Description
Renal angiomyolipoma (AML) is a benign kidney lesion consisting of blood vessels (angio-), smooth muscle (-myo-), and fat cells (-lipoma). They account for approximately 3% of all renal masses and are often associated with tuberous sclerosis, an autosomal dominant genetic condition.
Pathogenesis
The formation of angiomyolipoma is still under investigation, but it appears to be linked to the mutations in the TSC1 and TSC2 genes. These gene mutations are frequently identified in patients with tuberous sclerosis complex (TSC), with up to 80% of these patients developing AMLs.
Epidemiology, Risk Factors & Associations
- Renal angiomyolipoma (AML) is relatively common benign renal neoplasm, representing around 3% of all renal masses.
- Incidence increases with age, with the mean age at diagnosis being approximately 40 years.
- The condition shows a significant predilection for women with a female-to-male ratio of 4:1 in sporadic AMLs.
- Approximately 80-90% of individuals with tuberous sclerosis complex (TSC), a genetic disorder that causes benign tumours to develop in many parts of the body, will develop renal AMLs. AMLs tend to be larger, bilateral, and multifocal. They also present at a younger age compared to sporadic cases.
- Found in 20% of patients with pulmonary lymphangioleiomyomatosis, a diffuse cystic lung disease primarily affecting females of child-bearing age.
- Peutz-Jeghers syndrome (rare).
Clinical Features
- Most AMLs are asymptomatic and discovered incidentally.
- Larger AMLs (>4 cm) may present with flank pain, haematuria, or symptoms related to mass effect.
- Spontaneous rupture leading to retroperitoneal hemorrhage (Wunderlich syndrome) can occur, especially with lesions larger than 4 cm.
Complications
- Haemorrhage: The most significant complication. Most common in lesions larger than 4 cm, resulting in retroperitoneal haemorrhage (Wunderlich syndrome).
- Renal impairment: Rare but can occur due to mass effect on normal renal parenchyma.
Pathological Features
Histopathology
Characterised by the presence of three components – blood vessels, smooth muscle, and mature adipose tissue. The proportion of these components can vary, and occasionally one component may predominate.
Genetics
Mutations in TSC1 and TSC2 genes are noted in angiomyolipoma, particularly in cases associated with tuberous sclerosis.
Radiological Features
General Features
- Typically well-defined, round or oval mass within the renal parenchyma with evidence of macroscopic fat
- Intratumoral vessels
- Presence of solid components raises suspicion of dedifferentiation into more aggressive tumour.
CT
- Non-contrast: Shows macroscopic fat within the lesion, defined as areas of attenuation less than -10 Hounsfield units.
- Contrast: Heterogeneous enhancement, usually less than the renal cortex.
- CTA: Disorganised tumoural vessels, aneurysms, and pseudoaneurysms may be seen
MRI
- T1: High signal due to fat content.
- Fat-suppressed sequences: Loss of signal confirms the presence of macroscopic fat.
- T2: Variable signal depending on the amount of fat, smooth muscle, and vessels.
- In/Out of Phase: India-ink artifact demonstrated at the interface between macroscopic fat in AML and adjacent renal parenchyma
US
- Hyperechoic mass, secondary to fat content. Can be mistaken for a renal cell carcinoma due to high echogenicity.
FL
- Hypervascular with trotuous, irregular, aneurysmal vessels (95%)
Differential Diagnosis
- Renal Cell Carcinoma: Can mimic AML on ultrasound due to high echogenicity, but usually lacks macroscopic fat on CT/MRI.
- Renal Lipoma: Purely fatty mass with no vascular or muscular components. Rare.
- Fat-containing Renal Cell Carcinoma: Rare type of renal cancer that can contain fat, necessitating careful evaluation of imaging features.
Differential diagnosis of fat-containing retroperitoneal lesion includes;
- Liposarcoma – Large size (>10 cm) and presence of septa/nodular components favour malignant liposarcoma over a benign lipoma
- Lipoma – Exceedingly rare in the retroperitoneum.
- Retroperitoneal teratoma – Presents in children/young adults and is characterized by the presence of calcification and a cystic component (often with fat fluid level).
Management
- Small AMLs (<4 cm) are usually managed conservatively with regular imaging follow-up.
- Larger AMLs (>4 cm) or symptomatic AMLs can be treated with various methods, including embolisation, cryoablation, or surgery.
- Genetic counselling may be offered to patients with associated genetic conditions such as tuberous sclerosis.