Angiomyolipoma (AML): Understanding Your Pathology Report

Section Editor: Trevor Flood MD FRCPC
July 19, 2026

Angiomyolipoma is a benign (noncancerous) tumor, most often found in the kidney. It is composed of three types of tissue: fat, smooth muscle, and abnormally shaped blood vessels. These three parts give the tumor its name, from angio (blood vessels), myo (muscle), and lipo (fat). It is the most common noncancerous tumor of the kidney.

The most important thing to know about angiomyolipoma is that it is not cancer and does not spread to other organs the way a cancer does. The main concerns are its size and its tendency to bleed, rather than any risk of it turning into a cancer, which is very rare. A separate, uncommon variant composed mostly of a single cell type, called epithelioid angiomyolipoma, can behave differently and is described in its own article.

This article will help you understand the findings in your pathology report for angiomyolipoma, what each term means, and why it matters for your care.

What causes angiomyolipoma?

Angiomyolipoma, the most common noncancerous kidney tumor, is now understood to be a true tumor rather than a developmental quirk. Most develop because both copies of the gene TSC1 or TSC2 stop working in tumor cells. These genes normally act as a brake on cell growth by controlling a growth-signaling system inside the cell called the mTOR pathway. When the brake is lost, the mTOR pathway stays switched on and the cells grow. This same mechanism is the reason a class of drugs called mTOR inhibitors can shrink these tumors, described in the treatment section below.

Angiomyolipoma belongs to a family of tumors called PEComas (perivascular epithelioid cell tumors), which arise from a specialized cell normally found wrapped around small blood vessels. Hormonal factors may also play a role, which could help explain why angiomyolipomas are more common in women and can grow during pregnancy.

Inherited conditions associated with angiomyolipoma

Between 80 and 90 out of every 100 angiomyolipomas are sporadic, meaning they occur in people without an inherited condition; these are more common in women and in older adults. The rest occur in people with tuberous sclerosis complex, an inherited condition caused by a change in the TSC1 or TSC2 gene present from birth. Tuberous sclerosis affects cell growth throughout the body and causes changes in the kidneys, brain, skin, heart, and lungs. Angiomyolipomas in people with tuberous sclerosis tend to appear at a younger age, sometimes in childhood, to occur in both kidneys, to be multiple, and to be associated with kidney cysts. For this reason, a person found to have multiple angiomyolipomas, angiomyolipomas in both kidneys, or an angiomyolipoma at a young age may be asked about features of tuberous sclerosis and, in some cases, referred for genetic assessment.

What are the symptoms of angiomyolipoma?

Most angiomyolipomas, the most common noncancerous kidney tumor, cause no symptoms and are found by chance during imaging done for another reason. This is increasingly common as ultrasound and CT scans are used more often. Larger tumors are more likely to cause symptoms, which may include:

Pain or discomfort in the flank or abdomen.
A feeling of fullness.
Blood in the urine.

The most important complication is bleeding. The blood vessels inside an angiomyolipoma are thick-walled but fragile, and larger tumors, particularly those above about 4 cm, can bleed. Sudden bleeding into the abdomen, called Wunderlich syndrome, can be serious and occasionally requires urgent treatment. This risk of bleeding, rather than any risk of cancer, is the main reason larger angiomyolipomas are treated or watched closely.

Who gets angiomyolipoma?

Angiomyolipomas account for about 1 out of every 100 kidney tumors removed by surgery. Most are sporadic and 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 tumors.

Where does angiomyolipoma develop?

Angiomyolipomas usually arise in the kidney and may be single or multiple, sometimes in both kidneys. Less commonly, they arise in the soft tissue behind the abdominal organs (the retroperitoneum), with or without a connection to the kidney. Rarely, angiomyolipoma tissue is found in nearby lymph nodes. This looks alarming but does not mean the tumor has spread the way a cancer does; it reflects the tumor growing in more than one place at once (multifocal growth), and it does not change the benign nature of the tumor.

How is the diagnosis made?

The diagnosis of angiomyolipoma is often suggested by imaging alone because a CT or MRI can usually detect the fat within the tumor, which is a strong clue. When the tumor contains little fat, it can look like a kidney cancer on imaging, and in that situation a biopsy or surgery is needed so that a pathologist can confirm the diagnosis under the microscope. Under the microscope, angiomyolipoma shows a mixture, in varying proportions, of three components: mature fat cells; smooth muscle cells, which may be spindle-shaped or epithelioid; and thick-walled blood vessels that lack the normal elastic layer. The smooth muscle cells often fan outward from the blood vessels in a sunburst pattern. Some tumors are mostly fat and are called lipoma-like, while others are mostly muscle and are called leiomyoma-like. The border with the normal kidney is usually sharp.

Because a tumor made mostly of fat or mostly of muscle can resemble other tumors, the diagnosis is confirmed with immunohistochemistry, a test that uses special stains to detect proteins inside cells. Angiomyolipoma is characteristically positive for markers normally found in pigment-producing cells, such as HMB-45 and Melan-A, for smooth muscle markers such as smooth muscle actin and calponin, and for cathepsin K. Just as importantly, it is negative for the markers of epithelial cells, which is what separates it from kidney cancers such as clear cell renal cell carcinoma. This staining pattern also helps distinguish angiomyolipoma from fatty tumors such as liposarcoma and muscle tumors such as leiomyoma or leiomyosarcoma, which do not express pigment-cell markers. Molecular testing is not needed to make the diagnosis; when performed, it usually shows loss of TSC1 or TSC2, which supports the diagnosis but does not change management.

Uncommon variants

A few uncommon variants may be named in a report. Angiomyolipoma with epithelial cysts contains cysts lined by epithelial cells with a distinctive layer of tumor cells beneath. Oncocytoma-like angiomyolipoma is made of pink (eosinophilic) cells that can mimic other kidney tumors. Both are benign. The epithelioid variant is different and important, because unlike classic angiomyolipoma it has the potential to behave like a cancer, and it is covered in its own article.

Is angiomyolipoma cancer, and is it staged?

Classic angiomyolipoma is benign and is not cancer. For that reason it is not given a stage, unlike kidney cancers such as renal cell carcinoma, which are staged with the TNM system. Some findings can look concerning under the microscope or on imaging, such as growth into a nearby vein, tumor tissue in a lymph node, or a large size, but none of these means the tumor is malignant. True change into a cancer is extremely rare. The one variant that can behave in a cancer-like way is the epithelioid type described above, which is assessed differently.

What is the prognosis?

Prognosis means the expected course of a disease. The outlook for classic angiomyolipoma is excellent. It is benign, most people do very well, and small tumors in particular rarely cause any problem. The two situations that need attention are not about cancer:

Bleeding — Larger tumors, particularly those above about 4 cm, carry a risk of bleeding, which is the main reason for treating or closely watching a larger angiomyolipoma.
Kidney function — People with tuberous sclerosis who have many angiomyolipomas in both kidneys can, over time, have reduced kidney function, both from the tumors themselves and from treatments used to control them.

For most people with a single small angiomyolipoma, none of this applies, and the tumor simply needs occasional monitoring.

What happens after the diagnosis?

Once angiomyolipoma has been confirmed, the size of the tumor, whether there is more than one, and whether tuberous sclerosis is present shape the next steps. Care usually involves a urologist and a radiologist, with other specialists as needed.

Observation — Most small angiomyolipomas, generally those under about 4 cm and causing no symptoms, are simply monitored with periodic imaging to ensure they are not growing. No treatment is needed.
Procedures to control bleeding or reduce size — For a larger tumor, a tumor that has bled, or one causing symptoms, options include a procedure that blocks the tumor’s blood supply (embolization) to reduce the risk of bleeding, or surgery, usually a partial nephrectomy that removes the tumor while preserving the rest of the kidney.
mTOR inhibitor drugs — For people with tuberous sclerosis who have angiomyolipomas larger than about 3 cm that do not need immediate surgery, a drug that blocks the mTOR pathway, everolimus, is approved to shrink the tumors and lower the risk of bleeding. This reflects the biology described earlier, and it is particularly useful when tumors are multiple or in both kidneys, where repeated surgery would sacrifice too much kidney. Tumors tend to regrow if the drug is stopped, so treatment is often continued long term.
Monitoring and genetic assessment — Regular kidney imaging is used to follow tumor size over time. If tuberous sclerosis is suspected, a referral to a genetics clinic may be arranged so that the condition can be looked for and family members supported if needed.

Questions to ask your doctor

How large is my angiomyolipoma, and is it likely to grow?
Did imaging clearly show fat, or was a biopsy needed to confirm the diagnosis?
Am I at risk of bleeding, and do I need treatment now or only monitoring?
Is my tumor the classic type, or the epithelioid type that needs closer follow-up?
Do I have one tumor or several, and is more than one kidney involved?
Could this be related to tuberous sclerosis?
Should I be referred for genetic counseling, and should my family be assessed?
If treatment is needed, would embolization, surgery, or an mTOR inhibitor be most appropriate for me?
How often should my kidneys be monitored with imaging?
What symptoms, such as sudden pain, should prompt me to seek urgent care?