Deep Fibromatosis: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 21, 2026

Deep fibromatosis is a growth made of fibrous tissue that develops deep inside the body, in or near muscles and around internal organs. The current name for this tumor is desmoid-type fibromatosis, and your report may also call it a desmoid tumor. Older reports may use the names abdominal fibromatosis, extra-abdominal fibromatosis, or intra-abdominal fibromatosis, depending on where the tumor was found.

Deep fibromatosis is not a cancer, and it does not spread to other parts of the body. It does grow into the tissue around it, and it can come back in the same place after surgery. For this reason, the World Health Organization places it in a middle category between noncancerous and cancerous tumors.

This tumor can develop at any age but is most common in adults between 20 and 60 years old. It affects women more often than men. Deep fibromatosis is the other main group of fibromatosis, along with superficial fibromatosis, which develops just under the skin.

This article explains how deep fibromatosis is diagnosed and what the microscope findings, beta-catenin result, molecular testing, and margins in your pathology report mean.

Where does deep fibromatosis develop?

Deep fibromatosis can develop almost anywhere. The location is often part of the name used in your report:

Abdominal fibromatosis. This type develops in the muscles of the abdominal wall. It usually occurs in women during or after pregnancy, and it can develop in a cesarean section scar.
Extra-abdominal fibromatosis. This type develops in or near the muscles of the shoulder, chest wall, back, or thigh. It affects men and women.
Intra-abdominal fibromatosis. This type develops in the fat around the bowel, in the pelvis, or in the retroperitoneum, which is the space at the back of the abdomen.

What causes deep fibromatosis?

Most cases of deep fibromatosis are caused by a change in the CTNNB1 gene, which makes a protein called beta-catenin. This change is found in about 85% to 90% of tumors. It develops in the tumor cells during a person’s lifetime and is not inherited.

About 1 in 10 people with deep fibromatosis have an inherited condition called familial adenomatous polyposis, sometimes called Gardner syndrome. In these people, the tumor is caused by an inherited change in the APC gene, and the tumor is often inside the abdomen. Our article on familial adenomatous polyposis explains this condition.

Both gene changes affect the same pathway inside the cell, which normally controls when connective tissue cells grow and stop growing. Other factors are linked to deep fibromatosis. Some tumors develop at the site of previous surgery or injury, and some grow during pregnancy, which suggests that hormones play a role.

What are the symptoms of deep fibromatosis?

The symptoms of deep fibromatosis depend on where the tumor is and how large it is:

A firm lump. Many people notice a lump that grows slowly in the shoulder, chest wall, back, thigh, or abdominal wall.
Pain. Pain is common and can range from mild to severe.
Limited movement. A tumor near a joint or muscle can make the area stiff or hard to move.
Bowel symptoms. A tumor in the abdomen can press on the intestines and cause pain, bloating, or changes in bowel habits.
Nerve symptoms. A tumor pressing on a nerve can cause numbness, tingling, or weakness.

How is the diagnosis made?

The diagnosis of deep fibromatosis is usually made after a pathologist examines a sample of the tumor under the microscope. The sample is usually obtained by a core needle biopsy, which removes small pieces of the tumor with a needle. Making the diagnosis before surgery matters, because most people with this tumor are now watched rather than operated on right away.

Under the microscope, deep fibromatosis is made of long, thin spindle cells that look similar to the cells in normal connective tissue. The cells are evenly spaced in a background of collagen, they do not look abnormal, and very few of them are dividing. At the edge of the tumor, the cells extend into the surrounding muscle and fat instead of forming a clear border.

Because these features can also be seen in a scar and in other fibrous tumors, a small biopsy may not give a final answer. Your report may describe deep fibromatosis as a possible diagnosis to be confirmed with additional tests.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins tumor cells make. For deep fibromatosis, the most useful stain is beta-catenin.

Beta-catenin. In normal cells, this protein sits at the cell edge. In most deep fibromatosis tumor,s it builds up in the nucleus, the part of the cell that holds the genetic material. Your report may call this nuclear expression. The result supports the diagnosis but does not prove it, because some deep fibromatosis tumors show no nuclear staining and some other tumors do.
SMA. This muscle-type protein is often positive in some of the tumor cells, which fits with the myofibroblasts that make up the tumor.
Other stains. Stains such as CD34, S100, desmin, and ALK are usually negative. They are used to rule out other tumors made of spindle cells.

Molecular testing

Molecular testing looks for the gene changes that cause deep fibromatosis. It is used when the diagnosis is uncertain and to sort out which gene is involved:

CTNNB1 change found. This is the expected result for most tumors. It confirms the diagnosis and makes an inherited condition unlikely.
No CTNNB1 change found. This result raises the possibility of familial adenomatous polyposis, particularly for a tumor inside the abdomen. Your doctor may recommend a colonoscopy and a blood test for an inherited APC gene change.

One specific CTNNB1 change, written as S45F, has been linked to a higher chance that the tumor will come back after surgery. This testing is performed on the tumor tissue and can be done on a biopsy or on tissue removed at surgery.

Tumor extension

Deep fibromatosis has a poorly defined edge and grows into surrounding tissue. Your report describes which structures contain tumor cells, which may include muscle, bone, blood vessels, or nearby organs.

Extension matters for two reasons. A tumor that reaches widely into surrounding tissue is harder to remove completely, and its position determines which symptoms it causes. This information is used together with imaging when your team plans treatment.

Margins

A margin is the edge of tissue cut by the surgeon. The pathologist examines each margin to see whether tumor cells reach it.

Negative margin. No tumor cells are seen at the cut edge.
Positive margin. Tumor cells are present at the cut edge, and some tumor may remain.

Margins are interpreted differently for deep fibromatosis than for cancer. A positive margin raises the chance that the tumor will come back, but many tumors with a positive margin never do. Some tumors come back even after a negative margin. For this reason, a positive margin is often watched with imaging rather than treated with more surgery.

What is the prognosis?

Deep fibromatosis does not spread to other organs and is not a cancer. The outlook depends on whether the tumor keeps growing and where it is located.

The behavior of this tumor is hard to predict. Some tumors grow steadily, some stay the same size for years, and some shrink or disappear without any treatment. Because of this, doctors often watch the tumor for a period before starting treatment.

Features linked to a higher chance that the tumor will come back include:

Location. Tumors of the arms and legs come back more often than tumors of the abdominal wall.
Age. People aged 30 and younger have a higher chance of the tumor coming back.
Tumor size. Tumors 5 cm or larger come back more often.
Gene change. The S45F change in CTNNB1 has been linked to a higher chance of the tumor coming back.

What happens after the diagnosis?

Deep fibromatosis is usually managed by a team at a center experienced in treating sarcomas and related tumors. Treatment options are described in more detail in our article on desmoid tumor.

Active surveillance. Watching the tumor with regular imaging is now the usual first step, because some tumors stop growing or shrink on their own.
Medication. Treatment is started when a tumor keeps growing or causes symptoms.
Surgery. Surgery is used less often than in the past, because the tumor frequently comes back.
Other local treatments. Radiation therapy and freezing treatment, called cryoablation, may be considered for tumors that cannot be removed.

Questions to ask your doctor

Where is my tumor, and which structures does it involve?
Was beta-catenin staining performed, and what did it show?
Was molecular testing done, and was a CTNNB1 change found?
Should I be tested for familial adenomatous polyposis?
If the tumor was removed, were the margins negative, and does that change the plan?
Is active surveillance an option for me?
What would make you recommend starting treatment?
What is the chance that this tumor comes back?
How often will I need imaging, and for how long?
Which symptoms should prompt me to call sooner?