Renal angiomyolipoma

Renal angiomyolipomas (AMLs) are a type of benign renal neoplasm and are composed of vascular, smooth muscle and fat elements. They can spontaneously haemorrhage, which can be fatal. AMLs usually have characteristic radiographic appearances. 

Epidemiology

Angiomyolipomas are the most common benign solid renal lesion and also the most fat containing lesion of the kidneys. The majority of angiomyolipomas are sporadic (80%) and are typically identified in adults (mean age of presentation 43 years), with a strong female predilection (F:M of 4:1) 7,9

The remaining 20% are seen in association with phakomatoses, the vast majority in the setting of tuberous sclerosis, although they have also been described in setting of Von Hippel-Lindau syndrome (VHL) and neurofibromatosis type 1 (NF1) 5,7. In these cases they present earlier (usually identified by the age of 10 years), are larger, and are far more numerous. They are more likely to be fat-poor which accounts for their earlier presentation 2,6-7.

Clinical presentation

Angiomyolipomas are often found incidentally when the kidneys are imaged for other reasons, or as part of screening in patients with tuberous sclerosis.

Symptomatic presentation is most frequently with spontaneous retroperitoneal haemorrhage; the risk of bleeding being proportional to the size of the lesion (>4 cm diameter). Shock due to severe haemorrhage from rupture is described as Wunderlich syndrome 4-5,7.

Patients may present with numerous other symptoms and signs 2, e.g. palpable mass, flank pain, urinary tract infections, haematuria, renal failure, or hypertension 3.

Pathology

Angiomyolipomas are members of the perivascular epithelioid cells tumour group (PEComas) and are composed of variable amounts of three components; blood vessels (-angio), plump spindle cells (-myo) and adipose tissue (-lipo). Almost all classic angiomyolipomas are benign but they do have the risk of rupture with bleeding or secondary damage/destruction of surrounding structures as they grow. 

Variants

There is a special variant called an epithelioid angiomyolipoma, composed of more plump, epithelial looking cells, often with nuclear atypia, that have a described risk of malignant behaviour. This variant, unlike conventional AMLs, may mimic renal cell carcinoma.10 Metastases have also been described 9

Radiographic features

The cornerstone of diagnosis on all modalities is the demonstration of macroscopic fat, however in the setting of haemorrhage, or when lesions happen to contain little fat, it may be difficult to distinguish an AML from a renal cell carcinoma.

Ultrasound
  • tend to appear as hyperechoic lesions on ultrasound, located in the cortex and with posterior acoustic shadowing
  • in the setting of tuberous sclerosis, they may be so numerous that the entire kidney is affected, appearing echogenic with the loss of normal corticomedullary differentiation 7
  • contrast-enhanced ultrasound 12
    • tend to enhance peripherally
    • decreased central enhancement, compared with normal cortex
CT

Most lesions involve the cortex and demonstrate macroscopic fat (less than -20 HU). When small, volume averaging may make differentiation from a small cyst difficult.

It is important to realise that a proportion of angiomyolipomas are fat-poor. This is especially the case in the setting of tuberous sclerosis, where up to a third do not demonstrate macroscopic fat on CT 6. Calcification is rare.

MRI

MRI is excellent at evaluating fat-containing lesions, and two main set of sequences are employed. Firstly, and traditionally if you will, fat saturated techniques demonstrate high signal intensity on non-fat-saturated sequences, and loss of signal following fat saturation.

The second method is to use in and out of phase imaging which generates India ink artifact at the interface between fat and non-fat components. This can occur either at the interface between the angiomyolipoma and surrounding kidney or between fat and non-fat components of the mass 8

It is essential to remember that rarely renal cell carcinomas may have macroscopic fat components and as such the presence of fat is strongly indicative of an angiomyolipoma, but not pathognomonic. Since macroscopic fat in RCC almost always occurs in the presence of ossification/calcification, absence of ossification/calcification on imaging is in favour of AML.  

DSA - angiography

Angiomyolipomas are hypervascular lesions demonstrating often characteristic features:

  • arterial phase: a sharply marginated hypervascular mass with a dense early arterial network, and tortuous vessels giving the "sunburst" appearance
  • venous phase: whorled "onion peel" appearance of peripheral vessels
  • micro- or macro-aneurysms 2
  • absent AV shunting

Treatment and prognosis

Angiomyolipomas found incidentally usually require no therapy (when small), although follow-up is recommended to assess for growth. Small solitary AMLs (< 20 mm) probably do not require follow-up due to their slow growth 13

Larger AMLs, or those that have been symptomatic, can be electively embolised and/or resected with a partial nephrectomy.

Lesions that present with retroperitoneal haemorrhage often requires emergency embolisation as a life-saving measure.

Differential diagnosis

When an AML has typical appearances there is essentially no differential. If atypical, especially when fat-poor other lesions to consider include:

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Article Information

rID: 899
System: Urogenital
Section: Pathology
Synonyms or Alternate Spellings:
  • Renal angiomyolipoma
  • Renal angiomyolipoma (AML)
  • Angiomyolipoma
  • AML
  • Renal AML
  • Renal angiomyolipomas
  • Renal angiomyolipomata
  • Angiomyolipomas of the kidney
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    Macroscopic appea...
    Figure 1: gross pathology - renal angiomyolipoma
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    Axial CT

Note th...
    Case 1
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    High power of ang...
    Figure 2: histopathology - renal angiomyolipima
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    Case 2: with complicating haemorrhage
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    Case 3
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    Tuberous sclerosi...
    Case 4: with background tuberous sclerosis
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    Wunderlich syndro...
    Case 5: presenting with the Wunderlich syndrome
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    Fig. 1

Gradient ...
    Case 6
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    Case 7
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    Microcatheter in ...
    Case 8: showing feeder vessel prior to embolisation
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    Case 9: very large lesion
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    Case 10: large
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    Fatty and vascula...
    Case 11: giant renal angiomyolipoma
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    Case 12: with Wunderlich syndrome
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    Retroperitoneal h...
    Case 13: with Wunderlich syndrome
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    Case 14
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