Myofibroblastoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 28, 2026

Myofibroblastoma is an uncommon noncancerous tumor that develops from the connective tissue of the breast. It is made of fibroblasts and myofibroblasts, cells that normally build connective tissue and help wounds heal.

Myofibroblastoma does not spread to other parts of the body and does not turn into cancer. Removing it is curative for almost everyone.

Although most develop in the breast, the same tumor occurs in the groin, vulva, perineum, and scrotum. In those locations, it is often called a mammary-type myofibroblastoma because it looks the same as the breast tumor under the microscope.

This article explains how a myofibroblastoma is diagnosed and what the findings in your pathology report mean.

Who gets myofibroblastoma?

Myofibroblastoma occurs in both men and women, at ages ranging from childhood to late adulthood. It is most common in older men and in women after menopause.

What are the symptoms of myofibroblastoma?

Most people notice a painless lump that grows slowly. The lump is usually firm or rubbery, has a clear edge, and moves easily when pressed.

Because it grows slowly and does not hurt, a myofibroblastoma is often found during a physical examination or on imaging done for another reason. On a mammogram or ultrasound, it usually appears as a solid lump with a smooth border and none of the features that suggest cancer.

What causes myofibroblastoma?

The exact cause is unknown, although hormones appear to play a role. Most of these tumors make proteins that respond to estrogen, progesterone, and androgens. Myofibroblastoma is also associated with hormone-related conditions, including enlarged breast tissue in men and pseudoangiomatous stromal hyperplasia in women.

Most myofibroblastomas have lost a small part of chromosome 13, in a region known as 13q14. This region contains a gene called RB1, which normally helps control cell growth. The same change is found in several other noncancerous tumors, including spindle cell lipoma and cellular angiofibroma.

This change happens in the tumor cells during a person’s lifetime. It is not inherited, and it does not carry the risks associated with the inherited condition that affects the same gene in the eye.

Does a myofibroblastoma increase my risk of breast cancer?

No. A myofibroblastoma is not related to breast cancer and is not a precancerous condition. It does not raise your risk of developing breast cancer later.

Most people who have a myofibroblastoma removed have no further breast problems.

How is the diagnosis made?

Doctors usually confirm the diagnosis after removing the whole tumor and examining it under a microscope. A fine needle aspiration or a core needle biopsy may be done first, and can raise the possibility of a myofibroblastoma.

A small sample is often not enough for a final answer, because several breast lumps are made of similar-looking spindle-shaped cells. Your report may therefore describe a spindle cell lesion on the biopsy and give the final diagnosis after surgery.

What does myofibroblastoma look like under the microscope?

Under the microscope, a myofibroblastoma has a clear border separating it from the surrounding breast tissue. It consists of spindle-shaped and oval cells arranged in short bundles within a collagen background.

The cells look uniform, with small round or oval nuclei. Mitotic figures, which are cells in the process of dividing, are rare or absent. There is no dead tumor tissue, and the tumor does not grow into the surrounding breast.

Some myofibroblastomas contain fat, gel-like myxoid areas, or large cells with several nuclei arranged like flower petals. These variations are expected and do not change the diagnosis.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins tumor cells make. For a myofibroblastoma, it confirms the diagnosis and rules out other breast lumps made of spindle cells. Your report may include some of the following:

Desmin and CD34. Both are typically positive, and this combination is characteristic of a myofibroblastoma.
Hormone receptors. Estrogen, progesterone, and androgen receptors are often positive, which fits with the hormonal association.
RB1. This protein is lost in about 9 out of 10 myofibroblastomas, reflecting the loss of the RB1 gene.
S100 and SOX10. These are negative. They are used to rule out a nerve sheath tumor.
Cytokeratins and EMA. These are negative. They help rule out carcinoma, a cancer that starts in epithelial cells.
STAT6 and ALK. These are negative. They help rule out other soft tissue tumors.

How is this different from a breast cancer?

Several breast lumps are made of spindle-shaped cells, and a few of them are cancers. That is why pathologists often perform extra stains even when the tumor looks noncancerous.

A myofibroblastoma has a clear border, uniform cells, and almost no dividing cells, and it does not grow into the surrounding breast. The stains above separate it from a carcinoma and from other spindle cell tumors. A report naming a myofibroblastoma means the pathologist has considered and excluded these possibilities.

Margins

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

For a myofibroblastoma, a positive margin does not carry the meaning it does for cancer. It means only that a small amount of tumor may remain. Because these tumors have a clear border and grow slowly, more surgery is usually not needed.

What is the prognosis?

The prognosis is excellent. A myofibroblastoma is noncancerous, does not spread, and does not become cancer.

Complete removal is curative for almost everyone, and the tumor comes back only rarely, usually when some of it was left behind. A tumor that comes back is still noncancerous.

What happens after the diagnosis?

Surgery. Removing the tumor is the usual treatment and cures the condition. No further treatment is needed afterward.
Observation. A small tumor confirmed on biopsy and causing no symptoms may be watched rather than removed, particularly when surgery would be difficult.
Follow-up. Special imaging follow-up is not usually required. People continue with the routine breast screening recommended for their age.

Questions to ask your doctor

Was the whole tumor removed, and did it reach a margin?
Is there any chance the tumor will come back?
Do I need follow-up imaging, or should I continue with routine screening?
Which tests were used to rule out cancer?
Does this finding change my breast cancer risk?
What should I do if I notice a new lump?