Desmoid tumors, also known as aggressive fibromatosis, are rare, soft-tissue growths originating in the body’s connective tissues. They are classified as non-cancerous because they do not spread to distant organs (metastasize). However, they are locally aggressive, meaning they can invade and severely damage surrounding tissues and organs. This infiltrative nature makes them challenging to manage and often necessitates the involvement of specialists who treat malignant diseases.
Defining the Nature of Desmoid Tumors
Desmoid tumors arise from fibroblasts, the cells responsible for structural support and wound healing within connective tissue. Since connective tissue is found throughout the body, desmoid tumors can appear in almost any location, including the abdominal wall, limbs, and mesentery. The World Health Organization classifies them as intermediate tumors because they exhibit locally invasive growth without the potential for distant spread.
The primary concern is their tendency to infiltrate deeply into adjacent structures like muscle, nerves, and blood vessels. This local destruction can cause significant health problems and functional impairment, even though the tumor itself is not malignant. Desmoid tumors are distinct from true sarcomas, which are connective tissue cancers that can metastasize throughout the body.
These tumors are relatively rare, affecting an estimated two to six individuals per million worldwide. Their clinical course is highly variable; some may grow slowly or even spontaneously regress, while others exhibit rapid, aggressive growth. This unpredictable nature often complicates the decision-making process for treatment.
Identifying Causes and Risk Factors
The formation of a desmoid tumor is linked to a mutation that causes an overproduction of the protein beta-catenin, which drives cell growth and division. In most cases, this occurs sporadically, meaning the mutation is not inherited and happens during a person’s lifetime. The majority of these sporadic cases involve a somatic mutation in the CTNNB1 gene, which provides instructions for making beta-catenin.
A smaller, but significant, portion of desmoid tumors is associated with the inherited condition Familial Adenomatous Polyposis (FAP). FAP is caused by a germline mutation in the APC gene, which normally helps regulate beta-catenin levels. People with FAP have an increased risk of developing desmoid tumors, which often occur within the abdomen.
Beyond these genetic factors, certain physical and hormonal circumstances appear to trigger the development of sporadic desmoid tumors. Previous trauma or surgical procedures are recognized as potential risk factors. Hormonal factors also play a role, as desmoid tumors are more common in women, and pregnancy is a known predisposing factor, particularly for tumors found in the abdominal wall.
Common Symptoms and Tumor Locations
The most common initial indication of a desmoid tumor is a palpable mass or lump that feels firm and rubbery. These tumors often present without pain in their early stages, which can lead to a delay in seeking medical attention. As the tumor enlarges and begins to press against or infiltrate nearby structures, symptoms become more apparent.
The specific symptoms experienced depend entirely on the tumor’s location within the body. Desmoid tumors are broadly categorized as extra-abdominal (limbs, chest wall), abdominal wall, or intra-abdominal (mesentery). A tumor in an extremity may cause pain, swelling, or a tingling sensation due to pressure on nerves or blood vessels.
Intra-abdominal desmoid tumors, particularly those associated with FAP, can result in serious complications. When located in the mesentery, they can press on the intestines, leading to symptoms like nausea, cramping, constipation, or even bowel obstruction. Tumors near a joint can restrict movement, causing a loss of function in the affected limb.
Diagnosis and Modern Management Approaches
The pathway to a definitive diagnosis begins with imaging, typically using Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans. Imaging is essential for determining the tumor’s size, its exact relationship to surrounding organs, and the extent of its local invasion. Imaging alone cannot confirm the diagnosis because desmoid tumors can resemble other types of soft-tissue growths.
A core needle biopsy is the necessary next step, where a small tissue sample is extracted and examined by a specialized pathologist. This biopsy is crucial for confirming the presence of the characteristic fibroblastic cells and for genetic analysis to identify the specific CTNNB1 or APC gene mutation. Genetic information helps guide the choice of systemic therapy.
Modern management strategies have shifted away from immediate aggressive surgery due to the high likelihood of the tumor recurring, often at a larger size. Active surveillance, or “watchful waiting,” is now the preferred initial approach for many stable or minimally symptomatic tumors. This involves closely monitoring the tumor with regular imaging scans over one to two years.
If the tumor grows rapidly, causes significant symptoms, or threatens vital organs, active treatment becomes necessary. Systemic drug therapies are now frequently used as a first-line treatment instead of surgery. Targeted therapies, such as tyrosine kinase inhibitors (e.g., sorafenib) and gamma-secretase inhibitors (e.g., nirogacestat), have shown success by blocking the cellular pathways that drive tumor growth.
Other non-surgical options include low-dose chemotherapy regimens and local treatments like radiation therapy or cryoablation. Radiation therapy may be used to shrink the tumor or prevent recurrence, particularly when surgery would result in significant functional loss. The choice of treatment is individualized, depending on the tumor’s location, its growth rate, and the specific symptoms a person is experiencing.