Renal angiomyolipoma found on a scan: what it is and what happens next

A renal angiomyolipoma (AML) is a growth in the kidney. As the name suggests, it is made up of three components: blood vessels (angio), muscle-like spindle cells (myo) and fat (lipoma). A review in the Journal of Kidney Cancer and VHL describes it as, together with oncocytoma, among the most common benign tumours in surgical series. It is not a kidney cancer, although the review notes that a rare variant can have malignant features.

You may meet the term after a CT, MRI or ultrasound report mentions a "fat-containing lesion" or "angiomyolipoma". That wording is often reassuring, because the fat gives these growths a characteristic appearance on imaging that most kidney cancers do not have.

The review explains that, before scans were widely used, most AMLs were only found when they were large enough to cause symptoms. Today, with more frequent CT and ultrasound, most are found incidentally and tend to be asymptomatic. One study it cites looked at 61,389 people who had abdominal ultrasound and found an overall prevalence of 0.44%. The prevalence is higher in women, and the tumours tend to be found in the fourth or fifth decade of life.

Rarely, an angiomyolipoma causes symptoms such as abdominal or flank pain, blood in the urine or, in severe cases, a fall in kidney function. The review describes a sudden bleed into the space behind the abdomen (Wunderlich syndrome) as the way a large one can become a medical emergency.

Because fat has a distinctive appearance, most angiomyolipomas can be diagnosed from the imaging without a biopsy. The review explains that the fat-containing feature gives a unique appearance on imaging, and that on ultrasound they are almost always bright (hyperechoic). It describes three radiological types: fat-rich, fat-poor and fat-invisible, and says that in fat-rich lesions the diagnosis can be made confidently without a biopsy.

The difficulty is the small number with very little fat. The review says about 5% have so little fat that they can look like renal cell carcinoma, and in these a biopsy is usually warranted. Rarely there is a variant called epithelioid angiomyolipoma, which has malignant potential, and the review says malignant forms of this tumour family should be managed in a centre that specialises in sarcomas.

The main risk is bleeding. The review links bleeding risk to how vascular a tumour is and to the presence of aneurysms inside it larger than 0.5 cm. The TOSCA registry paper, which studied people with tuberous sclerosis, states that angiomyolipomas over 3 cm across have an increased risk of bleeding or of invading nearby kidney tissue and potentially causing kidney failure.

For sporadic tumours, the review says the decision to move from surveillance to treatment depends on symptoms and size, with 4 to 6 cm described as an accepted but controversial cut-off. It also notes that the evidence for how fast they grow, and whether larger ones grow faster, is inconsistent.

The review recommends surveillance in general, because behaviour varies, and says ultrasound is preferred for small low-risk lesions. It cites intervals ranging from every 3 to 4 years for tumours under 2 cm to every 6 to 12 months for larger ones, while stating there is still no consensus on how often sporadic AMLs should be monitored. That means the timetable you are given may differ from these figures, and it is a decision for your urology or kidney team.

The same paper suggests that people with multiple AMLs on both sides should be considered for genetic assessment, because around 20% are associated with tuberous sclerosis or related conditions. The TOSCA paper notes that new lesions can appear later in life, which is why lifelong surveillance is recommended in people with tuberous sclerosis.

When treatment is needed, the aim is to preserve kidney function. The review lists surgery and embolisation, in which the blood vessels feeding the tumour are blocked, as the main options, with minimally invasive methods such as transarterial embolisation and percutaneous cryoablation showing promise in case series. Urgent embolisation is the approach described for life-threatening bleeding.

Drugs that block the mTOR pathway, such as everolimus, are used for angiomyolipomas that are part of tuberous sclerosis. The review states there is no evidence to support using them in sporadic tumours. A few reports describe tumours growing or rupturing during pregnancy, so anyone planning a pregnancy with a known angiomyolipoma should raise it with their team.

Sudden severe pain in the side or abdomen, blood in the urine, or feeling faint or unwell need urgent medical assessment in anyone with a known kidney growth, because they may signal a bleed. Do not wait for the next scheduled scan. Otherwise most people with a small, incidentally found angiomyolipoma carry on normally and attend follow-up as arranged.