Myofibroblastoma

By Jason Wasserman MD PhD FRCPC
August 14, 2024


Background:

A myofibroblastoma is a rare, benign (noncancerous) tumour that typically develops in the soft tissues of the breast, although it can occur in other parts of the body as well. The tumour is made up of myofibroblasts, which are specialized cells that have features of both muscle cells and fibroblasts (cells that produce connective tissue). Myofibroblastomas are generally slow-growing and do not spread to other body parts.

What are the symptoms of myofibroblastoma?

Most people with a myofibroblastoma do not experience any symptoms. The tumour is often discovered incidentally during a physical exam or imaging test for another condition. When symptoms do occur, they may include:

  • A painless lump: Myofibroblastomas usually present as a painless lump or mass that can be felt under the skin. The lump may be firm or rubbery to the touch.
  • Pressure or discomfort: If the tumour grows large enough, it may cause pressure or discomfort in the affected area.

What causes a myofibroblastoma?

The exact cause of myofibroblastoma is not well understood. Most cases occur sporadically, meaning they happen by chance and are unrelated to known risk factors or inherited conditions. Myofibroblastoma is not associated with any specific lifestyle factors, and it is not known to be caused by environmental exposures.

What genetic changes are found in myofibroblastoma?

Some myofibroblastomas have been found to have genetic changes involving the 13q14 region of chromosome 13, which includes the RB1 gene. These genetic changes may play a role in the development of the tumour, but the exact mechanisms are still being studied. It is important to note that these genetic changes are not inherited but only present in the tumour cells.

Microscopic features of myofibroblastoma

Under the microscope, a myofibroblastoma is typically well-circumscribed, meaning it has a clear boundary separating it from the surrounding tissue, but it is not encapsulated (not surrounded by a distinct capsule of tissue). The tumour is made up of short spindle cells, which are elongated cells that resemble fibroblasts. These spindle cells show minimal mitotic activity, meaning they are not actively dividing, which is a sign of the tumour’s benign nature. Unlike more aggressive tumours, necrosis (dead tissue) is not seen in myofibroblastomas.

Immunohistochemistry

Immunohistochemistry (IHC) is a special test that pathologists use to help confirm the diagnosis of myofibroblastoma and to rule out other similar-looking tumours. In this test, special markers are applied to the tissue sample to detect specific proteins in the tumour cells. Most myofibroblastomas are positive for desmin and CD34, which are proteins commonly found in these tumours. The presence of these markers helps pathologists distinguish myofibroblastoma from other tumours that may look similar under the microscope.

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