A desmoid tumor develops from fibroblasts and myofibroblasts, cells that normally contribute to the structure and repair of connective tissues. It can occur in the abdominal wall, the deep abdomen, the limbs, the trunk, the head, or the neck.

Under a microscope, it does not exhibit the characteristics of metastatic cancer. However, it can be locally aggressive and encase or compress a nerve, a blood vessel, the intestine, the urinary tract, or other organs. A deep location can therefore become serious without ever producing metastases.

Some tumors remain stable for a long time, others shrink spontaneously, and still others progress in episodes. Size alone is not sufficient to determine a course of treatment. Symptoms, growth rate, location, the risk to function, and the impact on quality of life are essential factors.

In most cases, the tumor is sporadic. It most often involves an acquired mutation in the CTNNB1 gene, which encodes beta-catenin. This mutation is present only in the tumor and is not passed on to children.

A small proportion of desmoid tumors are associated with familial adenomatous polyposis caused by a constitutional mutation in the APC gene. These forms are often abdominal or multiple. A genetic consultation is considered based on the patient’s age, tumor location, family history, presence of polyps, and the results of tumor analyses.

The most common sign is a firm mass, which is sometimes painless and may adhere to surrounding tissues. Symptoms depend mainly on the location.

Imaging

MRI is the gold standard for imaging lesions in the extremities, trunk, head, and neck. CT scans may be useful for abdominal tumors or certain deep-seated lesions. Imaging clarifies the relationship with surrounding organs and serves as a baseline for follow-up.

Biopsy

The diagnosis must be confirmed by a biopsy performed at a specialized center. The pathologist examines the tumor’s appearance and the nuclear expression of beta-catenin. Testing for a CTNNB1 mutation helps confirm the diagnosis and may guide genetic testing in certain situations.

For a newly diagnosed, stable, or minimally symptomatic tumor, active surveillance is often the initial approach. It combines follow-up visits, assessment of pain and function, and repeated imaging. The frequency of follow-up is more frequent at the beginning and is then adjusted based on the tumor’s progression.

A moderate change in size does not automatically warrant treatment. The decision takes into account confirmed progression, the severity of symptoms, the risk of complications, and the impact on daily life.

Treatment may be recommended in cases of persistent disease progression, inadequately controlled pain, functional impairment, organ compromise, or a significant impact on quality of life. If the tumor is located in a high-risk area, the medical team may intervene before an irreversible complication occurs.

Treatments

There is no single treatment sequence that applies to all children. The strategy prioritizes the option most likely to control the disease with the least immediate and long-term toxicity.

Medical Treatments

In children, low-dose chemotherapy regimens—particularly those based on methotrexate combined with vinblastine or vinorelbine—may be recommended. Tyrosine kinase inhibitors may be considered for certain progressive forms of the disease. The choice depends on the child’s age, the location of the disease, the urgency of the situation, prior treatments, and expected side effects.

Gamma-secretase inhibitors

Nirogacestat demonstrated a benefit in a Phase III trial conducted in adults with progressive desmoid tumors and has been approved in the United States for this population. Its use in children should not be presented as standard of care and requires specialized consultation, an appropriate regulatory framework, or participation in a clinical trial.

Surgery

Surgery is no longer the routine first-line treatment. It remains an option when resection can be performed with limited morbidity, in certain locations, or in the face of a complication. Resection that is disfiguring or results in major functional loss is generally avoided when other options are available.

Cryoablation

Cryoablation destroys the tumor using cold applied via needles placed under image guidance. It may be considered for certain accessible extra-abdominal tumors, particularly when local treatment is desirable and surgery would be debilitating. The proximity of a nerve, skin, blood vessel, or organ may limit its use.

Radiation Therapy

Due to the risk of side effects on growing tissues and the risk of secondary cancer, radiation therapy is used only rarely in children. It is considered only after other options have failed or are not feasible in a life-threatening situation.

Pain does not always correlate exactly with the size of the tumor. It must be assessed and treated as a separate concern in its own right, using analgesics, management of neuropathic pain, physical therapy, adapted physical activity, and psychological support.

In children, the care plan must also take into account schooling, sleep, body image, activities, and the fatigue experienced by family members. Effective treatment of the tumor is not always enough to immediately eliminate all pain.

Follow-up care combines a clinical examination with an MRI or CT scan, depending on the tumor's location. Follow-up visits are generally scheduled more frequently at first and then spaced out if the condition remains stable. They monitor the tumor, pain, function, and the effects of treatment.

The Department of Pediatric and Adolescent Oncology coordinates pediatric care with the teams specializing in imaging, pathology, genetics, surgery, interventional radiology, medical treatment, pain management, and rehabilitation. Each case is discussed during a specialized multidisciplinary team meeting.

Gustave Roussy is part of the national NETSARC+ network. The treatment strategy may combine active surveillance, medical treatment, or cryoablation, with a personalized approach based on the tumor’s location and the patient’s age.