A desmoid tumor is an abnormal growth formed from the connective tissue that supports muscles, ligaments, organs, and bones throughout the body. Desmoid tumors occur in one or two of every 500,000 people worldwide, with between 900 and 1,500 new cases diagnosed annually in the U.S. Desmoid tumors are benign, which means they are not cancerous, but nor are they completely harmless.

Structure and Location

People usually have only one desmoid tumor, although multiple tumors can develop. The tumors can form almost anywhere in the body, but they are most common in the abdominal region, in the tissue along the abdominal wall. Intra-abdominal desmoid tumors form within connective tissues between abdominal organs. Experts refer to desmoid tumors anywhere else in the body as extra-abdominal. The shoulders, head, neck, upper legs, and upper arms are the most common sites outside of the abdominal region.

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Although desmoid tumors are not cancerous, they can spread aggressively throughout surrounding connective tissues. Symptoms develop due to growth and invasion of surrounding tissues. Pain is the most common symptom, and others depend on the location and size of the tumors. Intra-abdominal desmoid tumors may cause constipation by blocking parts of the bowel. A desmoid tumor in the arm or leg can hinder movement or cause limping by interfering with joints.

symptom, abdominal pain, constipation, limping Milos Dimic / Getty Images


Risk Factors and Causes

People with familial adenomatous polyposis or FAP, an inherited type of colon cancer, are at risk of developing desmoid tumors. Sporadic desmoid tumors are much more common, and somatic gene mutations is generally the cause. Somatic gene mutations occur at some point during a person's lifetime instead of being present at birth. A mutation in one of two genes, CTNNB1 gene or the APC gene, cause desmoid tumors. The former causes approximately 85% of sporadic desmoid tumors, while APC gene mutations are associated with FAP and about 15% of sporadic desmoid tumors.

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FAP is caused by an inherited mutation in one copy of the APC gene. It is a risk factor for abnormal growths or polyps and cancerous tumors in the colon. A somatic mutation in the second APC gene triggers desmoid tumors, which are almost always intra-abdominal in these cases. Experts refer to this condition as hereditary desmoid disease, aggressive fibromatosis, or familial infiltrative fibromatosis.

polyp, cancer, bowel, FAP, intra-abdominal laymul / Getty Images



The CTNNB1 gene and APC gene are both involved in signal pathways between cells that regulate cell growth and division. This is an important part of the differentiation process by which cells to reach maturity and perform specific functions. Cells in connective tissue, called myofibroblasts, are believed to be the source of desmoid tumors.

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The CTNNB1 gene controls the production of the protein beta-catenin, which interacts with other proteins in the cell-signaling pathway to control expression of specific genes. The genes affected by beta-catenin influence cell proliferation and differentiation. The CTNNB1 gene mutation causes abnormalities in this protein. Beta-catenin should break down when the body no longer needs it, but the abnormal proteins are not. This leads to a build-up in cells, and the protein continues to tell the cells to divide long after they should have stopped, leading to the uncontrolled growth of desmoid tumors.

protein, desmoid tumors, beta-catenin, cells enot-poloskun / Getty Images


APC Gene

The APC gene controls the production of a protein that regulates beta-catenin. The protein binds to beta-catenin when the differentiation and proliferation process is complete. The protein-binding signals that the process is finished and it is time to break down the beta-catenin. The APC gene mutation creates short proteins that cannot bind with beta-catenin, and as such, beta-catenin does not break down and uncontrolled growth continues.

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Doctors and other health care providers may not at first suspect a desmoid tumor when a person does not have a history of FAP or growths and polyps in the bowel. The first step is obtaining a medical history and description of reported symptoms. A desmoid tumor may be palpable if it is large enough or growing close to the surface. Imaging tests such as CT or MRI scans provide an image of the tumor. Doctors can confirm the diagnosis by removing the tumor, performing a biopsy for lab study.

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Doctors may choose to monitor desmoid tumors without intervention because there is a chance that the tumor will stop growing and will not require treatment. Radiation and chemotherapy can shrink or halt the growth of desmoid tumors, but both of those treatments have unpleasant side effects. Sometimes extreme heat or cold is effective, and many desmoid tumors respond to non-invasive treatments such as anti-inflammatory drugs or hormones.

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It is difficult to remove desmoid tumors surgically, and there is a high risk of recurrence. Surgery isn't always possible with deeply invasive desmoid tumors because removal would damage surrounding organs and structures. Superficial tumors that develop near the surface are less invasive, but abdominal and intra-abdominal tumors are considered deep tumors. Mesenteric tumors are a sub-type of intra-abdominal tumors that form in the mesentery tissues connecting the intestines to the abdominal wall. This type can cause severe complications including bowel blockages and even tissue death in segments of the intestines. Physicians usually treat mesentric tumors with surgical removal followed by frequent monitoring in case of recurrence.

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