Desmoid Tumour: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 21, 2026

A desmoid tumor is a growth made of fibrous tissue that develops deep inside the body, in or near muscles and around internal organs. The current medical name is desmoid-type fibromatosis, and your report may also call it deep fibromatosis.

A desmoid tumor is not a cancer. It does not spread to other parts of the body. It can grow into the surrounding tissue, 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.

Desmoid tumors are uncommon, affecting roughly 4 people per million each year. They can develop at any age but are most common in adults between 20 and 60 years old, and they affect women more often than men.

This article explains how a desmoid tumor behaves, what the findings in your pathology report mean, and how these tumors are managed today.

Where do desmoid tumors develop?

A desmoid tumor can develop almost anywhere in the body. The most common locations are the shoulder, chest wall, back, and thigh, the muscles of the abdominal wall, and inside the abdomen.

Tumors of the abdominal wall often occur in women during or after pregnancy, and some develop in a cesarean section scar. Tumors inside the abdomen are more common in people with an inherited condition called familial adenomatous polyposis.

What causes a desmoid tumor?

Most desmoid tumors 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 and is not inherited. It develops in the tumor cells during a person’s lifetime.

About 1 in 10 people with a desmoid tumor have familial adenomatous polyposis, an inherited condition caused by a change in the APC gene. It is sometimes called Gardner syndrome. In these people, the tumor is often inside the abdomen, and the condition also causes many polyps in the colon.

Previous surgery or injury at the same site, and pregnancy, are also linked to desmoid tumors. These factors do not explain every case, and many people have no identifiable trigger.

What are the symptoms of a desmoid tumor?

The symptoms of a desmoid tumor depend on its location and size:

A firm lump. A slow-growing lump is the most common finding, often in the shoulder, back, thigh, or abdominal wall.
Pain. Pain is common and can be severe enough to interfere with daily activities.
Limited movement. A tumor near a joint or muscle can cause stiffness or weakness.
Bowel symptoms. A tumor inside the abdomen can press on the intestines and cause pain, bloating, or a change in bowel habits.

Some desmoid tumors are found by chance on a scan done for another reason and cause no symptoms at all.

How is the diagnosis made?

A pathologist diagnoses a desmoid tumor after examining a tumor sample under a microscope, usually taken with a core needle biopsy. A biopsy before any surgery is important, because most desmoid tumors are now watched rather than removed right away.

Under the microscope, the tumor consists of long, thin spindle cells in a collagen background. The cells do not look abnormal, and few of them are dividing. A stain called beta-catenin supports the diagnosis when the protein builds up in the nucleus of the tumor cells. Molecular testing for a CTNNB1 gene change is used when the diagnosis is uncertain.

Our article on deep fibromatosis explains these pathology findings, including margins and tumor extension, in more detail.

How does a desmoid tumor behave?

Desmoid tumors behave unpredictably, and this is the single most important thing to understand about them:

Some grow. The tumor enlarges over months and presses on nearby structures.
Some stay the same. The tumor stops growing on its own and remains stable for years.
Some shrink. Several desmoid tumors shrink or disappear without treatment.

Because of this, a period of watching the tumor is now recommended before starting treatment for most people. Growth in the first months after diagnosis does not necessarily mean the tumor will keep growing.

What happens after the diagnosis?

A team at a center experienced in treating sarcomas and related tumors best manages desmoid tumors. The plan depends on where the tumor is, whether it is growing, and how much it affects your daily life.

Active surveillance. Watching the tumor with regular imaging, usually MRI, is the recommended first step for most people. National guidelines and international expert groups support this approach.
Medication. Treatment is started when the tumor keeps growing or causes symptoms. The FDA approves Nirogacestat for adults with a desmoid tumor that is growing. Other options include sorafenib, low-dose chemotherapy such as methotrexate with vinblastine, and hormone-blocking drugs. Availability differs between countries.
Surgery. Surgery is now used less often because these tumors come back frequently even after complete removal. It may still be considered for tumors of the abdominal wall or when other treatments have not worked.
Radiation and cryoablation. Radiation therapy and freezing the tumor through the skin may be considered for tumors that cannot be removed or that keep growing.
Pain management. Pain can persist even when the tumor is stable, and it deserves treatment in its own right.

What is the prognosis?

Desmoid tumor do not spread to other organs, and most people live a normal lifespan. The main issues are symptoms, recurrent growth, and treatment effects.After surgery, the tumor recurs in a substantial number of people. Features linked to a higher chance of the tumor coming 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. A specific CTNNB1 change, written as S45F, has been linked to a higher chance of the tumor coming back.

Tumors inside the abdomen, particularly in people with familial adenomatous polyposis, can be harder to treat because of their position near the bowel and its blood supply.

Questions to ask your doctor

Where is my tumor, and which structures does it involve?
Was the diagnosis confirmed with beta-catenin staining or molecular testing?
Should I be tested for familial adenomatous polyposis?
Is active surveillance appropriate for me, and how often would I be scanned?
What would make you recommend starting treatment?
Which medication would you consider first, and what are the side effects?
Is surgery an option in my case, and what is the chance the tumor returns?
What can be done about my pain while the tumor is being watched?
Are there clinical trials I could join?
Who should I contact if my symptoms change between appointments?