Myoepithelioma of soft tissue

by Jason Wasserman MD PhD FRCPC
August 3, 2023

What is a myoepithelioma?

A myoepithelioma is a non-cancerous type of soft tissue tumour. It made up of specialized myoepithelial cells that arise from cells within soft tissue such as muscle, cartilage, or connective tissue. This tumour should not be confused with the similarly named myoepithelioma that starts in the salivary glands.

Where is myoepithelioma normally found?

Most myoepitheliomas of soft tissue are found in the arms or legs or around the shoulders. Less common locations include the head and neck and chest. Rarely the tumour will start inside a bone.

Is myoepithelioma a type of cancer?

No. Myoepithelioma is a benign (non-cancerous) type of tumour.

What causes a myoepithelioma?

Most myoepitheliomas of soft tissue contain a genetic alteration involving the gene EWSR1. At present, doctors do not know what causes this genetic alteration to occur.

What are the symptoms of myoepithelioma?

The symptoms of myoepithelioma are typically a slow-growing, painless mass.

How is myoepithelioma diagnosed?

The diagnosis of myoepithelioma can be made after tissue from the tumour is examined under the microscope by a pathologist.

What does a myoepithelioma of soft tissue look like under the microscope?

When examined under the microscope, myoepithelioma of soft tissue is made up entirely of myoepithelial cells. The tumour cells may look spindled (long and thin), epithelioid (round and connected together), or plasmacytoid (similar to plasma cells). The stroma (connective tissue) surrounding the tumour cells may be described as myxoid, chondromyxoid, or fibrotic. The tumour cells usually show very little cytologic atypia and mitotic figures (cells dividing to create new cells) are rarely seen.

Myoepithelioma of soft tissue
Myoepithelioma of soft tissue. This picture shows a tumour made up of epithelioid cells arranged in small groups.

What other tests may be performed to confirm the diagnosis?


Immunohistochemistry (IHC) is a test that allows your pathologist to see markers such as proteins and other chemicals inside tumour cells. Cells that make (or express) a marker are called positive or reactive while those that do not make a marker are called negative or non-reactive. When immunohistochemistry is performed on a myoepithelioma of soft tissue, the tumour cells are usually positive for markers such as pan-cytokeratin, S100, SOX10, and GFAP.

Next-generation sequencing

Next-generation sequencing (NGS) is a test that allows your pathologist to look for genetic alterations in a tumour. When next-generation sequencing is performed on myoepithelioma of soft tissue, over half of the tumours will show a rearrangement involving the EWSR1 gene.

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