by Bibianna Purgina, MD FRCPC
December 6, 2022

What is fibromatosis?

Fibromatosis is a non-cancerous tumour made up of a specialized type of connective tissue called fibrous tissue. Pathologists divide fibromatosis into two categories depending on where the tumour develops. Tumours that develop just under the skin are called superficial fibromatosis. Tumours that develop deeper within the body are called deep fibromatosis.

Types of fibromatosis

Superficial fibromatosis

These tumours are described as ‘superficial’ because they develop just under the skin, often in the hands or feet. There are other names for superficial fibromatosis, based on where in the body it develops. When this condition develops in the palm of the hand it is called palmar fibromatosis (Dupuytren’s contracture). In this location, it can form hard bumps on the palm of the hand and puckering of the skin. This can make it difficult to extend your fingers. Dupuytren’s contracture is more common in older patients and can affect both hands. When this condition develops on the bottom of the foot (the sole) is called plantar fibromatosis or Ledderhose’s disease. This condition can also involve the penis and is called penile fibromatosis or Peyronie’s disease. Penile fibromatosis typically affects men over the age of 40.

Deep fibromatosis

These tumours are described as ‘deep’ because most tumours start in the wall of the abdomen or the tissues that cover the internal organs. Other names for deep fibromatosis are desmoid tumour, aggressive fibromatosis, abdominal fibromatosis, extra-abdominal fibromatosis, and intra-abdominal fibromatosis. The name used depends on where in the body the tumour was located.

Deep fibromatosis typically develops in teenagers and young adults and the tumour can sometimes cause pain.  Although deep fibromatosis is a non-cancerous tumour, it can grow back in the same area after surgery. This is called a local recurrence. The tumour cells in deep fibromatosis will not, however, spread to other parts of the body, as cancers are known to do. Deep fibromatosis can run in families and is seen in genetic syndromes, including familial Adenomatous Polyposis Syndrome (APC)/Gardner syndrome or familial desmoid syndrome.

Some types of deep fibromatosis are given a special name based on the location in the body where the tumour develops. Types of deep fibromatosis include:

  • Abdominal fibromatosis: This type develops in or near the muscles in the wall of the abdomen of women, usually during or after pregnancy and can develop in a Cesarean section scar.
  • Extra-abdominal fibromatosis: This type usually develops in or near the muscles of the shoulder, chest, back or thigh and can affect men and women equally.
  • Intra-abdominal fibromatosis: This type develops in the fat around the bowel or in the pelvis or the back of the abdomen (an area referred to as retroperitoneum).
How do pathologists make this diagnosis?

The diagnosis of fibromatosis is usually made after a small piece of the tumour is removed in a procedure called a biopsy or after the entire tumour is removed in a procedure called an excision. The tissue is then sent to a pathologist who examines it under a microscope. Sometimes additional tests such as immunohistochemistry or molecular testing may be performed to confirm the diagnosis.

Because deep fibromatosis can look like other tumours that develop from fibrous tissue, it can be difficult for your pathologist to make a definite diagnosis of deep fibromatosis with only the small amount of tissue provided with a biopsy.  However, your pathologist may suggest this diagnosis as a possibility to your clinician in the pathology report.

What does fibromatosis look like under the microscope?

When viewed under the microscope, both superficial and deep fibromatosis are made up of long thin spindle cells that look like the cells found in normal fibrous tissue. These cells are called fibroblasts and myofibroblasts and they form a mass that grows into the surrounding normal tissues.

The number of these fibroblasts and myofibroblasts cells in the tumour changes depending on how long the tumour has been there.  Usually, tumours that have been there for a long time have fewer cells.

Deep fibromatosis
This image shows the long thin spindle cells in fibromatosis.

What other tests may be performed to confirm the diagnosis of fibromatosis?


Your pathologist may also perform a test called immunohistochemistry to look at the proteins being made by the tumour cells. When this test is performed, the tumour cells in deep fibromatosis are often described as positive or reactive for the proteins smooth muscle actin and desmin. In addition, the cells in deep fibromatosis often show abnormal expression of a protein called beta-catenin. This protein is normally found in a part of the cell called the membrane. In deep fibromatosis, the beta-catenin protein does not move normally to the membrane of the cell. Instead, the beta-catenin protein builds up in a part of the cell called the nucleus. Pathologists often describe this as nuclear expression. If the beta-catenin protein is found mostly in the nucleus of the cell, this is considered abnormal and may be associated with a mutation in the genes for either APC or CTNNB1.

Molecular tests

Some people inherit particular genes that put them at a much higher risk of developing tumours such as fibromatosis. These people are said to have a syndrome and the most common syndromes associated with deep fibromatosis are Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome and familial desmoid syndrome.

Deep fibromatosis in people that have Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome is caused by inherited mutations in the APC gene. Most deep fibromatoses that develop in patients without a genetic syndrome have mutations in the CTNNB1 gene (also known as the beta-catenin gene).

Pathologists can test for these genetic changes by performing next-generation sequencing (NGS) on a piece of the tissue from the tumour. This type of testing is can be done on the biopsy specimen or when your tumour has been surgically removed.

What does tumour extension mean?

Deep fibromatosis starts in connective tissue but the tumour cells often grow into surrounding organs such as muscles, bone and blood vessels. This is called tumour extension. Tumour extension is important because tumours that extend very widely into surrounding tissues may be more difficult to remove fully and may grow back after surgery.

What is a margin?

The border between deep fibromatosis and the surrounding normal tissue is often not easy to see. For this reason, most surgeons will remove the tumour with some normal-looking tissue in order to make sure the entire tumour is removed. The normal tissue removed with the tumour is called a margin.

All margins will be very closely examined under the microscope by your pathologist to determine the margin status. A margin is considered negative when there are no tumour cells at the edge of the cut tissue. A margin is considered positive when there are tumour cells at the edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment. Margins are typically not examined for superficial fibromatosis.


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