Myofibroblastoma

By Jason Wasserman MD PhD FRCPC
July 6, 2025

Myofibroblastoma is a rare, non-cancerous tumour that develops from the connective tissue of the breast. It is made up of cells called fibroblasts and myofibroblasts, which are normal cells involved in wound healing and tissue repair.

Although myofibroblastomas most commonly form in the breast, they can also appear in other parts of the body, including the groin area, vulva, perineum, and scrotum. In these locations, they are often referred to as mammary-type myofibroblastomas.

Who gets myofibroblastoma?

Myofibroblastoma can occur in both men and women, with a wide age range from childhood to late adulthood. However, it is most commonly found in elderly men and postmenopausal women.

What are the symptoms of myofibroblastoma?

Most people notice a painless lump that slowly grows over time. The lump is usually:

  • Firm or rubbery.

  • Well-defined.

  • Non-tender.

  • Movable under the skin.

Because it grows slowly and does not cause pain, it is often found during a routine physical exam or imaging for another reason. Imaging (such as mammogram or ultrasound) usually shows a solid, well-circumscribed mass without concerning features like microcalcifications.

What causes myofibroblastoma?

The exact cause of myofibroblastoma is not known. However, research suggests that hormones may play a role. This is supported by the fact that many tumours produce proteins related to estrogen (ER), progesterone (PR), and androgen (AR) receptors. The tumour may also be associated with hormone-related conditions such as gynecomastia (enlarged breast tissue in men) and pseudoangiomatous stromal hyperplasia (PASH) in women.

Some genetic changes have also been found in tumour cells. Most myofibroblastomas show loss of a portion of chromosome 13 (13q14), which affects a gene called RB1, important in regulating cell growth. These changes are also seen in other similar benign tumours, such as spindle cell lipoma and cellular angiofibroma.

Is myofibroblastoma a cancer?

No. Myofibroblastoma is a benign tumour, not a cancer. The tumour cells do not invade surrounding tissue, do not spread to other parts of the body, and do not behave aggressively. Once removed, the tumour does not come back in most cases.

Does myofibroblastoma increase my chance of developing breast cancer in the future?

No. Myofibroblastoma does not increase your risk of developing breast cancer. It is not related to breast cancer and is not considered a pre-cancerous condition. Most people with myofibroblastoma do not develop any other breast problems after treatment.

What does myofibroblastoma look like under the microscope?

When examined under the microscope, myofibroblastoma shows the following features:

  • Well-circumscribed borders, meaning the tumour is clearly separated from the surrounding tissue.

  • Composed of spindle-shaped or oval cells arranged in short bundles or fascicles.

  • The background may contain collagen, a fibrous protein.

  • The cells are uniform and bland (non-cancerous), with small, round or oval nuclei.

  • Mitoses (dividing cells) are rare or absent.

  • No signs of aggressive behaviour, such as necrosis, infiltrating margins, or high-grade features.

The tumour may show a variety of growth patterns, and some tumours may contain fat cells, myxoid (gel-like) areas, or unusual cells with multiple nuclei (called floret-like cells). These variations are normal in this type of tumour and do not affect the diagnosis.

Immunohistochemistry

Immunohistochemistry is a special test that helps confirm the diagnosis of myofibroblastoma by detecting specific proteins made by the tumour cells.

Tumour cells in myofibroblastoma typically test positive for:

  • Desmin: a muscle-related protein.

  • CD34: a marker found in several types of soft tissue tumours.

  • Estrogen receptor (ER), progesterone receptor (PR), and androgen receptor (AR).

They are typically negative for:

  • S100 and SOX10 (markers seen in nerve-related tumours).

  • Cytokeratins and EMA (seen in carcinomas).

  • STAT6 and ALK (markers seen in other types of soft tissue tumours).

Loss of the RB1 protein is seen in about 90% of cases and can be tested by immunohistochemistry.

Can myofibroblastoma be diagnosed before surgery?

In some cases, a diagnosis may be suspected with a fine-needle aspiration (FNA) or core needle biopsy, which involves removing a small sample of tissue for examination. Under the microscope, the sample typically shows spindle-shaped cells arranged in small groups, which can raise the possibility of myofibroblastoma. However, a definite diagnosis usually requires examination of the entire tumour after surgical removal.

What is the treatment for myofibroblastoma?

Surgical removal (excision) is the treatment of choice. Because myofibroblastoma is benign, removing the tumour is curative. No additional treatment is required after surgery, and there is no risk of the tumour spreading to other parts of the body.

What is the prognosis for myofibroblastoma?

The prognosis is excellent. Myofibroblastoma does not come back (recur) after complete removal and does not transform into cancer. Follow-up after surgery is usually limited to routine exams to check for any new lumps, although recurrence is extremely rare.

Questions to ask your doctor

  • Was the entire tumour removed?

  • Do I need any follow-up or imaging?

  • Is there any chance the tumour could come back?

  • Should I be tested for hormone levels or genetic conditions?

  • What symptoms should I watch for in the future?

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