Myoepithelioma of Soft Tissue: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 3, 2026

A myoepithelioma of soft tissue is an uncommon tumor whose cells resemble myoepithelial cells. These are cells normally found in glands such as the breast and salivary glands, where they surround the gland and help push its secretions out.

Soft tissue does not normally contain myoepithelial cells, so the tumor is named for what its cells look like rather than where they came from. The normal cell of origin is unknown.

Most myoepitheliomas of soft tissue are noncancerous. A smaller number are cancerous and are called myoepithelial carcinoma. Your report will state which one you have, and this article explains how pathologists tell them apart.

This tumor is different from the similarly named myoepithelioma of the salivary glands, which is covered in its own article.

What are the types of myoepithelial tumor?

Pathologists group these tumors into a family with three members:

Myoepithelioma. Made entirely of myoepithelial-type cells that look uniform and only mildly abnormal. This is the noncancerous form and the most common one.
Mixed tumor. Similar to a myoepithelioma but also contains small gland-like structures. It is also noncancerous.
Myoepithelial carcinoma. The cancerous form. The cells look clearly abnormal, and this tumor can spread to other parts of the body. It is more common in children than in adults.

Where does a myoepithelioma of soft tissue develop?

Most develop in the arms or legs, including the hands and feet, or around the shoulders. Less common locations include the head and neck and the trunk. Rarely, the tumor starts inside a bone.

These tumors occur at all ages, from children to older adults, and affect men and women about equally. Many sit in the tissue just beneath the skin, while others are deeper.

What are the symptoms of a myoepithelioma?

The usual finding is a slow-growing lump that does not hurt. Most are a few centimeters across when found.

A myoepithelial carcinoma may grow more quickly or become painful, although this is not always the case. The symptoms alone cannot tell the two apart.

What causes a myoepithelioma?

About half of soft tissue myoepitheliomas have a fusion involving a gene called EWSR1. A fusion is an abnormal joining of two genes that are normally separate. A smaller number have a fusion involving a related gene called FUS.

Some tumors, particularly myoepithelial carcinomas in children, have lost a gene called SMARCB1. These changes develop in the tumor cells during a person’s lifetime. They are not inherited and cannot be passed on to children. Doctors do not know why they occur.

How is the diagnosis made?

A pathologist makes the diagnosis after examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that removes the whole tumor.

Under the microscope, the tumor cells may be spindle-shaped (long and thin) or epithelioid (round and joined together). Some resemble plasma cells, a type of immune cell, and are described as plasmacytoid. The cells are often arranged in cords or small groups.

The supporting tissue around the cells, called stroma, may be gel-like and described as myxoid, partly cartilage-like and described as chondromyxoid, or dense and fibrotic. Many tumors contain a mixture of these.

How do pathologists tell benign from malignant?

The single most important feature is how abnormal the cells look, which pathologists call cytologic atypia.

Mild or no atypia. The cells look uniform, with small, regular nuclei. The tumor is reported as a myoepithelioma, which is noncancerous.
Moderate or severe atypia. The cells have large, irregular nuclei with prominent centers. The tumor is reported as a myoepithelial carcinoma.

Other features, such as the number of dividing cells or areas of dead tumor tissue, are more common in carcinoma but do not decide the diagnosis on their own. The appearance of the cells is what matters most.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins tumor cells make. For this tumor, it is essential because the diagnosis depends on showing an unusual combination of proteins. Your report may include some of the following:

Cytokeratins and EMA. These proteins are normally found in epithelial cells. At least one is usually positive.
S100, SOX10, and GFAP. These proteins are often positive and, combined with an epithelial marker, support the diagnosis.
Calponin, SMA, and p63. These reflect the muscle-like and myoepithelial features of the cells and are positive in some tumors.
INI1. This protein is normally present. Loss of INI1 staining reflects loss of the SMARCB1 gene and is seen more often in myoepithelial carcinoma.

No single stain confirms the diagnosis. The pathologist looks for the combination of an epithelial marker with S100, SOX10, or GFAP.

Molecular testing

Molecular tests such as next-generation sequencing (NGS) or FISH may be used to look for an EWSR1 or FUS fusion. Finding one supports the diagnosis, especially when the microscopic appearance is unusual.

A negative result does not rule out a myoepithelioma, since about half of these tumors do not have a fusion. The molecular result supports the diagnosis rather than deciding whether the tumor is benign or malignant.

Margins

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

Margins matter for this tumor, even the noncancerous form, because about 1 in 5 benign-appearing myoepitheliomas come back in the same place. A positive margin raises that chance. For a myoepithelial carcinoma, a negative margin is the goal of surgery.

What is the prognosis?

For a myoepithelioma, the outlook is very good. The tumor does not spread, and complete removal is curative for most people. About 1 in 5 come back in the same place, usually when tumor was left behind, and further surgery is then needed.

A myoepithelial carcinoma behaves like a cancer. It can come back after surgery and can spread, most often to the lungs, lymph nodes, bone, or other soft tissue. The outlook depends on the size and location of the tumor and whether it has spread when it is found.

What happens after the diagnosis?

Myoepithelioma. Surgery to remove the tumor with clear margins is the usual treatment. Follow-up visits are recommended for a period afterward, because the tumor can come back.
Myoepithelial carcinoma. A team experienced in sarcomas plans treatment. It usually involves wider surgery, often with radiation before or after the operation, and imaging to check for spread.
Further surgery. If the tumor reaches a margin, a second operation may be recommended, particularly for a carcinoma.

Questions to ask your doctor

Does my report describe a myoepithelioma, a mixed tumor, or a myoepithelial carcinoma?
Was the whole tumor removed, and were the margins negative?
Was molecular testing done, and was a fusion found?
Is there a chance the tumor will come back?
Do I need further surgery or radiation?
How often will I need follow-up, and for how long?
Do I need imaging to check for spread?