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.
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:
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.
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.
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 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 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:
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.
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:
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 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.
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.
After a PEComa is confirmed, care is usually planned by a team at a center experienced in treating sarcomas and related tumors.
Availability of these drugs differs between countries, and your care team can explain what applies where you live.