Fibromatosis: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
September 21, 2026

Fibromatosis is a group of tumors made up of fibrous tissue, the tough connective tissue that supports and holds the body together. These tumors are made of cells called fibroblasts and myofibroblasts, which normally build and repair connective tissue.

No type of fibromatosis spreads to other parts of the body the way cancer does. Some types grow into surrounding tissue and can return to the same place after surgery. Pathologists divide fibromatosis into two main groups based on where the tumor develops.

This article gives an overview of fibromatosis and its types. It explains how doctors make the diagnosis and what the findings in your pathology report mean.

What are the types of fibromatosis?

Superficial fibromatosis. These tumors develop just under the skin, usually in the hands or feet. They are small, grow slowly, and do not spread. Each location has its own name. In the palm, it is called palmar fibromatosis, also known as Dupuytren contracture. On the sole, it is called plantar fibromatosis, also known as Ledderhose disease. In the penis it is called penile fibromatosis, also known as Peyronie disease. Our article on superficial fibromatosis covers this group.
Deep fibromatosis. These tumors develop deeper in the body, in or near muscles and around internal organs. The current name for this group is desmoid-type fibromatosis, and you may also see desmoid tumor on your report. These tumors grow into the surrounding tissue and often come back after surgery, but they do not spread to other organs. Our articles on deep fibromatosis and desmoid fibromatosis cover this group in more detail.

Deep fibromatosis is further named by where it develops in the body:

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, 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.

How are the types different?

Where they start: Superficial fibromatosis develops just under the skin of the hands, feet, or penis. Deep fibromatosis develops in or near muscles, in the abdominal wall, or around the internal organs.
Who they affect. Superficial fibromatosis is more common in older adults and often affects both hands or both feet. Deep fibromatosis affects adults of any age, most often between 20 and 60 years old.
How they behave. Superficial tumors stay small and mainly cause problems by tightening the surrounding tissue. Deep tumors can reach a large size, grow into nearby muscle and bone, and come back after surgery.
How they are treated. A hand or foot specialist treats superficial fibromatosis when it limits movement. Deep fibromatosis is usually watched closely at first, because some of these tumors stop growing or shrink on their own.

What causes fibromatosis?

Most cases of deep fibromatosis are caused by a change in a gene called CTNNB1, which makes a protein called beta-catenin. This change is found in about 85% to 90% of deep fibromatosis 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, which is sometimes called Gardner syndrome. In these people, the tumor is caused by a change in a different gene, called APC. Our article on familial adenomatous polyposis explains this condition.

Other factors are linked to fibromatosis. Deep fibromatosis sometimes develops at the site of previous surgery or injury, and it is more common in women. Some tumors grow during pregnancy, which suggests that hormones such as estrogen play a role. The cause of superficial fibromatosis is not well understood.

What are the symptoms of fibromatosis?

The symptoms of fibromatosis depend on the type and the location of the tumor:

Deep fibromatosis. A firm lump, often in the abdominal wall, shoulder, arm, or leg. The lump may be painful, and it can limit movement when it is near a joint or muscle.
Palmar fibromatosis (Dupuytren contracture). Lumps in the palm and thickened cords of tissue that pull the fingers toward the palm, making them hard to straighten.
Plantar fibromatosis (Ledderhose disease). Lumps in the arch of the foot that can cause pain when walking or standing.
Penile fibromatosis (Peyronie disease). Firm areas in the penis that cause curving during an erection and sometimes pain.

How is the diagnosis made?

A pathologist usually diagnoses fibromatosis after examining a tumor sample under a microscope. The sample may come from a biopsy, which removes a small piece of tissue, or from an excision, which removes the whole tumor.

Under the microscope, fibromatosis is made of long, thin spindle cells that look similar to the cells in normal fibrous tissue. The cells are evenly spaced within a collagen background, and few are dividing. In deep fibromatosis, the tumor cells extend into the surrounding muscle and fat rather than forming a clear border.

Because these features can also be seen in a scar or in other fibrous tumors, extra tests are often performed. A small biopsy may not be enough for a final diagnosis, and the pathologist may describe fibromatosis as a possible diagnosis to confirm after surgery.

Immunohistochemistry

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

Beta-catenin. In normal cells, this protein sits at the cell edge. In most deep fibromatosis tumors, it builds up inside the nucleus, the part of the cell that holds the genetic material. Your report may call this nuclear expression, and it supports the diagnosis.
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, and desmin help rule out other fibrous and nerve tumors that can look similar.

Beta-catenin staining is helpful but not perfect. Some deep fibromatosis tumors do not show nuclear staining, and some other tumors do.

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 deep fibromatosis 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 tumors 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 done on the tumor tissue and can be performed on a biopsy or on tissue removed at surgery.

Tumor extension and margins

Deep fibromatosis grows into surrounding tissue, including muscle, bone, and the tissue around blood vessels and nerves. Your report describes how far the tumor extends, because widely extending tumors are harder to remove completely.

A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge, and a positive margin means tumor cells reach it. For fibromatosis, a positive margin raises the chance that the tumor will come back, but many tumors with a positive margin never do. For this reason, doctors often watch a positive margin rather than treat it with more surgery.

What is the prognosis?

Fibromatosis does not spread to other organs, and it is not a cancer. Superficial fibromatosis is not life-threatening, although it can come back and can limit the use of a hand or foot.

Deep fibromatosis behaves less predictably. 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 treating it.

Features linked to a higher chance that deep fibromatosis 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?

Treatment depends on the type of fibromatosis, where it is, and whether it is causing symptoms. A team at a center experienced in treating sarcomas and related tumors usually manages deep fibromatosis.

Active surveillance. For most people with deep fibromatosis, watching the tumor with regular imaging is now the first step rather than surgery. This approach is recommended because some tumors stop growing or shrink on their own.
Medication. If a tumor keeps growing or causes symptoms, doctors may use medication. The FDA approves Nirogacestat for adults with a desmoid tumor that is growing. Other options include sorafenib, low-dose chemotherapy, and hormone-blocking drugs. Availability differs between countries.
Surgery. Surgery is used less often than in the past, because these tumors come back frequently. It may still be considered for tumors of the abdominal wall or when other treatments have not worked.
Radiation therapy and other local treatments. Radiation and freezing treatment, called cryoablation, may be considered for tumors that cannot be removed or that keep growing.
Superficial fibromatosis. Treatment is considered when the tumor limits movement. Options include injections and surgery performed by a hand or foot specialist.

Questions to ask your doctor

Do I have superficial fibromatosis or deep fibromatosis?
Was beta-catenin staining performed, and what did it show?
Was molecular testing done, and did it find a CTNNB1 change?
Should I be tested for familial adenomatous polyposis?
Where is my tumor, and what structures does it involve?
If the tumor was removed, were the margins negative?
Is active surveillance an option for me?
What would make you recommend starting treatment?
What is the chance this tumor comes back?
How often will I need imaging, and for how long?