Desmoid fibromatosis

by Bibianna Purgina, MD FRCPC
March 9, 2023

What is desmoid fibromatosis?

Desmoid fibromatosis or desmoid tumour is a type of non-cancerous type of tumour. It is considered locally aggressive because it can grow into surrounding tissues and organs. The tumour can also grow back if not completely removed. However, it will not metastasize (spread) to other parts of the body.

Is desmoid fibromatosis a type of cancer?

No, desmoid fibromatosis is not a type of cancer. The tumour, however, can grow into surrounding normal tissues and organs and can grow back if not fully removed.

What are the other names for desmoid fibromatosis?

Other names for desmoid fibromatosis are desmoid tumour, deep fibromatosis, aggressive fibromatosis, abdominal fibromatosis, extra-abdominal fibromatosis, and intra-abdominal fibromatosis. The name used depends on where in the body the tumour was located.

Where is desmoid fibromatosis normally found?

Desmoid fibromatosis can occur almost anywhere in the body. However, the most common locations include the extremities (arms and legs), retroperitoneum (the space at the back of the abdomen), abdominal cavity, and chest wall.

What causes desmoid fibromatosis?

Desmoid fibromatosis is associated with genetic conditions and physical factors including prior trauma, pregnancy, and surgery. People with Familial Adenomatous Polyposis (APC) syndrome, Gardener syndrome, and familial desmoid syndrome have a much higher risk of developing desmoid fibromatosis.

What are the types of desmoid fibromatosis?

Some types of desmoid fibromatosis are given a special name based on the location in the body where the tumour develops. Types of desmoid 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 is the diagnosis of desmoid fibromatosis made?

The diagnosis of desmoid fibromatosis is usually made after a small piece of the tumour is removed in a procedure called a biopsy.  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.

What does desmoid fibromatosis look like under the microscope?

When viewed under the microscope, the tumour is made up of long thin spindle cells that look like the cells found in normal fibrous tissue.  Most of these spindle cells are specialized fibroblasts and myofibroblasts and they form a mass that grows into the surrounding normal tissues.

Because desmoid fibromatosis can look like other tumours that develop from fibrous tissue, it can be difficult for your pathologist to make a definite diagnosis of desmoid fibromatosis with 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.

desmoid fibromatosis
Desmoid fibromatosis. The tumour is made up of long thin cells called spindle cells.

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


Immunohistochemistry is a test that allows pathologists to see different types of proteins made by the tumour cells. When this test is performed, the tumour cells in desmoid fibromatosis are often described as positive or reactive for the proteins smooth muscle actin and desmin. In addition, the cells 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 this tumour, 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 desmoid fibromatosis. These people are said to have a syndrome and the most common syndromes associated with desmoid fibromatosis are Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome and familial desmoid syndrome.

Desmoid fibromatosis in patients that have Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome is caused by inherited mutations in the APC gene. Most tumours 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?

Desmoid fibromatosis is usually a poorly defined tumour that grows into or around neighbouring muscles, bone and blood vessels.  Your pathologist will examine samples of the surrounding tissues under the microscope to look for tumour cells. Any surrounding organs or tissues that contain tumour cells will be described in your report.

What is a margin?

The border between the tumour 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 no tumour cells are at the cut tissue’s edge. 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 (local recurrence).


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