Desmoid

Desmoid tumors, also called desmoid fibromatosis or aggressive fibromatosis, are rare soft tissue neoplasms composed of fibroblastic and myofibroblastic cells. They are locally invasive and infiltrative but do not metastasize. Growth is variable; some lesions remain stable or regress, while others progress and cause functional impairment by invading surrounding structures such as muscles, nerves, or viscera.

Most cases are sporadic and driven by mutations in CTNNB1 (the gene encoding beta-catenin). A substantial minority occur in patients with familial adenomatous polyposis (FAP) or Gardner syndrome due to germline APC mutations. Hormonal factors, particularly pregnancy, may influence growth, and prior trauma or surgery has been implicated as a trigger in some individuals.

Desmoid tumors can arise in many locations. Common sites include the abdominal wall and intra-abdominal mesentery, as well as extra-abdominal regions such as the limbs, trunk, head, and neck. Abdominal-wall tumors are particularly associated with young women in the postpartum period; intra-abdominal disease can involve the mesentery and pelvis and may affect bowel function.

Diagnosis relies on imaging and tissue sampling. Magnetic resonance imaging is preferred for defining extent and relationships to nearby structures. Biopsy shows spindle-shaped fibroblasts in a densely collagenous stroma with infiltrative margins. Nuclear beta-catenin positivity on immunostaining supports a desmoid diagnosis and reflects CTNNB1 or APC pathway involvement.

Management is individualized. Active surveillance is appropriate for many stable or minimally symptomatic tumors. Treatment options include noncytotoxic approaches (nonsteroidal anti-inflammatory drugs, anti-estrogen therapy), low-dose chemotherapy (e.g., methotrexate with vinblastine), and targeted therapies such as tyrosine kinase inhibitors. Radiotherapy may be considered for unresectable or residual disease, and surgical excision with negative margins can be curative in selected cases, though recurrence is common. Overall prognosis is variable, with low disease-specific mortality but potential morbidity from local invasion and repeated relapse.