= INTRAOSSEOUS DESMOID TUMOR

= rare locally aggressive benign neoplasm of bone with borderline malignancy resembling soft-tissue desmoids / musculoaponeurotic fibromatosis

Prevalence: 107 cases in world literature

Histo: intracellular collagenous material in fibroblasts with small nuclei

Age: mean of 21 years (range, 15 months to 75 years); in 90% <30 years; M÷F = 1÷1

• slowly progressive pain + local tenderness; palpable mass

Location: mandible (26%), ilium (14%), >50% in long bones (femur [14%], humerus [11%], radius [9%], tibia [7%], clavicle), scapula, vertebra, calcaneus

Site: central meta- / diaphyseal (if growth plate open); may extend into epiphysis with subarticular location (if growth plate closed)

• geographic (96%) / moth-eaten (4%) bone destruction without matrix mineralization
• narrow (96%) / poorly defined (4%) zone of transition
• no marginal sclerosis (94%)
• residual columns of bone with “pseudotrabeculae” are CLASSIC (91%)
• bone expansion (89%); may grow to massive size (simulating aneurysmal bone cyst / metastatic renal cell carcinoma)
• breach of cortex + soft-tissue mass (29%)

Cx: pathologic fracture (9%)

Prognosis: 52% rate of local recurrence

Rx: wide excision

DDx:

1. Giant cell tumor (round rather than oval, may extend into epiphysis + subchondral bone plate)
2. Fibrous dysplasia (occupies longer bone, contains mineralized matrix, often with sclerotic rim)
3. Aneurysmal bone cyst (eccentric blowout appearance rather than fusiform)
4. Chondromyxoid fibroma (eccentric with delicate marginal sclerosis + scalloped border)