Smooth muscle neoplasm

A smooth muscle neoplasm is a term used to describe a group of tumours made up of smooth muscle cells. Smooth muscle is a type of tissue that helps control involuntary movements in the body, such as the contraction of blood vessels, the digestive tract, and the uterus. This group includes both benign (non-cancerous) tumours, such as leiomyomas, and malignant (cancerous) tumours, such as leiomyosarcomas.

The term “smooth muscle neoplasm” is often used when not enough tissue is available to determine whether the tumour is benign or malignant. Additional testing or a larger tissue sample may be needed for a more definitive diagnosis.

Where are smooth muscle cells normally found?

Smooth muscle cells are found in many parts of the body and play an important role in controlling involuntary movements. Unlike skeletal muscle, which moves under voluntary control, smooth muscle works automatically to perform essential functions.

Some of the most common locations for smooth muscle include:

• Walls of blood vessels: Smooth muscle cells help regulate blood pressure and flow by contracting and relaxing.
• Digestive tract: Smooth muscle moves food and liquids through the esophagus, stomach, intestines, and other parts of the digestive system.
• Uterus: In women, smooth muscle cells in the uterus contract during menstruation and childbirth.
• Bladder: Smooth muscle helps empty the bladder by contracting during urination.
• Respiratory system: Smooth muscle in the airways of the lungs helps control airflow.

These cells are essential for the normal functioning of these organs and systems. When they begin to grow abnormally, they can form tumours, which are referred to as smooth muscle neoplasms.

How is this diagnosis made?

The diagnosis of a smooth muscle neoplasm is usually made after a sample of tissue is removed and examined under a microscope by a pathologist. In some cases, if only a small tissue sample (such as a biopsy) is available, the pathologist may use the term “smooth muscle neoplasm” because the features needed to determine whether the tumour is benign or malignant may not be visible.

Additional tissue may need to be examined to clarify the diagnosis, or other tests, such as immunohistochemistry or next-generation sequencing, may be performed. These tests help pathologists identify specific characteristics of the tumour, allowing them to determine whether it is benign or malignant and guide further management.

Common benign and malignant smooth muscle neoplasms

• Leiomyoma is the most common benign (non-cancerous) smooth muscle tumour. Leiomyomas are most commonly found in the uterus, where they are often called fibroids. They can also occur in other locations, such as the skin or digestive tract.
• Leiomyosarcoma is the most common malignant (cancerous) smooth muscle tumour. Although rare, it can develop in various parts of the body, including the uterus, retroperitoneum (the area behind the abdominal organs), and deep soft tissues.

What are the microscopic features of a smooth muscle neoplasm?

All smooth muscle neoplasms are made up of spindle cells, which are long, thin cells that resemble the shape of a needle or spindle. These cells are typically arranged in bundles that may run in different directions when viewed under a microscope.

Pathologists use several features to determine whether a smooth muscle neoplasm is benign or malignant:

• Cellular atypia: This refers to abnormal changes in the shape or size of the cells or their nuclei. Atypical cells often have irregular, larger, or darker nuclei. Atypia is more common in malignant tumours, such as leiomyosarcomas.
• Increased cellularity: Malignant tumours often have a higher density of cells compared to benign tumours, meaning the cells are packed more closely together.
• Mitotic activity: Pathologists count the number of mitoses (dividing cells) under the microscope. A higher number of mitotic figures, especially abnormal ones, is a sign of a malignant tumour.
• Necrosis: This refers to areas of dead tumour cells, which are often seen in malignant tumours. Necrosis can indicate that the tumour is growing rapidly and outstripping its blood supply.

While benign smooth muscle neoplasms, such as leiomyomas, typically lack these concerning features, malignant tumours like leiomyosarcomas often show a combination of atypia, increased cellularity, mitotic activity, and necrosis. These findings help pathologists classify the tumour and guide treatment decisions.

What does it mean if a smooth muscle neoplasm is described as atypical or showing atypia?

When a smooth muscle neoplasm is described as atypical or showing atypia, it means that the cells in the tumour look abnormal under the microscope. These changes may include:

• Irregular cell shapes and sizes: The tumour cells may vary more in appearance than normal smooth muscle cells.
• Abnormal nuclei: The nuclei, which hold the genetic material of the cells, may appear larger, darker, or irregularly shaped.
• Increased mitotic activity: The tumour cells may show a higher number of mitoses (dividing cells) than is typical for benign smooth muscle tumours.

Atypia can occur in both benign and malignant smooth muscle neoplasms, but its presence often raises concern that the tumour might be more aggressive or have the potential to behave like a cancer. However, atypia alone does not confirm malignancy.

The pathologist considers other features, such as the tumour’s size, how quickly the cells are dividing, whether the tumour is invading surrounding tissues and whether there are areas of necrosis (dead tumour cells). These additional factors help determine whether the tumour is benign, malignant, or in a borderline category.

If your pathology report describes atypia, your doctor may recommend further testing or monitoring to better understand the tumour’s behaviour and guide treatment decisions.

What other tests may be performed to confirm the diagnosis?

In addition to examining the tissue under the microscope, pathologists often use other tests to confirm the diagnosis and gather more information about the tumour. These tests include:

• Immunohistochemistry: This test uses special dyes to look for specific proteins on the surface of the tumour cells. In smooth muscle neoplasms, the cells typically express proteins such as desmin, smooth muscle actin (SMA), and h-caldesmon. These findings help confirm that the tumour is made of smooth muscle cells and distinguish it from other types of tumours.
• Next-generation sequencing (NGS): This test examines the genetic material of the tumour cells for specific changes (mutations) that can help identify the type of tumour and guide treatment. Certain genetic alterations may suggest a leiomyosarcoma rather than another type of cancer.

These additional tests help the pathologist provide a more accurate diagnosis and assist doctors in planning the best treatment for the patient.