyma). This condition enhances identically to the normal
renal parenchyma. In these situations it proves appropriate to scan during the
corticomedullary and nephrographic phase.


## 4.2.9. Renal Mass Mimickers

These include inflammatory masses (including focal
pyelonephritis, renal abscess) and hematoma. A careful evaluation with
high-quality CT or MR examination combined with the clinical context of the
case and a familiarity with this group of “lesions” should reveal its true
nature. Most hematomas are perinephric and are surrounded by fat stranding.
They may occasionally have a masslike appearance, and some may not be
discovered until long time after the traumatism. Chronic hematomas can have
calcifications and do not enhance.


## 4.3. Diagnosis and Management of Renal Call Carcinoma with MD-CT

CT remains the most widely available and single most effective modality for staging renal call carcinoma  [38, 39]. 3DCT
combined with CT angiography has the potential to provide all the critical
information needed to plan the surgical procedure.  3DCT images can be viewed in multiples planes
and orientations to define the tumor and its relationship to the renal surface,
the collecting system, and adjacent organs. A 3DCT angiogram can be created to
delineate the renal arterial and venous anatomy.The anatomic extent of the tumor at the time of
diagnosis is the single most important factor in determining prognosis (see Table1)  [38].Table 1
Staging system for renal carcinoma and CT criteria.
Tumour positionRobsonTNMCT findingsConfined withinISoft-tissue mass
enhancesrenal capsuleless than normal renalSmall (<7 cm)T1parenchyma; centralLarge (≥7 cm)T2necrosis in large RCC.IIT3aPerinephric stranding;Spread to
perinephric fatPerinephric collateral
vessels;Soft-tissue mass in
perinephric spaceVenous
thrombusIII ARenal vein onlyT3bLow-attenuation
filling defect vein;IVC infradiaphragmaticT3cDirect continuity of
thrombus with primary mass;IVC supradiaphragmaticT4bEnhanced thrombusRegional
lymphIII BN1-N3Lymph nodes 1 cm in
diameter or largernode metastasesDirect invasion of adjacent organsIV AT4aObliteration of normal
soft-tissues planes between tumor and adjacent organsDistant
metastasesM1Metastases enhance with
IV contrast material;Hepatic metastases
best in arterial phaseIV: intravenous, IVC: inferior vena cava.Most
urological surgeons continue to refer to Robson’s classification, which is
essentially a surgical staging approach. This system includes the important
staging variables that have survived scrutiny over the years. Confinement
within the renal capsule, penetration into the perirenal fat, invasion into the
renal vein, and lymph node metastases are all important in determining the
prognosis.Under and overstating
of perinephric invasion are the most common staging errors at CT  [40]. The most specific finding of stage T3a, the presence of an enhanced
nodule in the perinephric space, is highly specific but also low sensitive. The
differentiation between stage T2 and T3a tumors is very problematic.If tumoral spread
within the IVC is identified, precise delineation of the superior extent of the
thrombus is essential for the surgeon to plan the optimal surgical strategy for
thrombectomy and minimize the risk of embolism. The level of involvement of the
IVC dictates the surgical approach. Involvement of the IVC is best shown during
corticomedullary phase. Because of its multiplanar capability, magnetic
resonance imaging is the preferred modality to image. However, the
three-dimensional CT with sagittal and coronal reconstructions is also
effective in depicting the superior extent of inferior vena cava thrombus (see Figure10), with the advantages of widespread availability, shorter
examination time, and lower cost in comparison with MR  [40].Renal cell carcinoma. (a) Three-dimensional
CT with coronal reconstruction, (b) sagittal oblique reconstruction.
(a)(b)


## 5. MR

### 5.1. Indications

Although
ultrasound and CT are often combined to reveal and characterize most renal
lesions, MR is sometimes required when indeterminate lesions are found  [41], or
when a hyperattenuating renal mass is observed on CT [42]. Furthermore, some
renal masses are incidentally discovered when MR imaging is performed to answer
questions other than urology ones.On the other hand, MR imaging has been considered the
most accurate method for those patients with contraindications to iodinated
contrast administration such as in patients with renal failure and those with
an allergy to iodinated contrast material [43].Recently,
nephrogenic systemic fibrosis was linked to gadolinium contrast agent exposure
in patients with renal failure; therefore its use should be reserved for
neurological and vascular cases where the quality of information gained was
sufficient to justify the risk of potential devastating adverse effects [44].
Nevertheless, the better contrast resolution of MR imaging, as opposed to CT,
is an undoubtful advantage in those patients for performing an MR exploration,
even though no contrast is provided.MR
imaging has been considered useful not only in characterization but also in evaluation of most
cystic renal masses with the Bosniak classification system [45]. In addition,
MR imaging can potentially improve staging and preoperative studying of a renal
mass [41, 46–51] as well as tumor multifocality, collecting
system invasion, and venous invasion [52]. Moreover, MRI is capable of showing
the patency of a blood vessel without the use of intravenous contrast medium.The accuracy in detection and characterization
of SRM carries great significance when nephron sparing surgery is being
considered for a renal cell carcinoma because synchronous lesions measuring 1–15 mm have been
reported to a frequency as high as 19.7%  [53].


### 5.2. Basic Technical Examination

A proper technical examination is needed for
studying a renal mass which is indeterminate after US and CT techniques. Owing to
intrinsic proprieties of magnetic resonance examination, high-quality renal MR
imaging is dependent on multiple factors, includingpatient
cooperation in holding breath. In patients referred for evaluation of a renal
mass, examinations are currently performed with a torso phased-array coil,
preferably during a more reproducibility end-expiratory breath hold  [29, 54].Different protocols can be used by different
institutions depending on their technical requirements. Essentially, the common
aspects are shown as follows. Prior to contrast administration, sequences are
used to obtain images weighted on T1 and T2, in and out of phase T1, and a precontrast 3D
weighted T1 with fat saturation can be acquired in different planes, basically
on axial plane but also coronal or sagittal can be employed to best depict the
mass, especially for such lesions located in renal poles. This approach is most
important when evaluating a patient with a solitary kidney that
contains a renal neoplasm that is amenable to partial nephrectomy.These sequences provide us with morphologic
information about the renal parenchyma and a renal mass (location, size, and
signal intensity), parenchymal structures and adjacent organs, vascular
structures and lymphadenopathies, and are designed to improve visualization of
tisular intrinsic characteristics (fluid, haemorrhage, fat, fibrosis), except
for detecting calcification.Use of gadolinium as a contrast agent has been
described as a higher detection and characterization of SRM with MR imaging to
a higher level than does contrast CT  [55, 56]. After contrast administration,
MR angiography, MR venography, and MR urography are performed by using an
oblique coronal breath-hold 3D fat-suppressed dynamically acquired T1-weighted
spoiled sequence. The imaging delay for MR angiography is based ona
bolus test with a power injector. MR venography and MR urography are
performed at approximately 30 seconds and 5 minutes after MR
angiography, respectively. The postcontrast acquisition is performed
between MR venography and MR urography. For the characterization of
renal masses and to determine the presenceor absence of
enhancement, most authors recommend an imaging delay of 3–5 minutes [29].These sequences are used to increase
conspicuity of a renal mass, and to depict the renal arteries and
veins, for the evaluation of the extent of the tumor in the perinephric fat and
the relationship of a renal tumor to the hilar vessels and
collecting system, which is helpful to the urologist in surgical
planning [49].


### 5.3. Important Features to Recognize on MR Imaging

#### 5.3.1. Signal Intensity

Most renal
masses are hypointense on T1 and hyperintense on T2-weighted images, reflecting
their water content (fluid, oedema, etc.). Haemorrhage or infection causes different and
heterogeneous signal intensities both on T1 and T2, depending on the age of the
bleed or the protein concentration, making difficult the characterization of
lesions.In some cases, T1 hyperintensity and T2 hypointensity renal lesions
are noticed (see Figure11), and most of them are also hyperattenuating renal
masses on unenhanced CT. These are either benign or malignant masses and
include blood breakdown products or proteinaceous cysts, haematomas and vascular
malformations, and oncocytomas or angiomyolipomas with minimal fat, but also
some malignant lesions such as RCC and lymphomas. Shinmoto et al.  [57] reported
that papillary RCC is associated with T2-hypointense appearance as well as
hemosiderin deposition, haemorrhage, and necrosis. When a solid hyperattenuated
renal mass is seen on unenhanced CT, an MR must be performed to characterize
the lesion looking for signal loss on T2-weighted images owing to blood
products, iron deposits, hypercellularity,
and proteinaceous content. MR is helpful to recognize lesions that are
otherwise impossible to differentiate by CT alone, such as AML with minimal fat
and clear cell renal cell carcinomas. However, differentiation between AML with
minimal fat and papillary RCC is often not always possible by MR, and a
percutaneous biopsy may be useful  [42]. When a cystic mass is evaluated, MR may
show additional thickness of the septa and wall on T2-weighted images than on
CT  [29].(a) Coronal haste T2 and  (b) axial precontrast T1: small lesion on medial aspect of the upper pole of the left kidney shows a low signal intensity on T1 and hyperintensity on T1,  revealing blood breakdown products consistent with haemorrhagic cyst.
(a)(b)


#### 5.3.2. Presence of Fatty Tissue

AMLs are the only solid renal tumors
that can be positively characterized using MR by demonstrating macroscopic fat
in the lesion, and this fact is basically useful in patients with tuberous
sclerosis, since they develop AML and an increasing risk of renal cell
carcinomas  [49] (see Figure 12).(a)  Axial T1 in phase,  (b)   axial T1 out of phase, (c) axial haste T2 with fat saturation, and ((d–g)  axial dynamic postcontrast T1  (d),  arterial; (e), venous; (f), nephrographic, and (g), excretory phase): right yuxtahilar renal lesion seen as a T1 hyperintense mass that shows gross fat suppression on T2 revealing macroscopic fat and also probably small foci of peripheral fat as there is a small amount of signal loss on out of phase sequence. On dynamic postcontrast images the mass shows a transitional highly peripheral enhancement in the non-fat components with poor enhancement on the remaining postcontrast study. An angiomyolipoma was found at surgery.
(a)(b)(c)(d)(e)(f)(g)Opposed phase images and spectral fat
suppression are useful in differentiating fat from haemorrhage containing
carcinomas causing high signal intensity on T1. Furthermore, in phase gradient
echo images are often helpful because the fatty portions of AML will be
hyperintense and renal clear cell carcinomas usually will not, although renal
cell carcinomas incidentally may be hyperintense on T1; in these cases spectral fat suppression
images should be used to prove the presence of macroscopic fat in the AML [49].Some renal lesions may contain very small
amount of fat, the so called “minimal fat AML,” with microscopic fat and
without demonstrable macroscopic fat (angiomyomas), and it is not possible to differentiate
from a renal neoplasm. Fat suppression techniques generally are not helpful for
detecting fat in AML with minimal fat, because such masses contain little or no
fat and often appear as isointense to the renal parenchyma. Chemical shift
imaging may be used to determinate a small amount of fat within a mass, and can
be used to differentiate AML with minimal fat from other renal neoplasms, with
a high sensitivity and specificity  [58].However, a renal mass that is suppressed
focally or diffusely on opposed-phase sequences and that does not exhibit fat
suppression should arouse suspicion about the possible presence of clear renal
cell carcinoma due to intracellular lipid [42]. Obviously, this differentiation
cannot be made only with this unique technique and information of other
sequences, such as signal intensity on T2 and dynamic gadolinium enhancement,
and proper followup should therefore take place. Kim et al. [58] suggested followup with CT or MR
imaging for two years after detection for such lesions.The presence of fat in clear cell carcinoma has been used to
differentiate subtypes of renal cell carcinoma because this fact does not occur
in oncocytoma and transitional cell carcinoma [49], but if the
tumor does not show signal loss, it can still be a clear cell carcinoma.


#### 5.3.3. Presence of Pseudocapsule

This is a pathologic feature composed of
fibrous tissue and compressed renal parenchyma, seen frequently in the early stages of a SRM. Although not specific (also
seen in some oncocytomas), it has been related to renal cell carcinomas
usually small and of low histologic grade, slow growing, and less likely to
metastasize  [15]. Their presence is an indicator of
prognostic value [59]. This condition allows renal
parenchyma-sparing surgery, especially if simple enucleation is considered in
patients with multiple tumors, Von Hippel-Lindau disease or familial tendency
for RCC.At MR, a
pseudocapsule was seen as a hypointense thin rim surrounding the tumor on both
T1- and T2-weighted images and is more difficult to detect in hypointense
tumors [60, 61]. With postcontrast
images, late enhancement of the pseudocapsule resulted in poor contrast
relative to the surrounding tissue, lessening its own visualization in this
sequence (see Figure 13). Some reports [61, 62] noticed that the presence of a
pseudocapsule offers an additional value for local staging. On that series, T2-weighted imaging was the most sensitive
technique for visualization of the pseudocapsule (sensitivity: 68%; specificity:
91%), and is corroborated by other authors [15, 63, 64]. For this reasons, MR shows a moderate to high
sensitivity in depicting the pseudocapsule than CT [49].  In
some large tumors, although tumor invasion was seen, a residual pseudocapsule
was found in some areas.(a) Axial stir and (b)
axial postcontrast T1. A pseudocapsule is seen as a hypointense rim surrounding the tumor on T2
and is better delineated than on postcontrast T1 owing to the late contrast
enhancement of the pseudocapsule.
(a)(b)


#### 5.3.4. Involvement of Perinephric Fat

This is a
key point in treatment planning in modifying the
surgical approach from conservative to radical nephrectomy (Robson’s stage I versus
stage II). If partial nephrectomy is considered, it is essential to know
preoperatively if the perinephric fat is invaded or not by the tumor. Although
MRI appears slightly more sensitive than CT, it is not specific in
distinguishing between these two stages.The
presence of an intact pseudocapsule is an indirect sign of lack of perinephric
fat invasion [61]. The overall sensitivity of CT in
detecting pseudocapsule is very low [62, 63, 65, 66], and MR had a pertinent accuracy for
evaluating possible involvement of perinephric fat using the aspect of the pseudocapsule
as an additional feature [61].Perirenal fat invasion diagnosis was made when there was loss of
capsular integrity indicated by interruption of the low signal intensity line
around the kidney on T1- and T2-weighted images and thick (>0.5 mm)
perinephric stranding. Thin perinephric stranding and collateral vessel
formation alone were not considered features of perinephric fat invasion. This
causes under- and overstaging on CT and MR and lacks good
specificity  [67]. It also may be caused by edema,
vascular engorgement, and/or inflammation [65]. The only highly specific finding was
the presence of an enhancing nodule or soft-tissue mass in the perinephric
space but this sign had only 46% sensitivity [38].


#### 5.3.5. Enhancement

Different patterns of enhancement have been observed, predominantly
peripheral, heterogeneous, and homogeneous. The dynamic evaluation appears to
be useful in the detection and characterization of simple renal cysts and solid
neoplasms [68].The presence or absence of enhancement within a
renal mass is the most important factor in its proper characterization. When a
predominant part of a renal mass enhances, the mass is considered solid and
likely neoplastic. Otherwise, it is important to be aware of the possibility of
pseudoenhancement and to know when to suspect it. All solid lesions
demonstrated gadolinium enhancement, not only RCC and invasive transitional
cell carcinomas but also oncocytomas and AML (see Figures12 and 14). Although
enhancement is sufficient for predicting malignancy, nonenhancement is not
sufficient to exclude malignancy, and again the integration of T2 appearance is
useful in improving the differentiation between benign from malignant renal
lesions [41].(a) Axial T1 in
phase, (b) axial T1 out of phase, (c) axial stir, and (d–g) axial dynamic
postcontrast T1 ((d) arterial; (e) venous; (f) nephrographic, and (g) excretory phase). A nonfatty
mass hyperintense on T2 and highly vascular on corticomedullary and
nephrographic phase with late mild washout as is shown on this clear renal cell
carcinoma.
(a)(b)(c)(d)(e)(f)(g)Hypervascular RCC can be easily differentiated
on dynamic contrast-enhanced MR. Hypovascular RCC, AML, and complicated cysts
enhanced significantly less than cortical and medullary tissue did (see Figure15). Furthermore, papillary RCC is typically hypovascular and shows mild
contrast enhancement, whereas AML with minimal fat is generally hypervascular and shows marked enhancement, but occasionally the
degree of enhancement varies, making this differentiation difficult  [42]. Also hypovascular RCC from the
first minute after gadolinium injection showed significantly greater
enhancement than complicated cyst  [69].(a) Cor haste T2,
(b) out of phase axial T1, (c) axial precontrast T1 with fat suppression, (d) early
postcontrast axial T1 with fat saturation, and (e) axial postcontrast T1 with
fat suppression.
Small renal lesion on the lateral aspect of the upper pole of the right kidney
shows a solid mass hypointense
on T2 that does not present a loss of signal out of phase sequence and only a
small enhancement on postcontrast sequences, consistent with a papillary renal carcinoma.
(a)(b)(c)(d)(e)Thus, areas with haemorrhage or infection
products do not enhance but their signal intensity remains higher than that of
simple cyst, making quantitative ROI measurements in these lesions essential  [70] for correct characterization. This
can be made by quantitative and qualitative methods. Ho et al.  [71] concluded that above 15% was the
optimal percentage of enhancement threshold for distinguishing cysts from
malignancies. Although usually quantitative and qualitative methods are
sensitive in the detection of enhancement, in hyperintense lesions on
unenhanced T1, qualitative assessment based on image subtraction should be
performed to avoid false negative quantitative results  [54].When a cystic mass is evaluated, MR imaging may
demonstrate definitive enhancement that shows only equivocal enhancement on CT.
In a cystic lesion with only a small solid component, subtraction images may
again be used to better assess the presence of enhancement  [49]. Furthermore, even if detecting a
calcification is a limitation on MR imaging, it is however an advantage to
determine whether enhancement is present in a heavily calcified cyst on CT
given that it could be better appreciated  [45]. The combination of mural
irregularity and intense mural enhancement is a strong predictor of malignancy
in renal cystic lesions (see Figure 16). However, the appearance of benign and
malignant lesions may overlap  [72, 73].(a) Coronal haste T2
and (b) late postcontrast coronal T1 with fat saturation. Cystic lesion with an
enhancing nodule on the caudal aspect of the kidney. Renal cell carcinoma was
found at surgery.
(a)(b)


#### 5.3.6. Other

Invasion
of Gerota’s fascia was diagnosed when continuity of the low signal intensity
line around the perinephric fat on T1-weighted images was disrupted by tumor.
Imaging of the ipsilateral adrenal gland and venous spread of tumors are out of
the scope of this chapter, thus the clinical setting of small renal tumors is
not found incidentally, as it is the case in the low probability of nodal
metastases in this stage of disease.On the other hand, advanced imaging techniques
led to improve the global accuracy for MDCT to adequately stage these clinical
aspects [38]. Hricak et al. [60] reported
accuracy rates for detecting adjacent organ invasion with MRI, although, in
their series, overstaging was caused by the presence of abnormal signal,
indistinct interface, and absence of a free fat plane between the tumor and the
adjacent organ.


### 5.4. Interpretation of Images and Surgical Criteria

When characterizing a renal mass, the major
question to be answeredis whether the mass represents a surgical or
nonsurgical lesion or, in some cases, if followup studies are
necessary.Magnetic resonance imaging allows an accurate
differentiation between solid and cystic masses, as a first approximation, but
angiomyolipomas are the only solid renal tumors that can be positively
characterized using MR. Nevertheless, Prasad et al. [74] reported that small renal medullary
tumors may be differentiated from the more common renal adenocarcinomas by
their central location and certain demographic characteristics. Furthermore,
some authors recently [75] analyzed the correlation between MR image features
and histopathological findings, giving value to subvoxel fat on chemical shift
imaging as a good correlation to clear cell type with a high specificity. These
authors reported that small size, peripheral location, low intratumoral signal
intensity on T2, and low level enhancement were also associated with low-grade
papillary carcinomas.Detection of macroscopicfat  is the key for diagnosing AML in the proper clinical
setting, and it is decisive because AML does not need to be surgically removed.
The diagnosis is made by demonstrating fat within a solid renal mass. One
pitfall of fat containing renal masses is the presence of a renal cell
carcinoma involving the perinephritic fat. This differentiation is easily made
if there is some calcification on CT, as do some RCC [29]. However, calcium is not always present in renal cell
carcinoma [76].The observation of apseudocapsule  surrounding a renal cell carcinoma is a sign of lack of perinephric fat invasion, and therefore it is
more likely to predict
that the tumor can be removed by nephron sparing, so partial nephrectomy or
simple enucleation may be indicated when a pseudocapsule is detected  [49, 62].The presence ofhaemorrhagic  products may obscure enhancement on dynamic postcontrast
T1 images, and it may also contribute to heterogeneity on T2 images. Thus, in
the setting of a T2 heterogeneous nonenhancing mass, careful followup after an
antibiotic trial may be a prudent recommendation to avoid nephrectomy of renal
abscess and to avoid misdiagnosis of a hemorrhagic renal cell carcinoma  [41].
Furthermore, a common occurrence of haemorrhage is described in patients with renal
cancer and/or in patients with renal insufficiency, and caution should be
exercised when evaluating haemorrhagic cystic lesions in these patients  [77].The most importantcriterion used in
differentiating surgical fromnonsurgical renalmasses is
the determination ofenhancement
[29]. Despite that, the lack of
enhancement of a renal lesion, particularly if small (<1 cm), is not
considered a sufficient criterion for excluding malignancy, as the haemorrhagic
lesions occur, and in this setting, T2-weighted images must be considered. Moreover,
it is important to combine the degree of enhancement with the morphologic
features of the lesion, such as homogeneity, wall thickening, and presence of
calcifications.When acystic  mass is evaluated, a surgical cyst can be suspected only if enhancement is
present. Calcification in a cystic renal mass is not as important in diagnosis
as the presence of associated enhancing soft-tissue elements  [78]. MR imaging
may depict additional septa, thickening of the wall and/or septa, or
enhancement, which may lead to an upgraded Bosniak cyst classification and can
affect case management   [31, 49].


### 5.5. Limitations

As with
all imaging techniques, it is extremely difficult with MR to determine whether
malignant tissue extends to adjacent normal tissue when strictly regular
margins are found because microscopic local invasion could have occurred
[61, 64]. Staging errors were
made because of limitations of the imaging technique: inability to detect
microscopic invasion of the perinephric fat, difficulty in differentiating
inflammatory changes from tumor infiltration, and insensitivity in
differentiating small collateral blood vessels from tumor extension in the
lymphatics  [67].There are
also limitations to the detection of a pseudocapsule by MR, mainly with
hypointense tumors because its detection may be less accurate on T2-weighted
owing to the lack of delineation of the surrounding rim, as is the case of some
papillary tumors. With SRM, partial volume averaging may obscure its
visualization, thus evaluation in three planes, and coronal, sagittal, or
oblique views are required to avoid this phenomenon on the upper and lower part
of the tumor [61]. Furthermore,
given that the pseudocapsule was also found in oncocytomas, it is not useful
for differentiating RCC from this benign solid tumor, and it cannot be used to
predict the nature of the lesion. But it can offer an additional value to the
performance of preoperative MR to stage renal tumors, aiding to make decisions
about the most appropriate surgical technique to employ.Concerning nodal staging, MR has the same
limitations as CT according to the nodal size over 1 cm in short-axis diameter,
being this an additional indication for the future use of new iron oxide-based
contrast agents on MR to improve specificity and accuracy in nodal staging
[49, 79].


## 5.1. Indications

Although
ultrasound and CT are often combined to reveal and characterize most renal
lesions, MR is sometimes required when indeterminate lesions are found  [41], or
when a hyperattenuating renal mass is observed on CT [42]. Furthermore, some
renal masses are incidentally discovered when MR imaging is performed to answer
questions other than urology ones.On the other hand, MR imaging has been considered the
most accurate method for those patients with contraindications to iodinated
contrast administration such as in patients with renal failure and those with
an allergy to iodinated contrast material [43].Recently,
nephrogenic systemic fibrosis was linked to gadolinium contrast agent exposure
in patients with renal failure; therefore its use should be reserved for
neurological and vascular cases where the quality of information gained was
sufficient to justify the risk of potential devastating adverse effects [44].
Nevertheless, the better contrast resolution of MR imaging, as opposed to CT,
is an undoubtful advantage in those patients for performing an MR exploration,
even though no contrast is provided.MR
imaging has been considered useful not only in characterization but also in evaluation of most
cystic renal masses with the Bosniak classification system [45]. In addition,
MR imaging can potentially improve staging and preoperative studying of a renal
mass [41, 46–51] as well as tumor multifocality, collecting
system invasion, and venous invasion [52]. Moreover, MRI is capable of showing
the patency of a blood vessel without the use of intravenous contrast medium.The accuracy in detection and characterization
of SRM carries great significance when nephron sparing surgery is being
considered for a renal cell carcinoma because synchronous lesions measuring 1–15 mm have been
reported to a frequency as high as 19.7%  [53].


## 5.2. Basic Technical Examination

A proper technical examination is needed for
studying a renal mass which is indeterminate after US and CT techniques. Owing to
intrinsic proprieties of magnetic resonance examination, high-quality renal MR
imaging is dependent on multiple factors, includingpatient
cooperation in holding breath. In patients referred for evaluation of a renal
mass, examinations are currently performed with a torso phased-array coil,
preferably during a more reproducibility end-expiratory breath hold  [29, 54].Different protocols can be used by different
institutions depending on their technical requirements. Essentially, the common
aspects are shown as follows. Prior to contrast administration, sequences are
used to obtain images weighted on T1 and T2, in and out of phase T1, and a precontrast 3D
weighted T1 with fat saturation can be acquired in different planes, basically
on axial plane but also coronal or sagittal can be employed to best depict the
mass, especially for such lesions located in renal poles. This approach is most
important when evaluating a patient with a solitary kidney that
contains a renal neoplasm that is amenable to partial nephrectomy.These sequences provide us with morphologic
information about the renal parenchyma and a renal mass (location, size, and
signal intensity), parenchymal structures and adjacent organs, vascular
structures and lymphadenopathies, and are designed to improve visualization of
tisular intrinsic characteristics (fluid, haemorrhage, fat, fibrosis), except
for detecting calcification.Use of gadolinium as a contrast agent has been
described as a higher detection and characterization of SRM with MR imaging to
a higher level than does contrast CT  [55, 56]. After contrast administration,
MR angiography, MR venography, and MR urography are performed by using an
oblique coronal breath-hold 3D fat-suppressed dynamically acquired T1-weighted
spoiled sequence. The imaging delay for MR angiography is based ona
bolus test with a power injector. MR venography and MR urography are
performed at approximately 30 seconds and 5 minutes after MR
angiography, respectively. The postcontrast acquisition is performed
between MR venography and MR urography. For the characterization of
renal masses and to determine the presenceor absence of
enhancement, most authors recommend an imaging delay of 3–5 minutes [29].These sequences are used to increase
conspicuity of a renal mass, and to depict the renal arteries and
veins, for the evaluation of the extent of the tumor in the perinephric fat and
the relationship of a renal tumor to the hilar vessels and
collecting system, which is helpful to the urologist in surgical
planning [49].


## 5.3. Important Features to Recognize on MR Imaging

### 5.3.1. Signal Intensity

Most renal
masses are hypointense on T1 and hyperintense on T2-weighted images, reflecting
their water content (fluid, oedema, etc.). Haemorrhage or infection causes different and
heterogeneous signal intensities both on T1 and T2, depending on the age of the
bleed or the protein concentration, making difficult the characterization of
lesions.In some cases, T1 hyperintensity and T2 hypointensity renal lesions
are noticed (see Figure11), and most of them are also hyperattenuating renal
masses on unenhanced CT. These are either benign or malignant masses and
include blood breakdown products or proteinaceous cysts, haematomas and vascular
malformations, and oncocytomas or angiomyolipomas with minimal fat, but also
some malignant lesions such as RCC and lymphomas. Shinmoto et al.  [57] reported
that papillary RCC is associated with T2-hypointense appearance as well as
hemosiderin deposition, haemorrhage, and necrosis. When a solid hyperattenuated
renal mass is seen on unenhanced CT, an MR must be performed to characterize
the lesion looking for signal loss on T2-weighted images owing to blood
products, iron deposits, hypercellularity,
and proteinaceous content. MR is helpful to recognize lesions that are
otherwise impossible to differentiate by CT alone, such as AML with minimal fat
and clear cell renal cell carcinomas. However, differentiation between AML with
minimal fat and papillary RCC is often not always possible by MR, and a
percutaneous biopsy may be useful  [42]. When a cystic mass is evaluated, MR may
show additional thickness of the septa and wall on T2-weighted images than on
CT  [29].(a) Coronal haste T2 and  (b) axial precontrast T1: small lesion on medial aspect of the upper pole of the left kidney shows a low signal intensity on T1 and hyperintensity on T1,  revealing blood breakdown products consistent with haemorrhagic cyst.
(a)(b)


### 5.3.2. Presence of Fatty Tissue

AMLs are the only solid renal tumors
that can be positively characterized using MR by demonstrating macroscopic fat
in the lesion, and this fact is basically useful in patients with tuberous
sclerosis, since they develop AML and an increasing risk of renal cell
carcinomas  [49] (see Figure 12).(a)  Axial T1 in phase,  (b)   axial T1 out of phase, (c) axial haste T2 with fat saturation, and ((d–g)  axial dynamic postcontrast T1  (d),  arterial; (e), venous; (f), nephrographic, and (g), excretory phase): right yuxtahilar renal lesion seen as a T1 hyperintense mass that shows gross fat suppression on T2 revealing macroscopic fat and also probably small foci of peripheral fat as there is a small amount of signal loss on out of phase sequence. On dynamic postcontrast images the mass shows a transitional highly peripheral enhancement in the non-fat components with poor enhancement on the remaining postcontrast study. An angiomyolipoma was found at surgery.
(a)(b)(c)(d)(e)(f)(g)Opposed phase images and spectral fat
suppression are useful in differentiating fat from haemorrhage containing
carcinomas causing high signal intensity on T1. Furthermore, in phase gradient
echo images are often helpful because the fatty portions of AML will be
hyperintense and renal clear cell carcinomas usually will not, although renal
cell carcinomas incidentally may be hyperintense on T1; in these cases spectral fat suppression
images should be used to prove the presence of macroscopic fat in the AML [49].Some renal lesions may contain very small
amount of fat, the so called “minimal fat AML,” with microscopic fat and
without demonstrable macroscopic fat (angiomyomas), and it is not possible to differentiate
from a renal neoplasm. Fat suppression techniques generally are not helpful for
detecting fat in AML with minimal fat, because such masses contain little or no
fat and often appear as isointense to the renal parenchyma. Chemical shift
imaging may be used to determinate a small amount of fat within a mass, and can
be used to differentiate AML with minimal fat from other renal neoplasms, with
a high sensitivity and specificity  [58].However, a renal mass that is suppressed
focally or diffusely on opposed-phase sequences and that does not exhibit fat
suppression should arouse suspicion about the possible presence of clear renal
cell carcinoma due to intracellular lipid [42]. Obviously, this differentiation
cannot be made only with this unique technique and information of other
sequences, such as signal intensity on T2 and dynamic gadolinium enhancement,
and proper followup should therefore take place. Kim et al. [58] suggested followup with CT or MR
imaging for two years after detection for such lesions.The presence of fat in clear cell carcinoma has been used to
differentiate subtypes of renal cell carcinoma because this fact does not occur
in oncocytoma and transitional cell carcinoma [49], but if the
tumor does not show signal loss, it can still be a clear cell carcinoma.


### 5.3.3. Presence of Pseudocapsule

This is a pathologic feature composed of
fibrous tissue and compressed renal parenchyma, seen frequently in the early stages of a SRM. Although not specific (also
seen in some oncocytomas), it has been related to renal cell carcinomas
usually small and of low histologic grade, slow growing, and less likely to
metastasize  [15]. Their presence is an indicator of
prognostic value [59]. This condition allows renal
parenchyma-sparing surgery, especially if simple enucleation is considered in
patients with multiple tumors, Von Hippel-Lindau disease or familial tendency
for RCC.At MR, a
pseudocapsule was seen as a hypointense thin rim surrounding the tumor on both
T1- and T2-weighted images and is more difficult to detect in hypointense
tumors [60, 61]. With postcontrast
images, late enhancement of the pseudocapsule resulted in poor contrast
relative to the surrounding tissue, lessening its own visualization in this
sequence (see Figure 13). Some reports [61, 62] noticed that the presence of a
pseudocapsule offers an additional value for local staging. On that series, T2-weighted imaging was the most sensitive
technique for visualization of the pseudocapsule (sensitivity: 68%; specificity:
91%), and is corroborated by other authors [15, 63, 64]. For this reasons, MR shows a moderate to high
sensitivity in depicting the pseudocapsule than CT [49].  In
some large tumors, although tumor invasion was seen, a residual pseudocapsule
was found in some areas.(a) Axial stir and (b)
axial postcontrast T1. A pseudocapsule is seen as a hypointense rim surrounding the tumor on T2
and is better delineated than on postcontrast T1 owing to the late contrast
enhancement of the pseudocapsule.
(a)(b)


### 5.3.4. Involvement of Perinephric Fat

This is a
key point in treatment planning in modifying the
surgical approach from conservative to radical nephrectomy (Robson’s stage I versus
stage II). If partial nephrectomy is considered, it is essential to know
preoperatively if the perinephric fat is invaded or not by the tumor. Although
MRI appears slightly more sensitive than CT, it is not specific in
distinguishing between these two stages.The
presence of an intact pseudocapsule is an indirect sign of lack of perinephric
fat invasion [61]. The overall sensitivity of CT in
detecting pseudocapsule is very low [62, 63, 65, 66], and MR had a pertinent accuracy for
evaluating possible involvement of perinephric fat using the aspect of the pseudocapsule
as an additional feature [61].Perirenal fat invasion diagnosis was made when there was loss of
capsular integrity indicated by interruption of the low signal intensity line
around the kidney on T1- and T2-weighted images and thick (>0.5 mm)
perinephric stranding. Thin perinephric stranding and collateral vessel
formation alone were not considered features of perinephric fat invasion. This
causes under- and overstaging on CT and MR and lacks good
specificity  [67]. It also may be caused by edema,
vascular engorgement, and/or inflammation [65]. The only highly specific finding was
the presence of an enhancing nodule or soft-tissue mass in the perinephric
space but this sign had only 46% sensitivity [38].


### 5.3.5. Enhancement

Different patterns of enhancement have been observed, predominantly
peripheral, heterogeneous, and homogeneous. The dynamic evaluation appears to
be useful in the detection and characterization of simple renal cysts and solid
neoplasms [68].The presence or absence of enhancement within a
renal mass is the most important factor in its proper characterization. When a
predominant part of a renal mass enhances, the mass is considered solid and
likely neoplastic. Otherwise, it is important to be aware of the possibility of
pseudoenhancement and to know when to suspect it. All solid lesions
demonstrated gadolinium enhancement, not only RCC and invasive transitional
cell carcinomas but also oncocytomas and AML (see Figures12 and 14). Although
enhancement is sufficient for predicting malignancy, nonenhancement is not
sufficient to exclude malignancy, and again the integration of T2 appearance is
useful in improving the differentiation between benign from malignant renal
lesions [41].(a) Axial T1 in
phase, (b) axial T1 out of phase, (c) axial stir, and (d–g) axial dynamic
postcontrast T1 ((d) arterial; (e) venous; (f) nephrographic, and (g) excretory phase). A nonfatty
mass hyperintense on T2 and highly vascular on corticomedullary and
nephrographic phase with late mild washout as is shown on this clear renal cell
carcinoma.
(a)(b)(c)(d)(e)(f)(g)Hypervascular RCC can be easily differentiated
on dynamic contrast-enhanced MR. Hypovascular RCC, AML, and complicated cysts
enhanced significantly less than cortical and medullary tissue did (see Figure15). Furthermore, papillary RCC is typically hypovascular and shows mild
contrast enhancement, whereas AML with minimal fat is generally hypervascular and shows marked enhancement, but occasionally the
degree of enhancement varies, making this differentiation difficult  [42]. Also hypovascular RCC from the
first minute after gadolinium injection showed significantly greater
enhancement than complicated cyst  [69].(a) Cor haste T2,
(b) out of phase axial T1, (c) axial precontrast T1 with fat suppression, (d) early
postcontrast axial T1 with fat saturation, and (e) axial postcontrast T1 with
fat suppression.
Small renal lesion on the lateral aspect of the upper pole of the right kidney
shows a solid mass hypointense
on T2 that does not present a loss of signal out of phase sequence and only a
small enhancement on postcontrast sequences, consistent with a papillary renal carcinoma.
(a)(b)(c)(d)(e)Thus, areas with haemorrhage or infection
products do not enhance but their signal intensity remains higher than that of
simple cyst, making quantitative ROI measurements in these lesions essential  [70] for correct characterization. This
can be made by quantitative and qualitative methods. Ho et al.  [71] concluded that above 15% was the
optimal percentage of enhancement threshold for distinguishing cysts from
malignancies. Although usually quantitative and qualitative methods are
sensitive in the detection of enhancement, in hyperintense lesions on
unenhanced T1, qualitative assessment based on image subtraction should be
performed to avoid false negative quantitative results  [54].When a cystic mass is evaluated, MR imaging may
demonstrate definitive enhancement that shows only equivocal enhancement on CT.
In a cystic lesion with only a small solid component, subtraction images may
again be used to better assess the presence of enhancement  [49]. Furthermore, even if detecting a
calcification is a limitation on MR imaging, it is however an advantage to
determine whether enhancement is present in a heavily calcified cyst on CT
given that it could be better appreciated  [45]. The combination of mural
irregularity and intense mural enhancement is a strong predictor of malignancy
in renal cystic lesions (see Figure 16). However, the appearance of benign and
malignant lesions may overlap  [72, 73].(a) Coronal haste T2
and (b) late postcontrast coronal T1 with fat saturation. Cystic lesion with an
enhancing nodule on the caudal aspect of the kidney. Renal cell carcinoma was
found at surgery.
(a)(b)


### 5.3.6. Other

Invasion
of Gerota’s fascia was diagnosed when continuity of the low signal intensity
line around the perinephric fat on T1-weighted images was disrupted by tumor.
Imaging of the ipsilateral adrenal gland and venous spread of tumors are out of
the scope of this chapter, thus the clinical setting of small renal tumors is
not found incidentally, as it is the case in the low probability of nodal
metastases in this stage of disease.On the other hand, advanced imaging techniques
led to improve the global accuracy for MDCT to adequately stage these clinical
aspects [38]. Hricak et al. [60] reported
accuracy rates for detecting adjacent organ invasion with MRI, although, in
their series, overstaging was caused by the presence of abnormal signal,
indistinct interface, and absence of a free fat plane between the tumor and the
adjacent organ.


## 5.3.1. Signal Intensity

Most renal
masses are hypointense on T1 and hyperintense on T2-weighted images, reflecting
their water content (fluid, oedema, etc.). Haemorrhage or infection causes different and
heterogeneous signal intensities both on T1 and T2, depending on the age of the
bleed or the protein concentration, making difficult the characterization of
lesions.In some cases, T1 hyperintensity and T2 hypointensity renal lesions
are noticed (see Figure11), and most of them are also hyperattenuating renal
masses on unenhanced CT. These are either benign or malignant masses and
include blood breakdown products or proteinaceous cysts, haematomas and vascular
malformations, and oncocytomas or angiomyolipomas with minimal fat, but also
some malignant lesions such as RCC and lymphomas. Shinmoto et al.  [57] reported
that papillary RCC is associated with T2-hypointense appearance as well as
hemosiderin deposition, haemorrhage, and necrosis. When a solid hyperattenuated
renal mass is seen on unenhanced CT, an MR must be performed to characterize
the lesion looking for signal loss on T2-weighted images owing to blood
products, iron deposits, hypercellularity,
and proteinaceous content. MR is helpful to recognize lesions that are
otherwise impossible to differentiate by CT alone, such as AML with minimal fat
and clear cell renal cell carcinomas. However, differentiation between AML with
minimal fat and papillary RCC is often not always possible by MR, and a
percutaneous biopsy may be useful  [42]. When a cystic mass is evaluated, MR may
show additional thickness of the septa and wall on T2-weighted images than on
CT  [29].(a) Coronal haste T2 and  (b) axial precontrast T1: small lesion on medial aspect of the upper pole of the left kidney shows a low signal intensity on T1 and hyperintensity on T1,  revealing blood breakdown products consistent with haemorrhagic cyst.
(a)(b)


## 5.3.2. Presence of Fatty Tissue

AMLs are the only solid renal tumors
that can be positively characterized using MR by demonstrating macroscopic fat
in the lesion, and this fact is basically useful in patients with tuberous
sclerosis, since they develop AML and an increasing risk of renal cell
carcinomas  [49] (see Figure 12).(a)  Axial T1 in phase,  (b)   axial T1 out of phase, (c) axial haste T2 with fat saturation, and ((d–g)  axial dynamic postcontrast T1  (d),  arterial; (e), venous; (f), nephrographic, and (g), excretory phase): right yuxtahilar renal lesion seen as a T1 hyperintense mass that shows gross fat suppression on T2 revealing macroscopic fat and also probably small foci of peripheral fat as there is a small amount of signal loss on out of phase sequence. On dynamic postcontrast images the mass shows a transitional highly peripheral enhancement in the non-fat components with poor enhancement on the remaining postcontrast study. An angiomyolipoma was found at surgery.
(a)(b)(c)(d)(e)(f)(g)Opposed phase images and spectral fat
suppression are useful in differentiating fat from haemorrhage containing
carcinomas causing high signal intensity on T1. Furthermore, in phase gradient
echo images are often helpful because the fatty portions of AML will be
hyperintense and renal clear cell carcinomas usually will not, although renal
cell carcinomas incidentally may be hyperintense on T1; in these cases spectral fat suppression
images should be used to prove the presence of macroscopic fat in the AML [49].Some renal lesions may contain very small
amount of fat, the so called “minimal fat AML,” with microscopic fat and
without demonstrable macroscopic fat (angiomyomas), and it is not possible to differentiate
from a renal neoplasm. Fat suppression techniques generally are not helpful for
detecting fat in AML with minimal fat, because such masses contain little or no
fat and often appear as isointense to the renal parenchyma. Chemical shift
imaging may be used to determinate a small amount of fat within a mass, and can
be used to differentiate AML with minimal fat from other renal neoplasms, with
a high sensitivity and specificity  [58].However, a renal mass that is suppressed
focally or diffusely on opposed-phase sequences and that does not exhibit fat
suppression should arouse suspicion about the possible presence of clear renal
cell carcinoma due to intracellular lipid [42]. Obviously, this differentiation
cannot be made only with this unique technique and information of other
sequences, such as signal intensity on T2 and dynamic gadolinium enhancement,
and proper followup should therefore take place. Kim et al. [58] suggested followup with CT or MR
imaging for two years after detection for such lesions.The presence of fat in clear cell carcinoma has been used to
differentiate subtypes of renal cell carcinoma because this fact does not occur
in oncocytoma and transitional cell carcinoma [49], but if the
tumor does not show signal loss, it can still be a clear cell carcinoma.


## 5.3.3. Presence of Pseudocapsule

This is a pathologic feature composed of
fibrous tissue and compressed renal parenchyma, seen frequently in the early stages of a SRM. Although not specific (also
seen in some oncocytomas), it has been related to renal cell carcinomas
usually small and of low histologic grade, slow growing, and less likely to
metastasize  [15]. Their presence is an indicator of
prognostic value [59]. This condition allows renal
parenchyma-sparing surgery, especially if simple enucleation is considered in
patients with multiple tumors, Von Hippel-Lindau disease or familial tendency
for RCC.At MR, a
pseudocapsule was seen as a hypointense thin rim surrounding the tumor on both
T1- and T2-weighted images and is more difficult to detect in hypointense
tumors [60, 61]. With postcontrast
images, late enhancement of the pseudocapsule resulted in poor contrast
relative to the surrounding tissue, lessening its own visualization in this
sequence (see Figure 13). Some reports [61, 62] noticed that the presence of a
pseudocapsule offers an additional value for local staging. On that series, T2-weighted imaging was the most sensitive
technique for visualization of the pseudocapsule (sensitivity: 68%; specificity:
91%), and is corroborated by other authors [15, 63, 64]. For this reasons, MR shows a moderate to high
sensitivity in depicting the pseudocapsule than CT [49].  In
some large tumors, although tumor invasion was seen, a residual pseudocapsule
was found in some areas.(a) Axial stir and (b)
axial postcontrast T1. A pseudocapsule is seen as a hypointense rim surrounding the tumor on T2
and is better delineated than on postcontrast T1 owing to the late contrast
enhancement of the pseudocapsule.
(a)(b)


## 5.3.4. Involvement of Perinephric Fat

This is a
key point in treatment planning in modifying the
surgical approach from conservative to radical nephrectomy (Robson’s stage I versus
stage II). If partial nephrectomy is considered, it is essential to know
preoperatively if the perinephric fat is invaded or not by the tumor. Although
MRI appears slightly more sensitive than CT, it is not specific in
distinguishing between these two stages.The
presence of an intact pseudocapsule is an indirect sign of lack of perinephric
fat invasion [61]. The overall sensitivity of CT in
detecting pseudocapsule is very low [62, 63, 65, 66], and MR had a pertinent accuracy for
evaluating possible involvement of perinephric fat using the aspect of the pseudocapsule
as an additional feature [61].Perirenal fat invasion diagnosis was made when there was loss of
capsular integrity indicated by interruption of the low signal intensity line
around the kidney on T1- and T2-weighted images and thick (>0.5 mm)
perinephric stranding. Thin perinephric stranding and collateral vessel
formation alone were not considered features of perinephric fat invasion. This
causes under- and overstaging on CT and MR and lacks good
specificity  [67]. It also may be caused by edema,
vascular engorgement, and/or inflammation [65]. The only highly specific finding was
the presence of an enhancing nodule or soft-tissue mass in the perinephric
space but this sign had only 46% sensitivity [38].


## 5.3.5. Enhancement

Different patterns of enhancement have been observed, predominantly
peripheral, heterogeneous, and homogeneous. The dynamic evaluation appears to
be useful in the detection and characterization of simple renal cysts and solid
neoplasms [68].The presence or absence of enhancement within a
renal mass is the most important factor in its proper characterization. When a
predominant part of a renal mass enhances, the mass is considered solid and
likely neoplastic. Otherwise, it is important to be aware of the possibility of
pseudoenhancement and to know when to suspect it. All solid lesions
demonstrated gadolinium enhancement, not only RCC and invasive transitional
cell carcinomas but also oncocytomas and AML (see Figures12 and 14). Although
enhancement is sufficient for predicting malignancy, nonenhancement is not
sufficient to exclude malignancy, and again the integration of T2 appearance is
useful in improving the differentiation between benign from malignant renal
lesions [41].(a) Axial T1 in
phase, (b) axial T1 out of phase, (c) axial stir, and (d–g) axial dynamic
postcontrast T1 ((d) arterial; (e) venous; (f) nephrographic, and (g) excretory phase). A nonfatty
mass hyperintense on T2 and highly vascular on corticomedullary and
nephrographic phase with late mild washout as is shown on this clear renal cell
carcinoma.
(a)(b)(c)(d)(e)(f)(g)Hypervascular RCC can be easily differentiated
on dynamic contrast-enhanced MR. Hypovascular RCC, AML, and complicated cysts
enhanced significantly less than cortical and medullary tissue did (see Figure15). Furthermore, papillary RCC is typically hypovascular and shows mild
contrast enhancement, whereas AML with minimal fat is generally hypervascular and shows marked enhancement, but occasionally the
degree of enhancement varies, making this differentiation difficult  [42]. Also hypovascular RCC from the
first minute after gadolinium injection showed significantly greater
enhancement than complicated cyst  [69].(a) Cor haste T2,
(b) out of phase axial T1, (c) axial precontrast T1 with fat suppression, (d) early
postcontrast axial T1 with fat saturation, and (e) axial postcontrast T1 with
fat suppression.
Small renal lesion on the lateral aspect of the upper pole of the right kidney
shows a solid mass hypointense
on T2 that does not present a loss of signal out of phase sequence and only a
small enhancement on postcontrast sequences, consistent with a papillary renal carcinoma.
(a)(b)(c)(d)(e)Thus, areas with haemorrhage or infection
products do not enhance but their signal intensity remains higher than that of
simple cyst, making quantitative ROI measurements in these lesions essential  [70] for correct characterization. This
can be made by quantitative and qualitative methods. Ho et al.  [71] concluded that above 15% was the
optimal percentage of enhancement threshold for distinguishing cysts from
malignancies. Although usually quantitative and qualitative methods are
sensitive in the detection of enhancement, in hyperintense lesions on
unenhanced T1, qualitative assessment based on image subtraction should be
performed to avoid false negative quantitative results  [54].When a cystic mass is evaluated, MR imaging may
demonstrate definitive enhancement that shows only equivocal enhancement on CT.
In a cystic lesion with only a small solid component, subtraction images may
again be used to better assess the presence of enhancement  [49]. Furthermore, even if detecting a
calcification is a limitation on MR imaging, it is however an advantage to
determine whether enhancement is present in a heavily calcified cyst on CT
given that it could be better appreciated  [45]. The combination of mural
irregularity and intense mural enhancement is a strong predictor of malignancy
in renal cystic lesions (see Figure 16). However, the appearance of benign and
malignant lesions may overlap  [72, 73].(a) Coronal haste T2
and (b) late postcontrast coronal T1 with fat saturation. Cystic lesion with an
enhancing nodule on the caudal aspect of the kidney. Renal cell carcinoma was
found at surgery.
(a)(b)


## 5.3.6. Other

Invasion
of Gerota’s fascia was diagnosed when continuity of the low signal intensity
line around the perinephric fat on T1-weighted images was disrupted by tumor.
Imaging of the ipsilateral adrenal gland and venous spread of tumors are out of
the scope of this chapter, thus the clinical setting of small renal tumors is
not found incidentally, as it is the case in the low probability of nodal
metastases in this stage of disease.On the other hand, advanced imaging techniques
led to improve the global accuracy for MDCT to adequately stage these clinical
aspects [38]. Hricak et al. [60] reported
accuracy rates for detecting adjacent organ invasion with MRI, although, in
their series, overstaging was caused by the presence of abnormal signal,
indistinct interface, and absence of a free fat plane between the tumor and the
adjacent organ.


## 5.4. Interpretation of Images and Surgical Criteria

When characterizing a renal mass, the major
question to be answeredis whether the mass represents a surgical or
nonsurgical lesion or, in some cases, if followup studies are
necessary.Magnetic resonance imaging allows an accurate
differentiation between solid and cystic masses, as a first approximation, but
angiomyolipomas are the only solid renal tumors that can be positively
characterized using MR. Nevertheless, Prasad et al. [74] reported that small renal medullary
tumors may be differentiated from the more common renal adenocarcinomas by
their central location and certain demographic characteristics. Furthermore,
some authors recently [75] analyzed the correlation between MR image features
and histopathological findings, giving value to subvoxel fat on chemical shift
imaging as a good correlation to clear cell type with a high specificity. These
authors reported that small size, peripheral location, low intratumoral signal
intensity on T2, and low level enhancement were also associated with low-grade
papillary carcinomas.Detection of macroscopicfat  is the key for diagnosing AML in the proper clinical
setting, and it is decisive because AML does not need to be surgically removed.
The diagnosis is made by demonstrating fat within a solid renal mass. One
pitfall of fat containing renal masses is the presence of a renal cell
carcinoma involving the perinephritic fat. This differentiation is easily made
if there is some calcification on CT, as do some RCC [29]. However, calcium is not always present in renal cell
carcinoma [76].The observation of apseudocapsule  surrounding a renal cell carcinoma is a sign of lack of perinephric fat invasion, and therefore it is
more likely to predict
that the tumor can be removed by nephron sparing, so partial nephrectomy or
simple enucleation may be indicated when a pseudocapsule is detected  [49, 62].The presence ofhaemorrhagic  products may obscure enhancement on dynamic postcontrast
T1 images, and it may also contribute to heterogeneity on T2 images. Thus, in
the setting of a T2 heterogeneous nonenhancing mass, careful followup after an
antibiotic trial may be a prudent recommendation to avoid nephrectomy of renal
abscess and to avoid misdiagnosis of a hemorrhagic renal cell carcinoma  [41].
Furthermore, a common occurrence of haemorrhage is described in patients with renal
cancer and/or in patients with renal insufficiency, and caution should be
exercised when evaluating haemorrhagic cystic lesions in these patients  [77].The most importantcriterion used in
differentiating surgical fromnonsurgical renalmasses is
the determination ofenhancement
[29]. Despite that, the lack of
enhancement of a renal lesion, particularly if small (<1 cm), is not
considered a sufficient criterion for excluding malignancy, as the haemorrhagic
lesions occur, and in this setting, T2-weighted images must be considered. Moreover,
it is important to combine the degree of enhancement with the morphologic
features of the lesion, such as homogeneity, wall thickening, and presence of
calcifications.When acystic  mass is evaluated, a surgical cyst can be suspected only if enhancement is
present. Calcification in a cystic renal mass is not as important in diagnosis
as the presence of associated enhancing soft-tissue elements  [78]. MR imaging
may depict additional septa, thickening of the wall and/or septa, or
enhancement, which may lead to an upgraded Bosniak cyst classification and can
affect case management   [31, 49].


## 5.5. Limitations

As with
all imaging techniques, it is extremely difficult with MR to determine whether
malignant tissue extends to adjacent normal tissue when strictly regular
margins are found because microscopic local invasion could have occurred
[61, 64]. Staging errors were
made because of limitations of the imaging technique: inability to detect
microscopic invasion of the perinephric fat, difficulty in differentiating
inflammatory changes from tumor infiltration, and insensitivity in
differentiating small collateral blood vessels from tumor extension in the
lymphatics  [67].There are
also limitations to the detection of a pseudocapsule by MR, mainly with
hypointense tumors because its detection may be less accurate on T2-weighted
owing to the lack of delineation of the surrounding rim, as is the case of some
papillary tumors. With SRM, partial volume averaging may obscure its
visualization, thus evaluation in three planes, and coronal, sagittal, or
oblique views are required to avoid this phenomenon on the upper and lower part
of the tumor [61]. Furthermore,
given that the pseudocapsule was also found in oncocytomas, it is not useful
for differentiating RCC from this benign solid tumor, and it cannot be used to
predict the nature of the lesion. But it can offer an additional value to the
performance of preoperative MR to stage renal tumors, aiding to make decisions
about the most appropriate surgical technique to employ.Concerning nodal staging, MR has the same
limitations as CT according to the nodal size over 1 cm in short-axis diameter,
being this an additional indication for the future use of new iron oxide-based
contrast agents on MR to improve specificity and accuracy in nodal staging
[49, 79].



---

*Source: 415848-2009-03-29.xml*