Desmoid tumour

What is a desmoid tumour?

A desmoid tumour is a type of non-cancerous type of tumour made up of spindle cells. Most tumours start in fibrous tissue such as the wall of the abdomen or the tissues that cover the internal organs. Other names for a desmoid tumour are 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.

Who develops a desmoid tumour?

Desmoid tumours typically develops in teenagers and young adults and the tumour can sometimes cause pain.  Although a desmoid tumour 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 a desmoid tumour will not, however, spread to other parts of the body, as cancers are known to do. Desmoid tumours can run in families and is seen in genetic syndromes, including familial Adenomatous Polyposis Syndrome (APC)/Gardner syndrome or familial desmoid syndrome.

Types of desmoid tumours

Some types of desmoid tumours are given a special name based on the location in the body where the tumour develops. Types of desmoid tumours 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 a desmoid tumour 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.

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.

Deep fibromatosis

Your pathologist may also perform a test called immunohistochemistry to confirm the diagnosis. This test allows your pathologist to see different types of proteins made by the tumour cells. When this test is performed, the tumour cells in a desmoid tumour are often described as positive or reactive for the proteins smooth muscle actin and desmin. In addition, the cells in a desmoid tumour 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 a desmoid 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.

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

Molecular tests

Each cell in your body contains a set of instructions that tell the cell how to behave. These instructions are written in a language called DNA and the instructions are stored on 46 chromosomes in each cell. Because the instructions are very long, they are broken up into sections called genes and each gene tells the cell how to produce a piece of the machine called a protein.

If the DNA becomes damaged or if it cannot be read accurately, the cell will be unable to produce the proteins it requires to function normally. An area of damaged DNA is called a mutation and mutations are one of the most common causes of cancer in humans.

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

A desmoid tumour in patients that have Familial Adenomatosis Polyposis Syndrome/Gardner Syndrome is caused by the inherited mutations in the APC gene. Most desmoid 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 to look for in your report after the tumour has been removed

Tumour size

After the tumour is completely removed your pathologist will measure it in three dimensions but only the largest dimension is typically included in your report. For example, if the tumour measures 5.0 cm by 3.2 cm by 1.1 cm, the report may describe the tumour size as 5.0 cm in the greatest dimension.

Tumour extension

A desmoid tumour 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.


The border between a desmoid 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 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 (local recurrence).


by Bibianna Purgina, MD FRCPC (updated August 30, 2021)
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