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.
Isolated angiomyolipoma occurs sporadically. It is often solitary and accounts for 80% of the angiomyolipomas.1 The mean age at presentation of patients with isolated angiomyolipoma is 43 years; 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 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.
Most small angiomyolipoma lesions are asymptomatic and are found incidentally on imaging studies. As many as 40% are symptomatic3 ; these may may manifest themselves as a palpable abdominal mass, and they may cause hematuria or flank pain. 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.
Related eMedicine topics:
Tuberous Sclerosis (from Radiology)
Tuberous Sclerosis (from Neurology)
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
More on Angiomyolipoma, Kidney |
 Overview: Angiomyolipoma, Kidney |
| Imaging: Angiomyolipoma, Kidney |
| Follow-up: Angiomyolipoma, Kidney |
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| References |
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References
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Further Reading
Related eMedicine topics
Kidney, Lymphoma
Autosomal Dominant Polycystic Kidney Disease
Autosomal Recessive Polycystic Kidney Disease
Guidelines and clinical studies
ACR Appropriateness Criteria® indeterminate renal masses. American College of Radiology. 1996 (revised 2007). 7 pages. NGC:005996
Efficacy and Safety of RAD001 in Patients Aged 18 and Over With Angiomyolipoma Associated With Either Tuberous Sclerosis Complex (TSC) or Sporadic Lymphangioleiomyomatosis (LAM)
RAD001 Therapy of Angiomyolipomata in Patients With TS Complex and Sporadic LAM
Sirolimus in Treating Patients With Angiomyolipoma of the Kidney
Rapamycin Therapy for Patients With Tuberous Sclerosis Complex and Sporadic LAM
Tuberous Sclerosis Complex Natural History Study: Renal Manifestations
Trial of Efficacy and Safety of Sirolimus in Tuberous Sclerosis and LAM
Keywords
angiomyolipoma of the kidney, benign mesenchymal tumor of the kidney, renal choristoma, renal hamartoma, isolated angiomyolipoma, tuberous sclerosis
