Your pathology report for leiomyoma

by Jason Wasserman MD PhD FRCPC
December 18, 2025

A leiomyoma is a benign (non-cancerous) tumour made of smooth muscle cells. Smooth muscle is the type of muscle found in structures such as blood vessels, hair follicles, the gastrointestinal tract, and the uterus. Leiomyomas can develop in the skin (cutaneous leiomyomas) or in deep soft tissues, including muscle and the retroperitoneum or abdominal cavity.

Leiomyoma belongs to a broader group of tumours called smooth muscle tumours. These tumours are different from leiomyosarcoma, which is malignant. Leiomyomas grow slowly and do not spread to other parts of the body.

Where is a leiomyoma found?

Leiomyomas can arise in several different locations, depending on the type.

Cutaneous leiomyomas develop in the skin. Most arise from the tiny smooth muscles attached to hair follicles, called arrector pili muscles. These are often called pilar leiomyomas or piloleiomyomas. Cutaneous leiomyomas most commonly involve the extensor surfaces of the arms and legs, followed by the trunk and the head and neck region. Another group, called genital leiomyomas, arises from smooth muscle in the scrotum, vulva, nipple, or areola.

Deep soft tissue leiomyomas usually occur in the arms or legs and arise from deep subcutaneous tissue or skeletal muscle. These tumours are uncommon.

Retroperitoneal and abdominal leiomyomas develop deep within the abdomen, including the retroperitoneum, abdominal wall, omentum, mesentery, or peritoneal surfaces. In the groin region, leiomyomas may arise from the round ligament. These tumours are often larger than cutaneous or limb lesions.

What are the symptoms of leiomyoma?

Symptoms depend on the location and number of tumours.

Cutaneous leiomyomas usually appear as firm, flesh-coloured to reddish-brown papules or nodules on the skin. Solitary lesions are often small and painless. When multiple tumours are present, pain is common and may be triggered by cold, pressure, or touch. Multiple lesions may be arranged in linear, segmental, or band-like patterns.

Genital leiomyomas are usually solitary and often asymptomatic. They may appear as raised or pedunculated nodules. Vulvar leiomyomas can enlarge during pregnancy.

Leiomyomas in deep soft tissue or the abdomen often cause no symptoms and are discovered on imaging studies. When symptoms do occur, they are usually related to mass effect, such as discomfort, pressure, or a slowly enlarging lump.

How common is leiomyoma?

Cutaneous leiomyomas are rare. Solitary skin lesions are more common in women, while cases with multiple tumours often begin earlier in life.

Deep soft tissue leiomyomas most often affect middle-aged adults and occur equally in men and women.

Leiomyomas of the retroperitoneum and abdominal cavity are much more common in women, particularly around the time of menopause.

What causes a leiomyoma?

Most leiomyomas occur sporadically, meaning they arise by chance and are not inherited.

Some cutaneous leiomyomas occur as part of an inherited condition called hereditary leiomyomatosis and renal cell cancer syndrome. This rare condition is caused by changes in a gene called FH (fumarate hydratase) and is inherited in an autosomal dominant pattern. People with this condition may develop multiple cutaneous leiomyomas, uterine leiomyomas, and have an increased risk of a specific type of kidney cancer.

Leiomyomas of the retroperitoneum and abdominal cavity often share genetic features with uterine leiomyomas. Many patients with these tumours have a history of uterine fibroids or prior surgery for fibroids, suggesting that these tumours may arise independently at multiple sites.

How is the diagnosis made?

The diagnosis of leiomyoma is made by examining a tissue sample under the microscope. This tissue is usually obtained by surgical removal or biopsy.

Microscopic features

Under the microscope, leiomyomas are composed of spindle-shaped smooth muscle cells. Spindle cells are long, narrow cells with tapered ends, resembling a stretched oval. This shape is typical of smooth muscle. The cells are arranged in orderly, intersecting bundles called fascicles and closely resemble normal smooth muscle.

The tumour cells have pink (eosinophilic) cytoplasm and blunt-ended, cigar-shaped nuclei. The cells appear uniform, with little or no variation in size or shape. Mitotic figures, which indicate cell division, are absent or very rare. Importantly, necrosis (tumour cell death) is not seen.

Cutaneous leiomyomas are often poorly circumscribed within the skin and may show occasional enlarged or dark-staining nuclei. In deep soft tissue and retroperitoneal leiomyomas, degenerative changes such as fibrosis, hyalinization, calcification, or myxoid (gel-like) change are common, especially in larger or long-standing tumours.

Immunohsitochemistry

Immunohistochemistry helps confirm the diagnosis. Leiomyoma cells typically express smooth muscle actin (SMA), desmin, and h-caldesmon, which are markers of smooth muscle. They are negative for S100, helping distinguish them from nerve sheath tumours.

Cutaneous leiomyomas associated with hereditary leiomyomatosis may show loss of FH expression and increased staining for a marker called 2-succinocysteine (2SC), which can suggest an inherited syndrome and prompt further evaluation.

Molecular tests

Molecular testing is not required for most sporadic leiomyomas. In patients with multiple cutaneous leiomyomas or a personal or family history suggestive of hereditary leiomyomatosis and renal cell cancer syndrome, additional testing may be recommended. Immunohistochemical stains for FH and 2SC can help identify tumours that may be associated with this condition, but genetic testing is needed to confirm an inherited diagnosis.

What is the prognosis for a person with a leiomyoma?

Leiomyomas are benign tumours, and complete surgical removal is considered curative. These tumours do not metastasize.

Cutaneous leiomyomas do not progress to cancer. However, when multiple cutaneous leiomyomas are present, especially at a young age, evaluation for hereditary leiomyomatosis and renal cell cancer syndrome may be important because of the associated kidney cancer risk.

Deep soft tissue leiomyomas do not recur after removal. Leiomyomas of the retroperitoneum or abdominal cavity may recur in a small number of cases, but even when recurrence occurs, the tumour does not behave aggressively.

Questions to ask your doctor

• Was the tumour completely removed?
• Is there a chance it could come back?
• Do I need follow-up imaging or skin exams?
• Do my findings suggest an inherited condition such as hereditary leiomyomatosis and renal cell cancer syndrome?