Leiomyoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 25, 2026

A leiomyoma is a noncancerous tumor made of smooth muscle cells. Smooth muscle works without conscious control and is found in blood vessels, hair follicles, the digestive tract, and the uterus.

Leiomyomas grow slowly and do not spread to other parts of the body. They differ from leiomyosarcoma, a cancer made of smooth muscle cells. A leiomyoma does not turn into a leiomyosarcoma.

This article covers leiomyomas of the skin, the soft tissue, and the abdomen. A leiomyoma in the uterus, often called a fibroid, is described in our article on leiomyoma of the uterus.

Where does a leiomyoma develop?

Skin. Most skin leiomyomas grow from the tiny muscles attached to hair follicles, which make hair stand up. These are called piloleiomyomas. They occur most often on the outer surfaces of the arms and legs, then the trunk, head, and neck.
Genital skin. A separate group develops from smooth muscle in the scrotum, vulva, nipple, or areola.
Deep soft tissue. These are uncommon and usually develop in an arm or leg, either beneath the skin or within muscle.
Abdomen and retroperitoneum. These develop deep in the abdomen, including the retroperitoneum, which is the space at the back of the abdomen. They are often larger than leiomyomas elsewhere and are much more common in women.

What are the symptoms of a leiomyoma?

Symptoms depend on where the tumor is and how many are present.

A single skin leiomyoma. A firm bump, flesh-colored to reddish-brown, usually small and painless.
Multiple skin leiomyomas. Pain is common and may be brought on by cold, pressure, or touch. The bumps may be grouped in a line or a band.
Genital leiomyoma. Usually a single painless nodule. A vulvar leiomyoma can enlarge during pregnancy.
Deep or abdominal leiomyoma. Often no symptoms, and many are found on imaging done for another reason. A large tumor may cause pressure or discomfort.

What causes a leiomyoma?

Most leiomyomas occur by chance and are not inherited.

Some skin leiomyomas occur as part of an inherited condition called hereditary leiomyomatosis and renal cell cancer syndrome. It is caused by a change in a gene called FH, which stands for fumarate hydratase. A parent with the condition has a 50% chance of passing it to each child. People with it may develop multiple skin leiomyomas and uterine leiomyomas, and they have a higher risk of a particular type of kidney cancer.

Leiomyomas of the abdomen and retroperitoneum share genetic features with uterine leiomyomas. Many people with these tumors have a history of uterine fibroids, which suggests that the tumors arise separately at more than one site.

How is the diagnosis made?

The diagnosis of a leiomyoma is made after a pathologist examines tumor tissue under the microscope. The tissue usually comes from surgery that removes the tumor, or from a biopsy.

Under the microscope, a leiomyoma is made of spindle cells that closely resemble normal smooth muscle. The cells are arranged in orderly bundles that cross one another. They have pink cytoplasm and blunt-ended nuclei that pathologists describe as cigar-shaped.

leiomyoma

The cells look uniform, with little variation in size or shape. Mitotic figures, which are cells in the process of dividing, are absent or very rare, and there is no necrosis (dead tumor tissue). Together, these three features separate a leiomyoma from a leiomyosarcoma.

Skin leiomyomas often blend into the surrounding tissue and may contain scattered enlarged or dark nuclei, which is not a sign of cancer. Deep and abdominal leiomyomas often show changes that develop over time, including scarring, calcification, or gel-like myxoid areas.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins cells make. For a leiomyoma, it confirms that the cells are smooth muscle. Your report may include some of the following:

SMA, desmin, and h-caldesmon. These smooth muscle proteins are positive in a leiomyoma.
S100. This protein is negative, which helps separate a leiomyoma from a nerve sheath tumor.
FH and 2SC. These stains are used when an inherited condition is suspected. Loss of FH staining, or increased 2SC staining, suggests hereditary leiomyomatosis and renal cell cancer syndrome and may prompt further assessment.

Molecular testing is not needed for most leiomyomas. It may be recommended when a person has multiple skin leiomyomas or a personal or family history suggesting the inherited condition. Stains can point toward that condition, but a blood test is needed to confirm it.

Margins

A margin is the edge of tissue cut by the surgeon. Your report may note whether the tumor reaches the cut edge.

For a leiomyoma, a positive margin does not carry the meaning it does for cancer. It means that a small amount of tumor may remain, which slightly raises the chance the lump comes back in that spot. Skin leiomyomas often blend into the surrounding tissue, so this finding is common and usually does not require more surgery.

What is the prognosis?

The outlook is excellent. A leiomyoma is noncancerous, does not spread, and complete removal is curative.

Skin leiomyomas do not become cancer, although new ones can develop elsewhere, particularly in people with the inherited condition. Deep soft tissue leiomyomas usually do not come back after removal. Leiomyomas of the abdomen and retroperitoneum come back in a small number of people, and even then they remain noncancerous.

What happens after the diagnosis?

Treatment depends on where the tumor is and what symptoms it causes.

No treatment. A single painless leiomyoma that has been removed needs no further treatment.
Surgery. Removal is the usual treatment for a tumor that is painful, is growing, or is causing pressure.
Pain control for multiple skin leiomyomas. When there are too many bumps to remove, medications can reduce the pain, and avoiding triggers such as cold and pressure often helps. Your dermatologist can explain the options.
Assessment for an inherited condition. This is considered for people with multiple skin leiomyomas, an early age at diagnosis, or a family history of leiomyomas or kidney cancer. People with hereditary leiomyomatosis and renal cell cancer syndrome are followed with regular kidney imaging, because the associated kidney cancer can develop at a young age.

Questions to ask your doctor

Was the tumor completely removed?
Is there a chance it could come back?
Do I have one leiomyoma or several?
Do my findings suggest an inherited condition such as hereditary leiomyomatosis and renal cell cancer syndrome?
Should I have genetic counseling or testing?
Do I need kidney imaging or skin examinations?
If my lesions are painful, what can be done?