Desmoid tumors are rare soft-tissue growths, also known as aggressive fibromatosis, that arise from the body’s connective tissues. Affecting an estimated two to six people per million worldwide, these tumors require specialized management. This article explores the nature of desmoid tumors, their underlying causes, common locations where they develop, and current diagnostic and treatment strategies.
Defining Desmoid Tumors
Desmoid tumors are growths originating from fibroblasts, the cells responsible for producing connective tissue like collagen. These tumors are classified as non-cancerous because they do not metastasize to distant parts of the body. They are sometimes referred to as deep or aggressive fibromatosis due to their behavior of local invasion.
The World Health Organization (WHO) classifies these tumors as intermediate, falling between benign and malignant growths. This classification reflects their aggressive nature, as they can grow into and damage surrounding tissues and organs. Their tendency to infiltrate nearby structures means they are often grouped with soft-tissue cancers, or sarcomas, and treated by similar specialized medical teams. Desmoid tumors are distinct from sarcomas because they cannot spread through the bloodstream or lymphatic system.
The clinical course of a desmoid tumor is highly unpredictable, with some growing rapidly and others remaining stable or even shrinking spontaneously. Approximately 20% of cases exhibit spontaneous regression, disappearing with minimal or no treatment. However, they have a high rate of local recurrence after surgical removal, which underscores the difficulty in managing this condition.
Understanding the Causes
The development of a desmoid tumor is rooted in a genetic change that causes connective tissue cells to multiply uncontrollably. About 90% of cases are sporadic, meaning they are not inherited. These sporadic tumors are associated with a somatic mutation in the CTNNB1 gene, which provides instructions for making the protein beta-catenin.
The remaining 5% to 10% of cases are hereditary and occur in individuals with a condition called Familial Adenomatous Polyposis (FAP). FAP is a genetic syndrome caused by an inherited mutation in the APC gene. Both the CTNNB1 and APC gene mutations lead to an accumulation of beta-catenin protein within the cell, which improperly signals the cell to grow and divide.
In FAP patients, desmoid tumors often appear after abdominal surgery, suggesting that trauma may act as a trigger. Pregnancy is also recognized as a potential risk factor, particularly for sporadic abdominal wall tumors, possibly due to hormonal changes or surgical trauma from cesarean sections.
Where Desmoid Tumors Form
Desmoid tumors can arise anywhere connective tissue is present, but they display a preference for certain anatomical locations. They are broadly categorized into abdominal wall, intra-abdominal, and extra-abdominal tumors. Extra-abdominal tumors commonly occur in the extremities (such as the upper arms and legs), the chest wall, or the back.
Tumors forming in the abdominal wall are the most frequent type, often presenting as a firm, palpable mass. Intra-abdominal desmoid tumors, which grow within the mesentery that surrounds the intestines, are particularly challenging due to their potential to press on vital organs. In FAP patients, intra-abdominal desmoids are the most common and are often more aggressive.
The symptoms experienced by a patient depend entirely on the tumor’s location and size. A tumor in an arm or leg may cause pain, swelling, and restricted joint movement. Intra-abdominal tumors can lead to severe symptoms like cramping, nausea, or bowel obstruction as they compress the intestines or other abdominal structures.
Diagnosis and Treatment Pathways
The diagnostic process typically begins with imaging studies to determine the tumor’s size and local extent. Magnetic Resonance Imaging (MRI) is the preferred method because it provides superior soft tissue contrast for evaluating the tumor against surrounding structures. A definitive diagnosis, however, requires a core needle biopsy, where a small tissue sample is examined by a pathologist.
Historically, surgical resection was the standard first-line treatment, but this approach has shifted dramatically due to high local recurrence rates (25% to 60%). Current international consensus guidelines now recommend active surveillance as the primary approach for asymptomatic or minimally symptomatic tumors. This involves regular monitoring with MRI scans, typically every three to six months, to track tumor size and growth rate.
Active treatment is initiated if the tumor is progressive, causes significant symptoms, or threatens nearby organs and function. Surgical removal is now generally considered a second-line option and is mostly reserved for smaller sporadic tumors in the abdominal wall where the procedure’s morbidity is low. For progressive disease in other locations, systemic therapies are often the first active treatment choice.
These medical therapies include chemotherapy, targeted therapies that inhibit the signaling pathway, and hormonal agents. The use of non-steroidal anti-inflammatory drugs (NSAIDs) can be effective in some cases, and newer targeted drugs have shown promise in slowing or shrinking the tumors. Local ablative treatments, such as cryoablation or radiation therapy, may also be considered for specific tumors that are not easily accessible for surgery or are located near structures that would be damaged by an operation. The treatment pathway is highly individualized, depending on the tumor’s behavior, location, and the patient’s overall health.