What is a smooth muscle neoplasm?

Smooth muscle neoplasm is a descriptive term used by pathologists to describe a tumour composed of smooth muscle cells. Smooth muscle is the type of muscle found in organs that move automatically, such as the walls of blood vessels, the digestive tract, and the uterus. Under the microscope, smooth muscle cells appear long and thin, and they often form bundles or intersecting patterns.

The term smooth muscle neoplasm describes how the tumour looks, not whether it is benign (non-cancerous) or malignant (cancerous). This term is often used when the tissue sample is too small or incomplete to determine the precise type of tumour present. Additional testing or a larger biopsy is usually required before a final diagnosis can be made.

Why might my pathology report use this term?

Pathologists use the term smooth muscle neoplasm when the tumour clearly appears to arise from smooth muscle, but there is insufficient information to determine whether it is benign or malignant. This situation commonly happens with small biopsies or samples with limited tissue.

Using this general term allows your healthcare team to continue evaluation while waiting for the results of special tests or until more tissue is collected.

Where are smooth muscle cells normally found?

Smooth muscle cells are located in many parts of the body and perform important involuntary functions.

Examples include:

• Walls of blood vessels: Smooth muscle helps regulate blood pressure and blood flow.

• Digestive tract: Smooth muscle moves food, fluid, and waste through the stomach and intestines.

• Uterus: Smooth muscle contracts during menstruation, pregnancy, and childbirth.

• Bladder: Smooth muscle helps empty the bladder during urination.

• Respiratory system: Smooth muscle in the airways helps control airflow.

Because smooth muscle is widespread, smooth muscle neoplasms can arise in many different parts of the body.

Are smooth muscle neoplasms cancer?

Not always. A smooth muscle neoplasm can be benign, malignant, or sometimes somewhere in between. The term on its own does not indicate cancer. A more specific diagnosis is necessary before a doctor can determine how the tumour will behave or what treatment is needed.

Examples of benign and malignant smooth muscle neoplasms

Leiomyoma

A leiomyoma is a benign tumour made of smooth muscle. The most common type occurs in the uterus, where it is often called a fibroid. Leiomyomas can also develop in the skin or digestive tract. They do not spread to other parts of the body but can sometimes cause symptoms depending on their size or location.

Leiomyosarcoma

A leiomyosarcoma is a malignant smooth muscle tumour. Although rare, it can occur in many body sites, including the uterus, deep soft tissues, and the retroperitoneum (the space behind the abdominal organs). Leiomyosarcomas grow more aggressively than leiomyomas and can spread to distant organs.

What do smooth muscle neoplasms look like under the microscope?

Smooth muscle neoplasms are made up of spindle-shaped cells, which are long, thin cells that resemble needles or tapered ribbons. These cells usually grow in bundles or intersecting patterns.

Pathologists look for several features to help determine whether the tumour is benign or malignant. These include:

• Atypia: Abnormal-looking cells with irregular or enlarged nuclei.

• Cellularity: The number of cells within a given area. Malignant tumours tend to be more densely packed.

• Mitotic activity: The number of cells seen dividing under the microscope. A high number of mitotic figures suggests faster growth.

• Necrosis: Areas of dead tumour tissue, which can indicate aggressive behaviour.

Benign smooth muscle neoplasms usually show none or only mild abnormalities, while malignant tumours such as leiomyosarcoma often show several concerning features.

What does atypia mean in a smooth muscle neoplasm?

Atypia refers to abnormal changes in the size, shape, or appearance of the tumour cells. Cells may have irregular borders, unusually large or dark nuclei, or other atypical features. Atypia can occur in both benign and malignant tumours, so it does not confirm cancer on its own.

When atypia is described in a pathology report, additional factors—such as mitotic activity, necrosis, and invasion—must be considered to determine the tumour’s behaviour. Your doctor may recommend further testing to understand the significance of these findings better.

How do pathologists determine the exact diagnosis?

To classify a smooth muscle neoplasm more precisely, pathologists combine several types of information:

• Microscopic appearance: Including the arrangement of cells and the presence of atypia, mitotic activity, or necrosis.

• Immunohistochemistry: Smooth muscle tumours typically show proteins such as desmin, smooth muscle actin (SMA), and h-caldesmon. These tests help confirm that the tumour truly arises from smooth muscle.

• Molecular tests: Next-generation sequencing (NGS) or other genetic tests may detect changes that support a diagnosis of leiomyosarcoma or another tumour type.

• Clinical and imaging findings: The tumour’s size, location, and behaviour help guide interpretation.

By integrating these findings, the pathologist can determine whether the tumour is benign, malignant, or borderline.

Why is identifying the specific tumour type important?

Different smooth muscle tumours behave differently. Some grow slowly and require little more than monitoring or surgical removal, while others grow quickly and require treatments such as chemotherapy or radiation. Identifying the exact tumour type helps your doctor:

• Understand how the tumour is likely to behave.

• Select the most appropriate treatment plan.

• Estimate your prognosis.

• Decide whether additional tissue or testing is required.

A precise diagnosis is key to avoiding unnecessary treatment for benign lesions and to ensuring timely treatment for malignant ones.

Questions to ask your doctor

• Do we have enough information to know whether the tumour is benign or malignant?

• Were immunohistochemistry or molecular tests performed?

• Do I need additional testing or a larger biopsy?

• What tumour types are being considered based on the current findings?

• What are the next steps for diagnosis, monitoring, or treatment?