Angiomyolipoma (AML): Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
January 12, 2026

Angiomyolipoma is a benign (non-cancerous) tumour, most often found in the kidney. It is composed of fat cells, smooth muscle cells, and abnormally shaped blood vessels. These three components give the tumour its name: angio (blood vessels), myo (muscle), and lipo (fat).

This article explains the pathology report for angiomyolipoma, including how it is diagnosed, what features pathologists look for, and how these findings relate to prognosis and treatment.

Where does angiomyolipoma develop?

Angiomyolipomas usually arise in the kidney, most commonly in the cortex or medulla. They may be solitary or multiple, sometimes affecting both kidneys.

Less commonly, angiomyolipomas can arise in the retroperitoneal soft tissues (the deep tissues behind the abdominal organs), with or without direct connection to the kidney. Rarely, angiomyolipoma tissue can be found in regional lymph nodes, indicating multifocal growth rather than spread, unlike cancer.

What are the symptoms of angiomyolipoma?

Most angiomyolipomas cause no symptoms and are discovered incidentally during imaging for another reason. This is increasingly common because ultrasound and CT scans are widely used.

Larger tumours are more likely to cause symptoms, which may include:

• Flank or abdominal pain.

• A feeling of fullness.

• Blood in the urine.

Large angiomyolipomas are at risk of bleeding because their blood vessels are thick-walled but fragile. Sudden bleeding into the abdomen (called Wunderlich syndrome) can be life-threatening and may require urgent treatment.

Some angiomyolipomas occur in people with tuberous sclerosis, a genetic condition. In these patients, angiomyolipomas:

• Often develop at a younger age (sometimes in childhood).

• They are more likely to be bilateral and multiple.

• It may be associated with kidney cysts or polycystic kidney disease.

Who gets angiomyolipoma?

Angiomyolipomas account for about 1% of surgically removed kidney tumours.

• 80–90% are sporadic, meaning they occur in people without an inherited condition. These are more common in women and older adults.

• A smaller proportion occurs in people with tuberous sclerosis, often at a younger age and with multiple tumours.

What causes angiomyolipoma?

Angiomyolipoma is now recognised as an actual tumour, not just a developmental abnormality.

Most cases develop because of changes in genes called TSC1 or TSC2. These genes normally help control cell growth. When they do not function properly, cells can grow in an uncontrolled way.

Hormonal factors may also play a role, which may explain why angiomyolipomas:

• Are more common in women.

• Can grow during pregnancy or in response to hormonal stimulation.

How does angiomyolipoma develop?

Angiomyolipoma belongs to a family of tumours called PEComas, which arise from special cells called perivascular epithelioid cells. These cells are usually located around blood vessels.

In most angiomyolipomas, both copies of TSC1 or TSC2 are inactivated. This leads to abnormal activation of a pathway called mTOR, which controls cell growth and metabolism. When this pathway is overactive, tumour cells can grow and survive longer than normal.

This understanding explains why some patients benefit from mTOR inhibitor medications when surgery is not possible or when tumours are large or multiple.

How is the diagnosis made?

The diagnosis is usually suggested by imaging, especially when fat is clearly seen within a kidney mass. In more difficult cases, a biopsy or surgical removal is examined under the microscope to confirm the diagnosis.

Microscopic (pathologic) features

Under the microscope, angiomyolipoma shows a variable mixture of three components:

• Mature fat cells.

• Smooth muscle cells, which may be spindle-shaped or epithelioid.

• Thick-walled blood vessels that lack the normal elastic layer.

The smooth muscle cells often radiate outward from blood vessels in a sunburst-like pattern. Some tumours contain mostly fat (lipoma-like), while others are dominated by smooth muscle (leiomyoma-like).

The blood vessels are structurally abnormal, which explains why angiomyolipomas can bleed. The border between the tumour and normal kidney is usually sharp, although normal kidney tubules may be trapped at the edges.

Subtypes

• Angiomyolipoma with epithelial cysts (AMLEC): A rare form with cysts lined by epithelial cells and a distinctive layer of tumour cells beneath the cyst lining.

• Oncocytoma-like angiomyolipoma: A sporadic form made of eosinophilic (pink) cells that can mimic other kidney tumours.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses special stains to detect specific proteins inside tumour cells. These stains help confirm the diagnosis and distinguish angiomyolipoma from other kidney tumours.

Angiomyolipomas typically show:

• Melanocytic marker positivity, such as HMB45 and melan-A.

• Smooth muscle marker positivity, such as SMA and calponin.

• Expression of cathepsin K.

Importantly, angiomyolipomas are negative for epithelial markers, which helps distinguish them from kidney cancers.

Molecular testing

Molecular testing looks for changes in the DNA of tumour cells. Routine molecular testing is not required to diagnose angiomyolipoma. When performed, testing often shows inactivation of TSC1 or TSC2, especially in tumours associated with tuberous sclerosis. These findings support the diagnosis but usually do not change management.

What conditions can look similar?

Angiomyolipoma is usually easy to recognize when all three components are present. However, specific patterns can resemble:

• Lipoma or liposarcoma.

• Leiomyoma or leiomyosarcoma.

• Some types of kidney cancer.

Immunohistochemistry is especially helpful in these situations, as angiomyolipoma has a distinctive staining pattern.

How is angiomyolipoma staged?

Angiomyolipoma is not staged because it is benign.

What is the prognosis for a person with angiomyolipoma?

Classic angiomyolipoma is benign. Most patients do very well, especially when tumours are small.

Potential complications include:

• Bleeding, particularly in large tumours.

• Kidney problems, especially in people with tuberous sclerosis and multiple tumours.

Features such as growth into nearby veins or lymph nodes may look concerning, but do not mean the tumour is malignant. True cancerous transformation is extremely rare.

Questions to ask your doctor

• How large is my angiomyolipoma, and is it likely to grow?
• Does imaging clearly show fat, or was a biopsy needed?
• Am I at risk for bleeding, and do I need treatment now?
• Could this be related to tuberous sclerosis?
• How often should my kidneys be monitored with imaging?