Desmoid Tumor of the Abdominal Wall

Desmoid tumor (aggressive fibromatosis) of the abdominal wall is a monoclonal fibroblastic proliferation arising in the rectus or oblique muscles and their fascia. It does not metastasize but infiltrates locally and recurs. Incidence is about 2 to 4 per million a year, with a peak at 30 to 40 years and a female to male ratio of about 2 to 1. The abdominal wall is the commonest site in women who are pregnant or postpartum, and the intra-abdominal mesenteric form is typical of familial adenomatous polyposis.

Biology

• Somatic CTNNB1 mutations (about 85%), most often T41A, S45F or S45P. The S45F mutation carries a higher risk of progression.
• APC mutations in familial adenomatous polyposis (desmoid tumors develop in 10 to 25% of these patients).
• Triggers: pregnancy, estrogen exposure, prior surgery or trauma at the site (scars).

Natural history is unpredictable. About 20 to 30% regress spontaneously, and many stabilize for years, so initial observation is acceptable. Complications are pain, functional limitation, bowel obstruction or fistula (when the tumor invades viscera), hernia and recurrence after excision (20 to 40%).

• A painless or mildly painful firm, fixed mass in the abdominal wall that grows slowly, often with no skin change
• A history of pregnancy, cesarean section or abdominal surgery in the scar
• Progression causing a fixed, tethered mass with discomfort, limited trunk movement and, in large tumors, compression of adjacent organs
• Features of familial adenomatous polyposis: multiple colorectal polyps, congenital hypertrophy of the retinal pigment epithelium, osteomas, epidermoid cysts
• Red flags: rapid growth over weeks, skin ulceration, overlying skin changes (suspect sarcoma), a mass in a young patient with a family history of polyposis

1. MRI of the abdominal wall: a well-circumscribed or infiltrative mass, intermediate to high signal on T2 with low signal bands (collagen), and variable enhancement. It defines extent and relation to fascia, peritoneum and vessels.
2. Ultrasound is a first-line test: a hypoechoic mass with fascial extension.
3. CT with intravenous contrast for intra-abdominal extension.
4. Core needle biopsy (image-guided, with a 14-gauge needle): bland spindle cells in long fascicles, nuclear beta-catenin staining in about 80%, and CTNNB1 sequencing for mutation type.
5. Colonoscopy and APC germline testing when disease is multifocal, in a young patient or with a family history.

Differentials: sarcoma (low-grade fibrosarcoma, dermatofibrosarcoma), hernia, scar keloid, lymphoma, metastasis, endometriosis in a cesarean scar, hematoma, nodular fasciitis.

Management is stepwise and stresses function.

1. Active surveillance for 1 to 2 years: MRI at 3 months, then every 3 to 6 months. Intervene on symptomatic or threatening progression (growth over 20% on two scans, pain, organ involvement).
2. Medical therapy for progressive disease:
   • Meloxicam 15 mg orally once daily for 6 months (with food, avoiding peptic ulcer disease and renal impairment), or sulindac 150 mg orally twice daily.
   • Tamoxifen 20 to 40 mg orally once daily in women (with endometrial surveillance).
   • Nirogacestat 150 mg orally twice daily, continuously (adverse effects: diarrhea, ovarian toxicity, rash and hypophosphatemia; contraindicated in pregnancy).
   • Sorafenib 400 mg orally once daily (adverse effects: hand-foot reaction, hypertension, diarrhea and raised transaminases; monitor blood pressure and liver tests every 2 weeks initially).
   • Low-dose chemotherapy: methotrexate 30 mg/m2 IV with vinblastine 5 mg/m2 IV weekly for 26 weeks, or doxorubicin 60 to 75 mg/m2 IV every 3 weeks (maximum cumulative 450 mg/m2) with dacarbazine for rapid progression.
3. Surgery for limited, resectable disease with symptoms: wide local excision with a 1 to 2 cm margin where possible, with mesh reconstruction for fascial defects of over 5 cm, cefazolin 2 g IV before incision, and a drain. Positive margins do not always lead to recurrence, so avoid disabling re-resection.
4. Radiotherapy (50 to 56 Gy in fractions of 1.8 to 2 Gy) for unresectable or recurrent disease, and cryoablation or high-intensity focused ultrasound for selected lesions.
5. Pregnancy: observe and defer treatment until after delivery. Avoid systemic therapy, and operate only for organ threat.
6. Children: methotrexate and vinblastine, and sorafenib 200 to 400 mg orally daily by size.

Follow-up with MRI every 3 to 6 months for 2 years, then annually. Counsel on pregnancy planning (tumors may flare), screen relatives for polyposis, and arrange regular colonoscopy in affected families.