Angiomyolipoma (AML)

by Jason Wasserman MD PhD FRCPC
February 9, 2023

What is angiomyolipoma?

Angiomyolipoma (AML)  is a non-cancerous tumour made up of a combination of adipose tissue (fat), smooth muscle, and blood vessels. The most common location for this type of tumour is the kidney although it can also be found in the liver and pancreas.

What is the difference between a PEComa and an angiomyolipoma?

PEComa is a type of tumour made up of specialized perivascular epithelioid cells that make proteins normally found in melanocytes (HMB45, melan-A, and MITF) and smooth muscle cells (SMA and calponin). Because angiomyolipoma is believed to start from perivascular cells and because it typically produces these proteins, it is considered a type of PEComa.

What causes angiomyolipoma?

Most angiomyolipomas contain a genetic alteration that renders the gene TSC2 (or less commonly TSC1) inactive. Both TSC2 and TSC1 are tumour suppressor genes which means they prevent cells from growing and dividing uncontrollably. For most people who develop an angiolipoma, the condition is sporadic which means the loss of TSC2 or TSC1 is not associated with any pre-existing genetic condition. What causes the loss of TSC2 or TSC1 in these people is still not known. Angiomyolipomas, however, are much more common in people with tuberous sclerosis, a genetic syndrome caused by the loss of TSC2 or TSC1.

What are the symptoms of angiomyolipoma?

Most angiolipomas do not cause any symptoms and the tumour is discovered incidentally (by accident) when imaging of the abdomen or pelvis is performed for another reason.

How is the diagnosis of angiomyolipoma made?

The diagnosis can be made after part or all of the tumour is removed and the tissue is sent to a pathologist for examination under the microscope.

What does angiomyolipoma look like under a microscope?

When examined under the microscope, angiomyolipoma is made up of a combination of mature adipocytes (fat cells), variably sized blood vessels, and smooth muscle cells. Pathologists often perform a test called immunohistochemistry to confirm the diagnosis. When this test is performed the tumour cells are typically positive for the melanocytic markers HMB45, melan-A, and MITF and the smooth muscle markers SMA and calponin. The tumour cells are typically negative for epithelial cell markers such as pan-cytokeratin.

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