Perivascular Epithelioid Cell Tumour (PEComa): Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 21, 2026

A perivascular epithelioid cell tumor, usually called a PEComa, is an uncommon tumor made of cells that grow around small blood vessels. These cells have features of both muscle cells and the pigment-producing cells of the skin. There is no normal cell in the body that matches them exactly, which is why the tumor is named for where the cells sit rather than for a normal tissue.

Most PEComas behave in a noncancerous way and are cured by surgery. A minority behave like a cancer by growing into nearby tissue, coming back after surgery, or spreading to other parts of the body. Your pathologist looks for specific features under the microscope to judge which group a tumor belongs to.

This article explains how a PEComa is diagnosed and what the immunohistochemistry results, size, mitotic count, and margins in your pathology report mean.

What tumors belong to the PEComa family?

PEComa is a family of tumors rather than a single diagnosis. All of them are made of the same type of cell, but they have different names depending on where they develop and what they look like:

Angiomyolipoma. This is the most common member of the family. It usually develops in the kidney and contains fat, blood vessels, and muscle-like cells. It is not a synonym for PEComa, although older reports sometimes used the words this way.
Lymphangioleiomyomatosis. This form affects the lungs, mostly in women, and causes cysts to form in the lung tissue.
Clear cell tumor of the lung. This is a rare tumor sometimes called a sugar tumor.
PEComa, not otherwise specified. This name is used for tumors in the soft tissue, uterus, abdomen, and digestive tract that do not fit the descriptions above. It is the form most often meant when a report says PEComa.

Where do PEComas develop?

PEComas can develop almost anywhere in the body. Common locations include the kidney, liver, uterus, and digestive tract. They also include the retroperitoneum, which is the space at the back of the abdomen, and the deep soft tissue of the trunk and limbs. Tumors in the soft tissue and uterus occur more often in women.

What causes a PEComa?

Most PEComas are caused by the loss of a gene called TSC2, or less often TSC1. These genes normally act as brakes on a growth signal inside the cell known as the mTOR pathway. When the brake is lost, the pathway stays switched on, and the cells grow without the usual controls. This change is found in about 90% of PEComas that occur on their own.

A separate group of PEComas has a rearrangement of a gene called TFE3 instead. These tumors tend to occur in younger people and behave somewhat differently, which matters when treatment is being chosen.

Most PEComas occur in people with no inherited condition. A minority occur in people with tuberous sclerosis complex, an inherited condition caused by a change in the TSC1 or TSC2 gene. People with tuberous sclerosis often develop several tumors, most commonly angiomyolipomas in the kidneys.

What are the symptoms of a PEComa?

Many PEComas cause no symptoms and are found by chance on a scan done for another reason. When symptoms occur, they depend on where the tumor is:

A painless lump. Soft-tissue tumors usually grow slowly and are not tender.
Abdominal or pelvic symptoms. Pain, fullness, or a change in bowel or bladder habits.
Bleeding. Kidney tumors can bleed, which may cause sudden pain or blood in the urine.
Uterine symptoms. Abnormal bleeding or pelvic pressure.

How is the diagnosis made?

A pathologist diagnoses a PEComa by examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that removes the whole tumor.

Under the microscope, a PEComa is made of round or slightly elongated cells with pink or clear cytoplasm, which is the body of the cell. The cells are arranged in nests and sheets around thin-walled blood vessels. In most PEComas, few cells are dividing, and there is no dead tumor tissue.

Because these features can also be seen in other tumors, immunohistochemistry is needed to confirm the diagnosis. The full extent of the tumor and the number of dividing cells can only be assessed once the whole tumor has been removed.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins tumor cells make. For a PEComa, it shows the unusual combination of pigment-cell and muscle proteins that defines these tumors. Your report may include some of the following:

HMB-45 and Melan-A. Pigment-producing cells normally make these proteins. At least one of them is positive in most PEComas, and HMB-45 is the most useful of the two.
SMA and desmin. These muscle proteins are positive in many PEComas, although not in all of them. Tumors with a TFE3 rearrangement often show little or no muscle protein staining.
TFE3. Strong staining in the nucleus suggests that the tumor belongs to the TFE3-rearranged group.
S100. This protein is usually negative in PEComa. A strongly positive result would raise the possibility of melanoma, which shares the pigment-cell markers.

Molecular testing may also be performed. It can confirm the loss of TSC1 or TSC2 or a rearrangement of TFE3, and the result may affect treatment if the tumor needs more than surgery.

How is the risk of cancerous behavior assessed?

Because PEComas range from harmless to cancerous, pathologists assess several features to judge how a tumor is likely to behave. The most widely used system counts the following worrisome features:

Size. The tumor is larger than 5 cm.
Growth pattern. The tumor grows into the surrounding tissue instead of pushing it aside.
Cell appearance. The cells are crowded and look very abnormal.
Mitotic count. More than one dividing cell is seen in a large area of tumor examined under the microscope.
Necrosis. The tumor contains dead tumor tissue.
Vascular invasion. Tumor cells are seen inside blood vessels.

Tumors with two or more of these features are usually reported as malignant. Tumors with one feature may be described as having uncertain malignant potential, and tumors with none are usually reported as benign. Pathologists apply these rules with caution for tumors of the uterus and other gynecologic organs, where they can overestimate risk, and newer scoring systems are being studied.

Tumor size, extension, and margins

Tumor size is the greatest dimension of the PEComa, measured in centimeters (cm), and it is one of the features used to judge risk. Size is measured after the whole tumor has been removed.

Tumor extension describes whether the PEComa has grown into nearby structures such as muscle, fat, or organs. Most PEComas have a clear border, and growth into surrounding tissue is one of the worrisome features described above.

A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge, and a positive margin means tumor cells reach it. A positive margin raises the chance that the tumor will come back in the same place, and further surgery may be considered, particularly for a tumor with worrisome features.

What is the prognosis?

Most people with a PEComa are cured by surgery, especially when the tumor is small, has a clear border, and shows none of the worrisome features. These tumors rarely come back.

A malignant PEComa behaves differently. It can come back in the same place and can spread to the lungs, liver, bone, or lymph nodes. Outcomes for people whose tumor has spread remain poor, although treatment has improved since drugs that block the mTOR pathway became available.

Because a PEComa can come back years later, follow-up with imaging is usually recommended, and the schedule depends on the features found in your report.

What happens after the diagnosis?

After a PEComa is confirmed, care is usually planned by a team at a center experienced in treating sarcomas and related tumors.

Surgery. Complete removal is the main treatment for a PEComa that has not spread, and it cures most of these tumors.
Observation. Small tumors without worrisome features may be monitored, particularly in the kidney of a person with tuberous sclerosis.
mTOR inhibitors. These drugs block the growth signal that is switched on when TSC1 or TSC2 is lost. The FDA approved nab-sirolimus in 2021 for adults with a malignant PEComa that cannot be removed or has spread. In the trial that led to approval, tumors shrank in about 39% of people treated. Oral drugs in the same family, such as sirolimus and everolimus, are also used.
Treatment for TFE3-rearranged tumors. These tumors respond less well to mTOR inhibitors, and other approaches are being studied.
Clinical trials. Because this tumor is rare, your oncologist may discuss a trial if it has spread or come back.

Availability of these drugs differs between countries, and your care team can explain what applies where you live.

Questions to ask your doctor

Does my report describe the tumor as benign, of uncertain malignant potential, or malignant?
Which worrisome features, if any, were found?
How large was the tumor, and were the margins negative?
Was molecular testing done, and was a TSC1, TSC2, or TFE3 change found?
Should I be assessed for tuberous sclerosis?
Do I need any treatment after surgery?
If treatment is needed, would an mTOR inhibitor be an option for me?
What is the chance this tumor comes back?
How often will I need imaging, and for how many years?
Which symptoms should prompt me to call sooner?