Desmoid tumor (aggressive fibromatosis)

Desmoid tumors (deep or aggressive fibromatosis) are clonal fibroblastic proliferations that arise in fascia, aponeuroses, and muscle, most often in the abdominal wall, mesentery, shoulder girdle, and chest wall. They never metastasize, but they infiltrate surrounding muscle, viscera, and neurovascular structures and can be relentlessly locally progressive. Some are associated with prior surgery, trauma, pregnancy, or familial adenomatous polyposis (Gardner syndrome).

Radiation arrests the proliferation of the fibroblastic clone and induces hyalinization and regression of the collagen matrix. Because desmoids often encase vessels and nerves in the extremity, shoulder, or mesentery, complete resection with negative margins may require amputation or disabling resection — radiation provides comparable local control while preserving anatomy. Radiation is used definitively for unresectable tumors, adjuvantly after R1/R2 resection, or when tumors progress on watchful waiting.

Positioning is site-specific: supine with arms akimbo or raised for truncal lesions, custom vacuum-lock immobilization for extremities, and prone with a belly board for intra-abdominal mesenteric desmoids. CT simulation with contrast defines the tumor on fused MRI, which is essential for delineating the extent along fascial planes. The clinical target volume adds 2–5 cm proximally and distally along the fascial plane (smaller with image guidance), and critical structures — bowel, spinal cord, kidneys, and neurovascular bundles — are contoured with dose constraints.

Standard regimens deliver 50–56 Gy in 1.8–2 Gy daily fractions over roughly six weeks, with IMRT preferred to sculpt dose away from bowel and viscera. Intra-abdominal desmoids may be treated to 45–50 Gy to respect small-bowel limits. Stable disease is a success — desmoids may shrink slowly for years, and treatment response is judged by growth arrest on serial MRI rather than by complete disappearance.