eMedicine Specialties > Radiology > Genitourinary

Angiomyolipoma, Kidney

Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, Consultant Radiologist and Honorary Professor, North Manchester General Hospital Pennine Acute NHS Trust, UK
Colm Boylan, MB, BCh, MRCP, FRCR, Assistant Professor of Radiology, McMaster University; Staff Radiologist, St Joseph's Hospital, Canada; Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute; Brendan Costello, MD, Clinical Director, Department of Urology, North Manchester General Hospital; Nigel Thomas, MBBS, Vice-Chair, Manchester (North) Research Ethics Committee; Honorary Lecturer, Visiting Professor, University of Salford, UK; Khalid Mahmood, MBBS, FCPS, Locum Appointment Training Specialist Registrar, Department of Radiology - Paediatric, Royal Liverpool (Alder Hey) Children's Hospital; Abdulrahim Salim Mohammad Bawazier, MB, MCh, FRCR, Assistant Consultant, Department of Radiology, King Abdul Aziz Medical City for National Guard Hospital

Updated: Feb 3, 2009

Introduction

Background

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

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.

Angiomyolipoma that is associated with tuberous sclerosis accounts for 20% of angiomyolipomas³ . The lesions are typically larger than isolated angiomyolipomas, and they are often bilateral and multiple. Angiomyolipomas occur in 80% of patients with tuberous sclerosis.³ The male-to-female sex distributions of angiomyolipoma in patients with tuberous sclerosis are nearly equal, but the prevalence is higher in women. Angiomyolipomas 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 angiomyolipomas are considered benign, rare cases that are possibly related to multicentric disease have been reported regarding extension into the renal vein, the inferior vena cava (IVC), or both; deposits in the regional lymph nodes have also been reported.^4,5,6

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.

Contrast-enhanced axial computed tomography scan obtained through the kidneys in the same patient as in Image above. The image shows patchy tumor enhancement, with displacement of part of the normal lateral aspect of the renal cortex.

Pathophysiology

Angiomyolipomas are benign tumors; the majority 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, vascular, and smooth muscle elements; however, angiomyolipomas are not hamartomas, 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 may be detected only at microscopy⁷ ; this limitation makes accurate characterization on the basis of radiologic findings alone impossible.

The blood vessels in angiomyolipomas 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.⁷ In addition, angiomyolipomas are slow growing and are truly space occupying; angiomyolipomas displace the renal parenchyma and distort 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.

A case of angiomyolipoma of renal sinus origin that was associated with an inferior vena cava tumor thrombus reaching the right atrium and Budd-Chiari syndrome has been reported. Extremely rarely, angiomyolipoma may present as Budd-Chiari syndrome. Because of potentially fatal cardiopulmonary embolism, surgical treatment (radical nephrectomy in combination with tumor thrombectomy) of these lesions is indicated even when they are asymptomatic.⁸

A malignant epithelioid angiomyolipoma has been reported in a 36-year-old male patient with tuberous sclerosis. The patient was diagnosed 20 years previously as having a bilateral renal tumor. The patient's left kidney was surgically resected when he was 34 years of age; the left renal tumor was pathologically diagnosed as classic angiomyolipoma and epithelioid angiomyolipoma. The patient died suddenly of cardiac arrest. At autopsy, massive necrosis of the right kidney tumor was found to be invading the inferior vena cava; the tumor was not hemorrhagic. Metastatic lesions were identified in the right lung, liver, diaphragm, and mesentery. Although microscopic examination revealed a benign epithelioid angiomyolipoma, this case proved to involve a malignancy, owing to the occurrence of distant metastases; the case demonstrated that p53 mutations are not always associated with malignant transformation in cases of epithelioid angiomyolipoma.⁹

Tumor-to-tumor metastases have been described with angiomyolipoma in association with a primary tumor in the breast, neuroendocrine carcinoma of the pancreas, and adenocarcinoma of the lung.

Frequency

United States

Angiomyolipoma occurs with a frequency of 0.3-3%.³ Isolated angiomyolipoma occurs sporadically. It is often solitary and accounts for 80% of the angiomyolipomas.¹ Angiomyolipoma that is associated with tuberous sclerosis accounts for 20% of angiomyolipomas.³

Mortality/Morbidity

Most cases of angiomyolipoma follow a benign course. If the diagnosis is certain, patients usually may be treated conservatively. Follow-up consists of ultrasonography. However, a subset of patients for whom severe hemorrhage is the first sign of angiomyolipoma may have a life-threatening condition. In cases in which patients have recurrent episodes of hemorrhage or massive bleeding, the tumor may be resected. Surgery, whether planned or performed emergently, has a certain risk of morbidity and mortality. Renal arterial embolization may be used to control hemorrhage.¹⁰

Sex

The male-to-female ratio for isolated angiomyolipoma ranges from 1:4 to 1:8.⁷ The male-to-female ratio for angiomyolipomas that are associated with tuberculous sclerosis ranges from 1:1 to 1:2.

Age

Most patients with isolated angiomyolipma are 27-72 years of age; the mean age is 43 years.⁷ The mean age of patients with angiomyolipoma that is associated with tuberous sclerosis is 17 years.

Presentation

Findings

The majority (60%) of angiomyolipomas are asymptomatic.⁷ In a case series that included patients with tuberous sclerosis, 82% of patients with a tumor ³ 4 cm were symptomatic, whereas only 23% of patients with tumors <4 cm were symptomatic.¹¹ With developments in abdominal ultrasonography, CT scanning, and magnetic resonance imaging (MRI), the incidental detection of asymptomatic angiomyolipomas has increased. 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; 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 women 40-50 years of age who have a sporadic unilateral tumor.³ In younger patients 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; 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.¹² Nephron conservation is of even greater importance in patients with tuberous sclerosis, in whom the tumors are often bilateral. In patients with recurrent episodes of hemorrhage or massive bleeding, the tumor may be resected. In acute, severe cases in which more conservative methods fail, radical nephrectomy may be the only option.

Partial nephrectomy is ideal for masses with a diameter <3 cm; partial nephrectomy may be possible for 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, because the symptomatic masses are often larger. Renal arterial embolization may be used to control hemorrhage.¹⁰

There is no universal agreement 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 angiomyolipomas 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.¹³ 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; renal cell carcinoma has the same appearance. In a case series with pathologically proven renal cell carcinomas, 32% of tumors with a diameter £ 3 cm were echogenic.¹⁴ 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.¹⁵

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 may contain fat. Rarely, but most importantly, fat may 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; the risk is probably very low in tumors that are <3 cm in diameter. Therefore, consideration of nephron-sparing surgery may be reasonable in patients with tuberous sclerosis who have lesions without demonstrable fat that are enlarging and are close to or >3 cm in diameter.

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

Differential Diagnoses

Other Problems to Be Considered

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

Radiography

Findings

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 Images below). 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).

Renal ultrasonogram depicting many tumors in the right kidney. The arrow marks an echogenic 1-cm lesion. (Image above and 2 below were obtained in the same patient.)

Selective right renal angiogram showing multiple avascular tumors. The tumors are small. (Images above and one below were obtained in the same patient.)

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 above were obtained in the same patient.)

Degree of Confidence

Plain radiography and IV urography are not useful in the diagnosis of angiomyolipoma, because neither modality has enough sensitivity to demonstrate fat within the tumor. Moreover, there are other causes of the occurrence of fat within renal masses, although such cases are rare. Multiple angiomyolipomas that distort the collecting system may be indistinguishable from polycystic disease.

False Positives/Negatives

A false-positive diagnosis may occur in cases involving other renal tumors containing fatty tissue. Only larger angiomyolipomas contain a sufficient amount of fat to be visible on plain radiographs.

Computed Tomography

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.

Computed tomography (CT) scan obtained in the same patient as in Image above. The image shows the technique of CT scanning-guided biopsy with a Tru-cut needle.

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.

Contrast-enhanced axial computed tomography scan obtained through the kidneys in the same patient as in Image above. The image shows patchy tumor enhancement, with displacement of part of the normal lateral aspect of the renal cortex.

Findings

Angiomyolipomas are usually well-marginated, cortical heterogeneous tumors with predominantly fatty attenuation; rarely, higher attenuation is seen in patients who have tumors with 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 may be useful. Attenuations of less than –20 Hounsfield units (HU) are widely accepted as confirming the presence of fat; 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; significant calcification should prompt the reconsideration of angiomyolipoma as a diagnosis.^{16,17,18,19,20,21,22}

The widespread availability of spiral and multisection CT scanning has made the characterization of angiomyolipoma tumors more accurate (see Images 5-10, 13).²³ 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. It is important to review the nonenhanced images for fat because 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.²⁴ 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; sensitivity is further limited by 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 calyces, contribute to the heterogeneity of these lesions. With CT scanning, particularly spiral and multisection CT scanning, optimal visualization of blood vessels and aneurysms in the vascular phase of the bolus of injected contrast agent may be achieved.

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.²⁵

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 alconcluded that biphasic helical CT scanning is useful in the differentiation of angiomyolipoma with minimal fat from renal cell carcinoma.²⁵ 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.²⁶  Angiomyolipomas had an early peak mean percentage of enhancement at 30 s, even though the tumors remained hypointense in comparison 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 radiologic characteristics of renal masses in 12 individuals with tuberous sclerosis complex (TSC).²⁷ Using serial CT scans 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 scans or MRIs and interval renal ultrasonography. Rapidly growing indeterminate masses were considered suspicious for renal cell carcinoma; biopsy or nephrectomy of the masses followed. A median of 4 years of comparative data were available.²⁷

Of the solid masses, 133 were typical angiomyolipomas and 52 were indeterminate. On follow-up, 3 indeterminate masses showed rapid growth (>0.5 cm/y), 1 of which proved to be a renal cell carcinoma on biopsy.The remaining 2 indeterminate masses were found to be minimal-fat angiomyolipomas; the remainder of the masses showed either no growth 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 recommended that individuals with TSC have annual radiologic follow-up of indeterminate renal masses.²⁷

A study assessed the diagnostic accuracy of ROI density measurements and pixel mapping for angiomyolipomas using CT scanning. An ROI threshold value of £10 units was found to have a very high specificity (100%), although the sensitivity was only 73%.²⁸ Pixel mapping was found to be more sensitive for recognizing small clusters of fat; however, the authors recommended that both methods be employed when the diagnosis of angiomyolipomas is suspected.

Simpson and Patel recommended that a diagnostic threshold of ≤-10 units be used for ROI density measurements; this measurement is convenient for analyzing large areas of suspected fat. In cases with small lucent areas or indeterminate values after ROI analysis, pixel mapping is recommended; the recommended discriminating threshold is a line of 4 pixels £ -10 units or a square of 4 pixels £ -10 units.

Degree of Confidence

The characterization of angiomyolipomas with CT is dependent on 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.²³

False Positives/Negatives

When negative attenuation values of less than 20 HU are recorded in renal tumors, angiomyolipomas may 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 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. A false-negative diagnosis is made in the 5% of patients with angiomyolipomas that contain only microscopically visible fat.

Magnetic Resonance Imaging

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. On T2-weighted images, the signal remains isointense relative to that of perinephric fat. However, areas of high signal intensity on T1-weighted images are not pathognomonic of fat, and blood and pockets of fluid of high protein content may have 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.²⁹ MRI studies may show the rare complication of regional lymph node involvement and invasion of the renal vein and IVC.

On MRI, changes in signal intensity that occur as a result of the intrinsic differences in the resonant frequencies of precessing protons are known as the chemical shift phenomenon.^30,31 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 may 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; 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.

Traditionally, angiomyolipoma has been diagnosed by comparing T1-weighted images incorporating frequency-selective fat suppression with T1-weighted images without frequency-selective fat suppression. Angiomyolipomas may also be diagnosed using opposed-phase chemical shift artifact. Israel et al investigated the use of opposed-phase chemical shift MRI in the diagnosis of renal angiomyolipoma.³²

Two types of edge artifacts have been described with chemical shift MRI: the chemical shift artifact and the India ink artifact. The chemical shift artifact is dependent on the receiver bandwidth and the shape and orientation of the fat–water interface. The India ink artifact is caused by the presence of fat and water protons within the same imaging voxel; this results in signal loss. The India ink artifact may be recognized on opposed-phase MRIs as a characteristic sharp black line at fat–water interfaces. Because the 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, not only in the frequency-encoding direction.

In addition, because most angiomyolipomas contain macroscopic fat, the India ink artifact appears 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; for that reason, the India ink artifact appears at the interface of the renal mass with perinephric fat when the mass is exophytic. Thus, the diagnosis of angiomyolipoma is indicated when the 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 remains the modality of choice for the detection of fat in renal tumors.

False Positives/Negatives

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

Ultrasonography

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 Images below). 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).

Renal ultrasonogram depicting many tumors in the right kidney. The arrow marks an echogenic 1-cm lesion. (Image above and 2 below were obtained in the same patient.)

Selective right renal angiogram showing multiple avascular tumors. The tumors are small. (Images above and one below were obtained in the same patient.)

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 above were obtained in the same patient.)

Findings

Renal angiomyolipomas are intensely echogenic and may cause acoustic shadowing (see Images 1-2, 11, 14).¹³ They are round or oval cortical tumors; they tend to be well circumscribed, with an echogenicity similar to that of the echogenic renal sinus. Because of their intense echogenicity, angiomyolipomas as small as a few millimeters in diameter may be identified. Less echogenic areas within the tumor are related to hemorrhage, necrosis, or dilated calyces. A reduction of echogenicity in angiomyolipomas is thought to be related to a decrease in the quantity of fat and to an increase in the prominence of myogenic components. Doppler ultrasonography may be used to confirm the rare complication of extension into the renal vein and the IVC.

Degree of Confidence

Marked echogenicity in a renal mass is not pathognomonic of angiomyolipomas. Other tumors, including renal cell carcinoma, may be hyperechoic. Tumors that are echogenic on ultrasonograms should be further investigated with CT scans, and the fat content of the tumor should be assessed.

False Positives/Negatives

A false-negative diagnosis may occur in cases involving a hemorrhagic or infarcted tumor, as well as in cases involving a tumor that has little fat. Also, a scar in the periphery of the kidney may be filled with intraperitoneal or omental fat; such a scar may have ultrasonographic features typical of an angiomyolipoma.

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 Imaging

Findings

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

Angiography

Findings

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

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

Degree of Confidence

CT scanning has superseded angiography as a diagnostic tool; CT scanning is noninvasive, its results are more reproducible, and it is 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 cases of angiomyolipoma is the lack of AV shunting on angiograms. However, AV shunting has been reported in cases of angiomyolipoma; therefore, it cannot be used to distinguish an angiomyolipoma from renal cell carcinoma. A reduction in the blood flow of a mass in response to epinephrine is characteristic of a benign lesion. The absence of this reduction in response to epinephrine was thought to be specific for malignant tumors, but such reduction in blood flow has been demonstrated to occur in cases of angiomyolipoma.

Intervention

Regarding percutaneous fine-needle biopsy or aspiration cytologic evaluation of renal angiomyolipomas, variable success has been reported with aspiration biopsy.³³ A diagnosis of angiomyolipoma is suggested if smooth muscle cells and mature fat cells are present 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 cases of tuberous sclerosis, because of its nephron-sparing potential in the acute setting. Angiomyolipomas are often multiple in tuberous sclerosis, and nephrectomy is undertaken only when other reasonable options fail or are impossible.

There is a risk of subsequent hemorrhage from any tumors remaining in the contralateral kidney. Many reports of successful embolization of bleeding angiomyolipomas have been published. Various embolic materials have been used, including ethanol, Gelfoam, and polyvinyl alcohol (PVA).³⁴ PVA appears to be the agent of choice; 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 has been reported.³⁵

CT-guided radiofrequency ablation has been used to ablate an angiomyolipoma in a patient with a solitary kidney. No complications occurred and no recurrence of angiomyolipoma was observed during the 12-month follow-up period.³⁶

Byrd et al studied laparoscopic cryoablation for 7 patients with renal angiomyolipoma. CT and MRI confirmed reduced lesion size and lack of regrowth in all 7 patients. The investigators therefore concluded that laparoscopic cryoablation is safe and feasible as a minimally invasive, nephron-sparing alternative treatment.³⁷

Medicolegal 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, 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; there were 25 false-negative findings (8.3%).³⁸

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

Multimedia

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 Images below). 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).

Media file 2: Renal ultrasonogram depicting many tumors in the right kidney. The arrow marks an echogenic 1-cm lesion. (Image above and 2 below were obtained in the same patient.)

Media file 3: Selective right renal angiogram showing multiple avascular tumors. The tumors are small. (Images above and one below were obtained in the same patient.)

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 above were obtained in the same patient.)

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.

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

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.

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

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).

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.

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.

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).

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).

Media file 14: Axial fat-suppressed magnetic resonance image obtained through the upper poles of the kidneys in the patient in Images 11-13. Image shows a lesion of high signal intensity 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.

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Keywords

angiomyolipoma of the kidney, benign mesenchymal tumor of the kidney, renal choristoma, renal hamartoma, isolated angiomyolipoma, tuberous sclerosis

Contributor Information and Disclosures

Author

Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, Consultant Radiologist and Honorary Professor, North Manchester General Hospital Pennine Acute NHS Trust, UK
Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR is a member of the following medical societies: American Association for the Advancement of Science, American Institute of Ultrasound in Medicine, British Medical Association, British Society of Interventional Radiology, Royal College of Physicians, Royal College of Physicians and Surgeons of the United States, Royal College of Radiologists, and Royal College of Surgeons of England
Disclosure: Nothing to disclose.

Coauthor(s)

Colm Boylan, MB, BCh, MRCP, FRCR, Assistant Professor of Radiology, McMaster University; Staff Radiologist, St Joseph's Hospital, Canada
Colm Boylan, MB, BCh, MRCP, FRCR is a member of the following medical societies: Royal College of Radiologists
Disclosure: Nothing to disclose.

Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute
Sumaira MacDonald, MBChB, PhD, MRCP, FRCR is a member of the following medical societies: British Medical Association, Royal College of Physicians, and Royal College of Radiologists
Disclosure: Nothing to disclose.

Brendan Costello, MD, Clinical Director, Department of Urology, North Manchester General Hospital
Brendan Costello, MD is a member of the following medical societies: British Medical Association
Disclosure: Nothing to disclose.

Nigel Thomas, MBBS, Vice-Chair, Manchester (North) Research Ethics Committee; Honorary Lecturer, Visiting Professor, University of Salford, UK
Disclosure: Nothing to disclose.

Khalid Mahmood, MBBS, FCPS, Locum Appointment Training Specialist Registrar, Department of Radiology - Paediatric, Royal Liverpool (Alder Hey) Children's Hospital
Disclosure: Nothing to disclose.

Abdulrahim Salim Mohammad Bawazier, MB, MCh, FRCR, Assistant Consultant, Department of Radiology, King Abdul Aziz Medical City for National Guard Hospital
Abdulrahim Salim Mohammad Bawazier, MB, MCh, FRCR is a member of the following medical societies: Royal College of Radiologists
Disclosure: Nothing to disclose.

Pharmacy Editor

Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand
Disclosure: Nothing to disclose.

Managing Editor

Arnold C Friedman, MD, FACR, Associate Chairman, Department of Radiology, University of Florida Health Science Center; Chief, Department of Radiology, Shands-Jacksonville Hospital
Arnold C Friedman, MD, FACR is a member of the following medical societies: American College of Radiology, American Institute of Ultrasound in Medicine, American Roentgen Ray Society, Association of University Radiologists, and Radiological Society of North America
Disclosure: Nothing to disclose.

CME Editor

Robert M Krasny, MD, Resolution Imaging Medical Corporation
Robert M Krasny, MD is a member of the following medical societies: American Roentgen Ray Society and Radiological Society of North America
Disclosure: Nothing to disclose.

Chief Editor

Eugene C Lin, MD, Consulting Radiologist, Virginia Mason Medical Center; Clinical Assistant Professor of Radiology, University of Washington School of Medicine
Eugene C Lin, MD is a member of the following medical societies: American College of Nuclear Medicine, American College of Radiology, Radiological Society of North America, and Society of Nuclear Medicine
Disclosure: Nothing to disclose.

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