AUTHOR AND EDITOR INFORMATION

Section 1 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

INTRODUCTION

Section 2 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Background

Angiomyolipoma is a benign renal neoplasm composed of fat, vascular, and smooth muscle elements. It has an incidence of about 0.3-3%, and 2 types are described: isolated angiomyolipoma and angiomyolipoma that is associated with tuberous sclerosis.¹

Isolated angiomyolipoma occurs sporadically, is often solitary, and accounts for 80% of the tumors.¹ The mean patient age at presentation of isolated angiomyolipoma is 43 years, and this neoplasm is about 4 times more common in women than in men.² Interestingly, 80% of the cases involve the right kidney.¹

Angiomyolipoma that is associated with tuberous sclerosis accounts for 20% of these tumors³; the lesions are typically larger than isolated angiomyolipomas, and they are often bilateral and multiple. Angiomyolipomas occur in 80% patients with tuberous sclerosis.³ The male-to-female sex distributions of angiomyolipoma in patients with tuberous sclerosis are nearly equal, but women outnumber men in terms of prevalence. Angiomyolipomas also occur in young women with lymphangiomyomatosis without other stigmata of tuberous sclerosis. Angiomyolipomas and lymphangiomyomatosis are sometimes considered the forme fruste of tuberous sclerosis. Although an angiomyolipoma is considered benign, rare cases, possibly related to multicentric disease, have been reported regarding extension into the renal vein and/or inferior vena cava (IVC), as well as deposits in the regional lymph nodes.⁴

Most small angiomyolipoma lesions are asymptomatic and found incidentally on imaging studies. As many as 40% are symptomatic³; these can cause a palpable abdominal mass, hematuria, or flank pain. The solitary sporadic tumors may cause an acute abdomen and shock as a result of spontaneous hemorrhage in the tumor. The demonstration of fatty attenuation in renal tumor on computed tomography (CT) scanning studies is virtually diagnostic of angiomyolipomas.

Pathophysiology

An angiomyolipoma is a benign tumor, and the majority of angiomyolipomas are small and single, although they vary in size from a few millimeters to larger than 20 cm. At gross pathologic examination, the tumors are round or lobulated and yellow to gray on cut sections because of their fat content. Angiomyolipomas are composed of fat and vascular and smooth muscle elements; however, these are not hamartomas by definition, because fat and smooth muscle are not normal constituents of the renal parenchyma. Rather, an angiomyolipoma is a choristoma; that is, as the name implies, it is composed of variable amounts of vascular, muscular, and fatty tissues. In 5% of the tumors, fatty elements can be detected only at microscopy⁵; this limitation makes accurate characterization on the basis of radiologic findings alone impossible.

The blood vessels in angiomyolipoma tumors frequently have an angiomatous arrangement, wherein the vessels are tortuous and thick walled. The blood vessels do not have elastic tissue, but they do have a disorganized adventitial cuff of smooth muscle. Angiomyolipomas do not have a capsule, but they often are well marginated; the majority (88%) extend through the renal capsule into the perinephric space.⁵ In addition, the tumor is slow growing and truly space occupying; an angiomyolipoma displaces the renal parenchyma and distorts the collecting system, sometimes causing renal destruction. The characteristic absence of elastic tissue in the tumor vessels predisposes the patient to aneurysm formation and spontaneous hemorrhage.

Frequency

United States

The frequency is 0.3-3%.³ Approximately 80% of cases occur in patients with tuberous sclerosis.³

Mortality/Morbidity

Most of the angiomyolipoma tumors have a benign course. If the diagnosis is certain, patients can usually be treated conservatively, and their follow-up can consist of ultrasonography. However, a subset of patients who present with severe hemorrhage as the first sign of the angiomyolipoma may have a life-threatening condition. In cases in which patients have recurrent episodes of hemorrhage or massive bleeding, the tumor can be resected. Surgery, whether planned or performed emergently, has a certain risk of morbidity and mortality. Renal arterial embolization may also be used to control hemorrhage.⁶

Sex

The male-to-female sex ratio for isolated angiomyolipoma is 1:4 to 1:8.⁵ The male-to-female sex ratio for angiomyolipomas that are associated with tuberculous sclerosis is 1:1 to 1:2.

Age

With isolated angiomyolipomas, most patients are aged 27-72 years, with a mean age of 43 years.⁵ With angiomyolipomas that are associated with tuberous sclerosis, the mean age of patients is 17 years.

Clinical Details

Findings

The majority (60%) of angiomyolipomas are asymptomatic.⁵ In a case series that included patients with tuberous sclerosis, 82% of patients with a tumor >4 cm were symptomatic, whereas only 23% of patients with tumors <4 cm were symptomatic.⁷ The development of abdominal ultrasonography, CT scanning, and magnetic resonance imaging (MRI) in recent years has increased the incidental detection of asymptomatic angiomyolipomas. Recent imaging experience shows that the presence of renal angiomyolipomas is more common than previously reported.

When angiomyolipomas are symptomatic, the main presenting symptoms are related to intratumoral or retroperitoneal hemorrhage. Symptoms related to hemorrhagic complications occur in 87% of patients, and hematuria is reported in 40%.³ A palpable abdominal mass is present in 47% of symptomatic patients.³ Shock resulting from massive retroperitoneal hemorrhage most commonly occurs in white females aged 40-50 years with a sporadic unilateral tumor.³ In younger patients diagnosed with angiomyolipomas and in those with associated tuberous sclerosis, tumor size and symptoms are significantly correlated.

Other symptoms reported in patients with associated tuberous sclerosis are flank or abdominal pain, weight loss, hypertension, fever, and nausea. In almost half of these cases, the tumors are bilateral, illustrating the importance of follow-up in this group of patients.³

Treatment

With advances in cross-sectional imaging, the diagnosis of renal angiomyolipoma can usually be established without surgery, and most of these tumors can be managed conservatively, particularly if they are asymptomatic. For suitable patients in whom the diagnosis of renal angiomyolipoma is not established with imaging findings, partial nephrectomy enables pathologic diagnosis with a minimal loss of function.⁸ Nephron conservation is of even greater importance in patients with tuberous sclerosis, where the tumors are often bilateral. In patients with recurrent episodes of hemorrhage or massive bleeding, the tumor can be resected. In the acute severe presentation, when more conservative methods fail, radical nephrectomy may be the only option.

Partial nephrectomy is ideal for masses with a diameter <3 cm but may be possible in masses with a diameter <5 cm that do not abut the hilum. Partial nephrectomy may be more suitable for the removal of indeterminate masses, rather than symptomatic masses, for the diagnosis of renal angiomyolipoma because the symptomatic masses are often larger. Renal arterial embolization can be used to control hemorrhage.⁶

No universal agreement exists regarding the follow-up of angiomyolipomas. Because the risk of hemorrhage is greater in masses with a diameter >4 cm, the restriction of follow-up to patients with multiple angiomyolipomas (ie, tuberous sclerosis) or those with tumors >4 cm may be reasonable. Small (<4 cm in diameter) solitary angiomyolipoma masses are the most common; they have little growth (<5% per year), are less likely to be complicated, and do not require follow-up unless the diagnosis is in doubt.

Preferred Examination

The preferred examinations are the following:

• Plain abdominal radiography
• Ultrasonography
• CT scanning
• Intravenous (IV) urography
• MRI
• Angiography
• Isotope renography and dimercaptosuccinic acid (DMSA) scanning
• Percutaneous renal biopsy
• Renal arterial embolization

Limitations of Techniques

The most characteristic ultrasonographic feature of an angiomyolipoma is its echogenicity; however, angiomyolipomas may also cause acoustic shadowing.⁹ The echogenic appearance of the tumor is thought to be related to its fat content and the presence of multiple tissue interfaces within it. However, this appearance in a mass is not pathognomonic for angiomyolipomas, and it is described in renal cell carcinoma. In a case series with pathologically proven renal cell carcinomas, 32% of tumors with a diameter <3 cm were echogenic.¹⁰ Not all angiomyolipomas are hyperechoic because the tumor constituents vary, and the fat content may be low. Hemorrhage, necrosis, and dilated calyces also may alter the echogenicity of the tumor.

In a mass with typical ultrasonographic features (see Ultrasound, Findings), fatty attenuation on plain abdominal radiographs, planar tomograms, and CT scans is virtually diagnostic of angiomyolipomas. However, an angiomyolipoma is not the only renal tumor that can contain fat. Rarely, but most importantly, fat can be found in a renal cell carcinoma because of the invasion of perinephric fat or metaplasia in the tumor. Intratumoral fat has also been found in renal lipomas, liposarcomas, Wilms tumors, teratomas, xanthogranulomatous pyelonephritis, and oncocytomas (engulfing of renal sinus fat).

If fat cannot be ultrasonically demonstrated within a renal mass in a patient with tuberous sclerosis, a diagnosis of renal cell carcinoma must be considered. Rapidly enlarging lesions or dystrophic calcification within a mass may suggest the diagnosis of angiomyolipoma. Although the natural history of renal cell carcinoma in tuberous sclerosis is not well known, the risk of metastases likely increases with the size of the tumor, and the risk is probably very low in tumors that are <3 cm in diameter. Therefore, the consideration of nephron-sparing surgery may be reasonable in patients with tuberous sclerosis who have lesions without demonstrable fat that are enlarging and close to or >3 cm in diameter.

Although routine follow-up seems reasonable in all patients with tuberous sclerosis, more intensive follow-up is necessary to establish the need for diagnostic resection in patients with tuberous sclerosis who have small masses (<3 cm in diameter) that are radiologically atypical for angiomyolipomas. Apart from this group with tuberous sclerosis, follow-up may be reasonably restricted to patients with sporadic tumors >4 cm in diameter who have a higher incidence of hemorrhagic complications.

DIFFERENTIALS

Section 3 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Other Problems to Be Considered

Renal cell carcinoma
Teratoma
Oncocytoma
Xanthogranulomatous pyelonephritis
Wilms tumor
Renal lipoma
Renal and retroperitoneal liposarcoma

RADIOGRAPH

Section 4 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

Angiomyolipomas of sufficient size may be appreciated on a plain abdominal radiograph or an IV urogram. A large, extrarenal, exophytic component is present in 25% of cases and may be visualized with both a plain abdominal radiograph and an IV urogram. If planar tomographic images are obtained before the administration of IV contrast material and if a large quantity of fat is present within the tumor, radiolucency may be evident. This finding suggests the diagnosis of angiomyolipoma and is present in <10% of cases. With multiple large angiomyolipomas, particularly those in patients with tuberous sclerosis, an IV urogram may demonstrate distortion of the renal collecting system that is indistinguishable from polycystic renal disease. On CT scans, visible calcification within the tumor is present in as many as 6% of cases.

Degree of Confidence

Plain radiography and IV urography are not sensitive techniques in the diagnosis of angiomyolipoma, because both modalities do not have enough sensitivity to demonstrate fat within the tumor. Moreover, other causes of fat within renal masses exist, although these causes are rare. Multiple angiomyolipomas that distort the collecting system can be indistinguishable from polycystic disease.

False Positives/Negatives

A false-positive diagnosis can occur with other renal tumors containing fatty tissue. Only larger angiomyolipomas contain sufficient fat to be visible on plain radiographs.

CT SCAN

Section 5 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

Although an angiomyolipoma is usually a well-marginated cortical heterogeneous tumor with predominantly fatty attenuation, in rare cases, it can have higher attenuation in patients who have tumors with a minimal fat content. The average attenuation depends on the relative proportions of fat and other soft tissue in the angiomyolipoma. In small masses, fat may be averaged out with region-of-interest (ROI) circles, and pixel maps can be useful. Attenuations of less than –20 Hounsfield units (HU) are widely accepted to confirm the presence of fat, and this finding virtually confirms the diagnosis of angiomyolipoma. Nonfatty angiomyolipomas are rare, but renal cell carcinoma may engulf fat. Angiomyolipomas may calcify and cause the HU value to increase out of the range for fat. However, this effect is rare, and significant calcification should prompt the reconsideration of angiomyolipoma as a diagnosis.^{11, 12}

The widespread availability of spiral and multisection CT scanning has made the characterization of angiomyolipoma tumors more accurate.¹³ Nonenhanced spiral CT scanning with 5-mm collimation and a pitch of 1.5 that is followed by nephrographic contrast-enhanced phase imaging (120-180 s after injection) is ideal for characterization. Reviewing the nonenhanced images for fat is important because the contrast enhancement averages out the appearance of fat. In small tumors, thin reconstructions may be necessary to optimize fat sensitivity.

In older, nonspiral CT scanners, contiguous 5-mm transaxial sections, imaged both before and after contrast enhancement, are used in the typical examination.¹⁴ In smaller lesions, even thinner sections (1.5- or 3.0-mm sections) may be required to improve fat sensitivity. Achieving good fat sensitivity is more difficult with slower scanning times and the limitations of breath-hold techniques.

Spiral CT scanning enables continuous scanning of complete anatomic volumes, which is particularly important for the characterization of small angiomyolipomas. Enhancement varies because of the vascular and muscle components. Hemorrhagic, necrotic, and cystic areas, as well as distorted or dilated calices, contribute to the heterogeneity of these lesions. CT scanning, particularly spiral and multisection CT scanning, can optimize visualization of blood vessels and aneurysms in the vascular phase of the bolus of injected contrast agent.

AML With Minimal Fat: Differentiation From Renal Cell Carcinoma at Biphasic Helical CT

Differentiating angiomyolipoma with minimal fat from a renal cell carcinoma on biphasic CT scanning may be problematic. Kim et al classified the renal tumor enhancement patterns following the administration of iodinated contrast as follows¹⁵:

An early washout pattern was considered to be present when a tumor showed peak enhancement in the corticomedullary phase and then demonstrated a washout of at least 20 HU in the early excretory phase; a gradual enhancement pattern was considered to be present when the tumor attenuation value in the early excretory phase was at least 20 HU greater than it was in the corticomedullary phase; and a prolonged enhancement pattern was considered to be present when the difference in tumor attenuation between the corticomedullary and early excretory phases ranged from 20 to 20 HU. The authors found that the most significant predictors of angiomyolipoma with minimal fat were homogeneous enhancement and a prolonged enhancement pattern.

The positive and negative predictive values of homogeneous enhancement for differentiating angiomyolipoma with minimal fat from renal cell carcinoma were 83% and 94%, respectively. For the prolonged enhancement pattern, the positive and negative predictive values were 65% and 88%, respectively. Kim et al concluded that biphasic helical CT scanning is useful in the differentiation of angiomyolipoma with minimal fat from renal cell carcinoma.¹⁵ Homogeneity of tumor enhancement and a prolonged enhancement pattern are the most valuable CT scan findings for differentiating between angiomyolipoma with minimal fat and renal cell carcinoma. Other findings, including tumor attenuation on unenhanced scans, amount of tumor enhancement, intratumoral calcification, and patient sex provide supplementary information.

Scialpi et al found quantitative analysis of signal intensity variations during dynamic contrast-enhanced MRI with fat suppression useful in the characterization of small renal lesions.¹⁶ Angiomyolipomas had an early peak mean percentage of enhancement at 30 s, even though the tumors remained hypointense when compared with the renal cortex. This finding was caused by the shift from low signal intensity on baseline fat-suppressed sequences to high signal intensity for lesion vascularity as soon as the contrast material (gadolinium) was injected. Subsequently, a vascular washout of gadolinium demonstrated a decline in the percentage of enhancement to values similar to that of hypovascular renal cell carcinomas from 90 to 210 s.

Patel et al studied the radiological characteristics of renal masses in 12 individuals with tuberous sclerosis complex (TSC).¹⁷ Using serial CT scan studies to examine how renal cell carcinoma may be differentiated from other masses, the authors measured the CT scanning density of all masses and categorized the masses as simple cysts, complex cysts, angiomyolipomas, or indeterminate solid masses. Subjects underwent regular follow-up with repeat CT scan studies or MRIs and interval renal ultrasonography. Rapidly growing indeterminate masses were considered suspicious for renal cell carcinoma, and biopsy or nephrectomy of the masses followed. A median of 4 years of comparative data were available with the following results: of the solid masses, 133 were typical angiomyolipomas and 52 were indeterminate. On follow-up, there were 3 indeterminate masses that showed rapid growth (>0.5 cm/y), 1 of whichprovedtobea renalcellcarcinoma onbiopsy.The remaining 2 indeterminate masses were found to be minimal-fat angiomyolipomas; the remainder of the masses showed no or slow growth. The authors concluded that many renal masses associated with TSC are radiologically indeterminate. A growth threshold of >0.5cm/y identified the only renal cell carcinoma in the study (0.5% of all masses). Patel et al recommend that individuals with TSC have annual radiological follow-up of indeterminate renal masses.

In a study of the diagnostic accuracy of ROI density measurements and pixel mapping for angiomyolipomas using CT scanning, the ROI threshold value of ≤−10 units had a very high specificity (100%), whereas the sensitivity was only 73%.¹⁸ Pixel mapping was found to be more sensitive for recognizing small clusters of fat; however, the authors recommended both methods when the diagnosis of angiomyolipomas is suspected. In addition, Simpson and Patel recommended a diagnostic threshold of ≤−10 units should be used for the ROI density measurement; this measurement is convenient for analysing large areas of suspected fat. In cases with small lucent areas or indeterminate values after ROI analysis, pixel mapping is recommended with the discriminating thresholds as a line of 4 pixels ≤−10 units or a square of 4 pixels ≤−10 units.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble movingorstraightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Degree of Confidence

CT scan study characterization of angiomyolipomas is dependent on its spatial resolution and accurate determination of attenuation values; newer spiral scanners meet these criteria. As a result, CT scanning is highly accurate in the characterization and diagnosis of angiomyolipoma lesions.¹³

False Positives/Negatives

When negative attenuation values of less than –20 HU are recorded in renal tumors, angiomyolipomas can be reliably diagnosed in the appropriate clinical setting, and the diagnosis of a renal cell carcinoma can generally be ruled out. However, isolated reports of renal cell carcinoma with demonstrable fat content have appeared in the recent literature. These renal carcinomas may entrap surrounding perinephric fat or undergo fatty change because of metaplasia. Intratumoral fat is also reported in Wilms tumors, oncocytoma, xanthogranulomatous pyelonephritis, renal and retroperitoneal liposarcoma, and teratoma. With angiomyolipomas, a false-negative diagnosis may occur in the 5% of patients with angiomyolipomas who have only microscopically visible fat within their tumors.

MRI

Section 6 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

The characteristic appearances of angiomyolipomas with MRI include variable areas of high signal intensity within the tumor on both T1-weighted and T2-weighted images. On a nonenhanced T1-weighted image, high signal intensity is present because of the fat content. The signal remains isointense relative to that of perinephric fat on T2-weighted images. However, areas of high signal intensity on T1-weighted images are not pathognomonic of fat, and blood and pockets of fluid with a high protein content can cause a similar appearance. Intratumoral fat is best demonstrated with fat-suppression techniques. The in-phase and out-of-phase T1-weighted imaging technique is extremely sensitive to small quantities of fat.¹⁹ MRI studies may show the rare complication of regional lymph node involvement and invasion of the renal vein and IVC.

On MRI studies, changes in signal intensity as a result of the intrinsic differences in the resonant frequencies of precessing protons are known as the chemical shift phenomenon.^{20, 21} This phenomenon has been used as a diagnostic tool; by demonstrating the inherent differences in resonant frequencies of fatty tissue and water, lipid-containing tumors can be identified. Although some success has been achieved in the diagnosis of fatty tumors such as angiomyolipomas, chemical shift (in-phase and out-of-phase imaging) probably is less likely to depict fat in an angiomyolipoma, because fat is macroscopic, and chemical shift imaging is more likely to demonstrate microscopic fat, such as that in a renal cell carcinoma. A standard chemically selective fat-saturated technique better depicts the signal-intensity dropout of fat in an angiomyolipoma because this technique better depicts macroscopic fat.

Comparing T1-weighted images both with and without frequency-selective fat suppression has traditionally been used to diagnose angiomyolipoma. However, angiomyolipomas can also be diagnosed using opposed-phase chemical shift artifact. Israel et al looked at the use of opposed-phase chemical shift MRI in the diagnosis of renal angiomyolipoma.²²

Two types of edge artifacts have been described with chemical shift MRI: the chemical shift artifact and India ink artifact. The chemical shift artifact is dependent on the receiver bandwidth and the shape and orientation of the fat–water interface. Alternatively, the India ink artifact is due to the presence of fat and water protons within the same imaging voxel, resulting in signal loss; this artifact can be recognized on opposed-phase MRIs as a characteristic sharp black line at fat–water interfaces. Because India ink artifact is a result of fat- and water-proton phase cancellation in all directions, the artifact occurs along the entire border of fat–water interface and not only in the frequency-encoding direction.

In addition, because most angiomyolipomas contain macroscopic fat, the India ink artifact will appear at all interfaces of the tumor with the kidney or at the interfaces of the fatty and nonfatty portions of the mass. Other renal masses do not contain macroscopic fat, and for that reason, the India ink artifact would appear at the interface of the renal mass with perinephric fat when the mass is exophytic. Thus, the diagnosis of angiomyolipoma is indicated when India ink artifact is present at a renal mass–kidney interface or within a renal mass.

Degree of Confidence

Because of its wide availability, CT scanning remains the examination of choice for detection of fat in renal tumors.

False Positives/Negatives

Hemorrhagic cysts and cysts with a high protein content may mimic fat within renal tumors.

ULTRASOUND

Section 7 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

Angiomyolipomas are intensely echogenic renal tumors and can cause acoustic shadowing.⁹ They are round or oval cortical tumors, and they tend to be well circumscribed, with an echogenicity similar to that of the echogenic renal sinus. Because of their intense echogenicity, angiomyolipoma tumors as small as a few millimeters in diameter can be identified. Less echogenic areas within the tumor are related to hemorrhage, necrosis, or dilated calyces. The reduced echogenicity in angiomyolipomas is thought to be related to a smaller quantity of fat and its more prominent myogenic components. Doppler ultrasonography may be used to confirm the rare complication of extension into the renal vein and IVC.

Degree of Confidence

Marked echogenicity in a renal mass is not pathognomonic of angiomyolipomas. Other tumors, including renal cell carcinoma, can be hyperechoic. Echogenic tumors depicted on ultrasonograms should be further characterized with CT scans, and the fat content in the tumor should be assessed.

False Positives/Negatives

A false-negative diagnosis can occur with a hemorrhagic and/or infarcted tumor and tumors that have little fat. Also, a scar in the periphery of the kidney may be filled in with intraperitoneal or omental fat, which can cause the typical ultrasonographic features of angiomyolipomas.

The differential diagnosis of a hyperechoic renal tumor includes the following:

• Angiomyolipoma
• Renal cell carcinoma
• Liposarcoma
• Atypical Wilms tumor
• Lipoma
• Oncocytoma
• Cavernous hemangioma
• Renal infarction
• Renal sinus
• Lipomatosis
• Fat-filled postoperative renal cortical defects

See Other Problems to Be Considered, above.

NUCLEAR MEDICINE

Section 8 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

The role of nuclear medicine is restricted to isotope renography with technetium-99m (^99mTc) mercaptoacetythiglycine (MAG3) to assess relative renal function before nephron-sparing surgery. Also, ^99mTc DMSA scanning may be used as a functional examination to assess differential renal function; ^99mTc DMSA may be more useful in differentiating true masses and pseudomasses in cases in which other imaging findings are equivocal.

ANGIOGRAPHY

Section 9 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Findings

Before the advent of ultrasonography, CT scanning, and MRI, much effort went into identifying angiographic characteristics that allow the distinction of angiomyolipomas from renal cell carcinoma. Among radiologists, controversy regarding the reliability of angiography in the differentiation of these tumors exists.

About 95% of angiomyolipomas are hypervascular, with enlarged interlobar and interlobular arteries. The intratumoral arteries are tortuous, irregular, and aneurysmal. Venous pooling exists, with a sunburst, whorled, and onion-peel appearance. Usually, no arteriovenous (AV) shunting is present. The presence of multisacculated pseudoaneurysms, a sunburst appearance of the capillary nephrogram, an onionskin appearance of the peripheral vessels in the venous phase, and the absence of AV shunting suggest an angiomyolipoma.

Degree of Confidence

CT scanning has now superseded angiography as a diagnostic tool; CT scanning is noninvasive, more reproducible, and less dependent on operator technique and interpretation. Currently, angiography is reserved for use in transcatheter embolization to control bleeding caused by angiomyolipomas.

False Positives/Negatives

A significant finding in angiomyolipomas is the lack of AV shunting on angiograms. However, AV shunting in angiomyolipomas has been described; therefore, it cannot be used to distinguish an angiomyolipoma from a renal cell carcinoma. A reduction in the blood flow of a mass in response to epinephrine is a feature of a benign lesion. The absence of this reduction in response to epinephrine was thought to be specific for malignant tumors, but this phenomenon was demonstrated in angiomyolipomas.

INTERVENTION

Section 10 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Regarding percutaneous fine-needle biopsy or aspiration cytologic evaluation of renal angiomyolipomas, variable success is reported with aspiration biopsy.²³ A diagnosis of angiomyolipoma is suggested if smooth muscle cells and mature fat cells can be identified in the aspirated renal material. Fine-needle biopsy generally is not considered to be a reliable technique in the characterization of renal masses.

The inherent weakness in the walls of the vessels of angiomyolipomas and the presence of pseudoaneurysms in these tumors make them particularly prone to spontaneous hemorrhage. Transcatheter arterial embolization (TCE) is an attractive alternative to surgery, particularly in tuberous sclerosis, because of its nephron-sparing potential in the acute situation. Angiomyolipomas are often multiple in tuberous sclerosis, and nephrectomy is undertaken only when other reasonable options fail or are impossible.

A risk of subsequent hemorrhage from any remaining tumors in the contralateral kidney exists. Many reports of successful embolization of bleeding angiomyolipomas are published. Various embolic materials have been used, including ethanol, Gelfoam (Pharmacia & Upjohn Company, Kalamazoo, Mich), and polyvinyl alcohol (PVA).²⁴ PVA appears to be the agent of choice because ethanol administration can be painful, and with Gelfoam, the effects are temporary and the vessels recanalize. A reduction in the size of angiomyolipomas after embolization is reported.

Medical/Legal Pitfalls

• In angiomyolipomas, smooth muscle cells may have bizarre morphologic features. The rare sarcomatoid renal cell carcinoma may have atypical cells that are indistinguishable from the smooth muscle cells in angiomyolipomas. Such cells can usually be differentiated from other varieties of renal cell carcinoma and oncocytoma.

• In a case series,²⁵ 14 of 301 (4.7%) ultrasonographically guided fine-needle aspirations of renal masses resulted in false-positive findings that were misinterpreted as those of a relatively well-differentiated adenocarcinoma. Another 25 (8.3%) findings were false negative.

• The complications of renal arterial embolization are postinfarction syndrome and standard angiography, as well as nontargeted embolization of healthy parts of the kidney.

MULTIMEDIA

Section 11 of 12  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

Media file 1:  Renal ultrasonogram obtained in a 12-year-old boy with known tuberous sclerosis. Note the multiple echogenic tumors of varying sizes in both kidneys (see also Image 2). This oblique sagittal scan through the left kidney shows a 4-cm echogenic mass (arrow) on the inferior aspect of the kidney that anteriorly displaces the renal sinus (S).
	View Full Size Image
Media type:  Image

Media file 2:  Renal ultrasonogram depicting many tumors in the right kidney. The arrow marks an echogenic 1-cm lesion. (Images 1, 3-4 were obtained in the same patient.)
	View Full Size Image
Media type:  Image

Media file 3:  Selective right renal angiogram showing multiple avascular tumors. The tumors are small. (Images 1-2, 4 were obtained in the same patient.)
	View Full Size Image
Media type:  X-RAY

Media file 4:  Selective left renal angiogram showing 2 tumors, which are larger than those in Image 3. The final diagnosis was multiple renal angiomyolipomas in a patient with tuberous sclerosis. (Images 1-3 were obtained in the same patient.)
	View Full Size Image
Media type:  X-RAY

Media file 5:  Computed tomography scan obtained in a 15-year-old boy with tuberous sclerosis (under surveillance). The image shows rapid growth in a right renal lesion with mixed attenuation. The final diagnosis was tuberous sclerosis–associated angiomyolipoma.
	View Full Size Image
Media type:  CT

Media file 6:  Computed tomography (CT) scan obtained in the same patient as in Image 5. The image shows the technique of CT scanning–guided biopsy with a Tru-cut needle.
	View Full Size Image
Media type:  CT

Media file 7:  Nonenhanced axial computed tomography scan through the kidneys. The image shows a space-occupying lesion of mixed attenuation interspersed with areas of fat attenuation. The final diagnosis was sporadic angiomyolipoma.
	View Full Size Image
Media type:  CT

Media file 8:  Contrast-enhanced axial computed tomography scan obtained through the kidneys in the same patient as in Image 7. The image shows patchy tumor enhancement, with displacement of part of the normal lateral aspect of the renal cortex.
	View Full Size Image
Media type:  CT

Media file 9:  Nonenhanced computed tomography (CT) scan obtained in a 28-year-old pregnant woman (at 26 weeks' gestation) who presented with sudden-onset right upper quadrant abdominal pain and hypotension. An ultrasonogram (not shown) depicted a complex mass that replaced the right kidney; this finding was suggestive of a tumor that had ruptured into the perinephric space and retroperitoneum. This CT scan, obtained through the mid portions of the kidneys, shows a mixed-attenuation mass interspersed with areas of low attenuation (fat) and areas of high attenuation (blood).
	View Full Size Image
Media type:  CT

Media file 10:  Enhanced computed tomography scan obtained in the same patient as in Image 9. The image shows patchy renal cortical enhancement around the tumor. Note the retroperitoneal stranding behind the tumor. The final diagnosis was spontaneous rupture of an angiomyolipoma.
	View Full Size Image
Media type:  CT

Media file 11:  Ultrasonogram obtained in a 48-year-old man who presented with dyspepsia and right upper quadrant discomfort. The patient was referred for a gallbladder ultrasonogram. The gallbladder was normal, but a solid 18-mm mass was present in the upper pole of the right kidney; it was isoechoic relative to the renal sinus.
	View Full Size Image
Media type:  Image

Media file 12:  Part of an intravenous urogram series obtained in the same patient as in Images 11, 13-14. The radiograph shows a hypoattenuating exophytic mass (arrow).
	View Full Size Image
Media type:  X-RAY

Media file 13:  Nonenhanced axial computed tomography scan obtained through the upper pole of the kidneys in the same patient as in Images 11-12 and 14. Image shows a hypoattenuating (15 HU) exophytic mass (arrow).
	View Full Size Image
Media type:  CT

Media file 14:  Axial fat-suppressed magnetic resonance imaging obtained through the upper poles of the kidneys in the patient in Images 11-13. Image shows a high signal-intensity lesion with tiny areas of intratumoral fat (which had low signal intensity on fat-suppressed images). The final diagnosis was renal cell carcinoma with fatty metamorphosis.
	View Full Size Image
Media type:  MRI

REFERENCES

Section 12 of 12

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Multimedia
• References

1. Rakowski SK, Winterkorn EB, Paul E, Steele DJ, Halpern EF, Thiele EA. Renal manifestations of tuberous sclerosis complex: Incidence, prognosis, and predictive factors. Kidney Int. Nov 2006;70(10):1777-82. [Medline].
2. Blute ML, Malek RS, Segura JW. Angiomyolipoma: clinical metamorphosis and concepts for management. J Urol. Jan 1988;139(1):20-4. [Medline].
3. Cohen MD. Genitourinary tumors. In: Cohen MD, ed. Imaging of Children With Cancer. St Louis, Mo: Mosby Year Book; 1992:552-88.
4. Baert J, Vandamme B, Sciot R, et al. Benign angiomyolipoma involving the renal vein and vena cava as a tumor thrombus: case report. J Urol. Apr 1995;153(4):1205-7. [Medline].
5. Dähnert W. Radiology Review Manual. 4^th ed. Baltimore, Md: Lippincott Williams & Wilkins; 1999:761.
6. Earthman WJ, Mazer MJ, Winfield AC. Angiomyolipomas in tuberous sclerosis: subselective embolotherapy with alcohol, with long-term follow-up study. Radiology. Aug 1986;160(2):437-41. [Medline].
7. Steiner MS, Goldman SM, Fishman EK, Marshall FF. The natural history of renal angiomyolipoma. J Urol. Dec 1993;150(6):1782-6. [Medline].
8. Kutikov A, Fossett LK, Ramchandani P, Tomaszewski JE, Siegelman ES, Banner MP. Incidence of benign pathologic findings at partial nephrectomy for solitary renal mass presumed to be renal cell carcinoma on preoperative imaging. Urology. Oct 2006;68(4):737-40.
9. Siegel CL, Middleton WD, Teefey SA, McClennan BL. Angiomyolipoma and renal cell carcinoma: US differentiation. Radiology. Mar 1996;198(3):789-93. [Medline].
10. Forman HP, Middleton WD, Melson GL, McClennan BL. Hyperechoic renal cell carcinomas: increase in detection at US. Radiology. Aug 1993;188(2):431-4. [Medline].
11. Helenon O, Merran S, Paraf F, et al. Unusual fat-containing tumors of the kidney: a diagnostic dilemma. Radiographics. Jan-Feb 1997;17(1):129-44. [Medline].
12. Strotzer M, Lehner KB, Becker K. Detection of fat in a renal cell carcinoma mimicking angiomyolipoma. Radiology. Aug 1993;188(2):427-8. [Medline].
13. Silverman SG, Pearson GD, Seltzer SE, Polger M, Tempany CM, Adams DF, et al. Small (< or = 3 cm) hyperechoic renal masses: comparison of helical and convention CT for diagnosing angiomyolipoma. AJR Am J Roentgenol. Oct 1996;167(4):877-81. [Medline].
14. Bosniak MA, Megibow AJ, Hulnick DH, et al. CT diagnosis of renal angiomyolipoma: the importance of detecting small amounts of fat. AJR Am J Roentgenol. Sep 1988;151(3):497-501. [Medline].
15. Kim JK, Park SY, Shon JH, Cho KS. Angiomyolipoma with minimal fat: differentiation from renal cell carcinoma at biphasic helical CT. Radiology. Mar 2004;230(3):677-84. [Medline]. [Full Text].
16. Scialpi M, Di Maggio A, Midiri M, Loperfido A, Angelelli G, Rotondo A. Small renal masses: assessment of lesion characterization and vascularity on dynamic contrast-enhanced MR imaging with fat suppression. AJR Am J Roentgenol. Sep 2000;175(3):751-7. [Medline]. [Full Text].
17. Patel U, Simpson E, Kingswood JC, Saggar-Malik AK. Tuberose sclerosis complex: analysis of growth rates aids differentiation of renal cell carcinoma from atypical or minimal-fat-containing angiomyolipoma. Clin Radiol. Jun 2005;60(6):665-73; discussion 663-4. [Medline].
18. Simpson E, Patel U. Diagnosis of angiomyolipoma using computed tomography-region of interest < or =-10 HU or 4 adjacent pixels < or = -10 HU are recommended as the diagnostic thresholds. Clin Radiol. May 2006;61(5):410-6. [Medline].
19. Martin J, Puig J, Falco J, et al. Hyperechoic liver nodules: characterization with proton fat-water chemical shift MR imaging. Radiology. May 1998;207(2):325-30. [Medline].
20. Hood MN, Ho VB, Smirniotopoulos JG, Szumowski J. Chemical shift: the artifact and clinical tool revisited. Radiographics. Mar-Apr 1999;19(2):357-71. [Medline]. [Full Text].
21. Kido T, Yamashita Y, Sumi S, et al. Chemical shift GRE MRI of renal angiomyolipoma. J Comput Assist Tomogr. Mar-Apr 1997;21(2):268-70. [Medline].
22. Israel GM, Hindman N, Hecht E, Krinsky G. The use of opposed-phase chemical shift MRI in the diagnosis of renal angiomyolipomas. AJR Am J Roentgenol. Jun 2005;184(6):1868-72. [Medline].
23. Zardawi IM. Renal fine needle aspiration cytology. Acta Cytol. Mar-Apr 1999;43(2):184-90. [Medline].
24. Rimon U, Duvdevani M, Garniek A, Golan G, Bensaid P, Ramon J. Ethanol and polyvinyl alcohol mixture for transcatheter embolization of renal angiomyolipoma. AJR Am J Roentgenol. Sep 2006;187(3):762-8. [Full Text].
25. Juul N, Torp-Pedersen S, Grønvall S, Holm HH, Koch F, Larsen S. Ultrasonically guided fine needle aspiration biopsy of renal masses. J Urol. Apr 1985;133(4):579-81. [Medline].
26. Bret PM, Bretagnolle M, Gaillard D, et al. Small, asymptomatic angiomyolipomas of the kidney. Radiology. Jan 1985;154(1):7-10. [Medline].
27. Byrd GF, Lawatsch EJ, Mesrobian HG, Begun F, Langenstroer P. Laparoscopic cryoablation of renal angiomyolipoma. J Urol. Oct 2006;176(4 Pt 1):1512-6; discussion 1516.
28. Chiang IC, Jang MY, Tsai KB, Hsieh TJ. Huge renal lipoma with prominent hypervascular non-adipose elements. Br J Radiol. Oct 2006;79(946):e148-51.
29. Davidson AJ, Davis CJ Jr. Fat in renal adenocarcinoma: never say never. Radiology. Aug 1993;188(2):316. [Medline].
30. Ditonno P, Smith RB, Koyle MA, et al. Extrarenal angiomyolipomas of the perinephric space. J Urol. Feb 1992;147(2):447-50. [Medline].
31. Kurosaki Y, Tanaka Y, Kuramoto K, Itai Y. Improved CT fat detection in small kidney angiomyolipomas using thin sections and single voxel measurements. J Comput Assist Tomogr. Sep-Oct 1993;17(5):745-8. [Medline].
32. Lemaitre L, Claudon M, Dubrulle F, Mazeman E. Imaging of angiomyolipomas. Semin Ultrasound CT MR. Apr 1997;18(2):100-14. [Medline].
33. Lemaitre L, Robert Y, Dubrulle F, et al. Renal angiomyolipoma: growth followed up with CT and/or US. Radiology. Dec 1995;197(3):598-602. [Medline].
34. Mitchell TL, Pippin JJ, Devers SM, et al. Incidental detection of preclinical renal tumors with electron beam computed tomography: report of 26 consecutive operated patients. J Comput Assist Tomogr. Nov-Dec 2000;24(6):843-5. [Medline].
35. Paivansalo M, Lahde S, Hyvarinen S, et al. Renal angiomyolipoma. Ultrasonographic, CT, angiographic, and histologic correlation. Acta Radiol. May 1991;32(3):239-43. [Medline].
36. Tello R, Blickman JG, Buonomo C, Herrin J. Meta analysis of the relationship between tuberous sclerosis complex and renal cell carcinoma. Eur J Radiol. May 1998;27(2):131-8.
37. Wills JS. Management of small renal neoplasms and angiomyolipoma: a growing problem. Radiology. Dec 1995;197(3):583-6. [Medline].

Article Last Updated: May 9, 2007