= GIANT OSTEOID OSTEOMA = OSTEOGENIC FIBROMA OF BONE = OSSIFYING FIBROMA

= rare benign locally aggressive tumor with unlimited growth potential + capability of malignant transformation

Frequency:<1% of all primary bone tumors; 3% of all benign bone tumors

Mean age: 6–19 years; 6–30 years (90%); 2^nd decade (55%); 3^rd decade (20%); M÷F = 2÷1

Size: lesion >1.5 cm

N.B.: smaller lesions are classified as osteoid osteoma

Histo: numerous multinucleated giant cells (osteoclasts), irregularly arranged osteoid + bone; very vascular connective tissue stroma with interconnecting trabecular bone; trabeculae broader + longer than in osteoid osteoma

• dull localized pain of insidious onset (84%), worse at night in 7–13%; asymptomatic in <2%
• response to salicylates in 7%
• localized swelling, tenderness, decreased range of motion (29%)
• painful scoliosis in 50% (with spinal / rib location) ← muscle spasm, may be convex toward side of tumor
• paresthesias, mild muscle weakness, paraparesis, paraplegia (← cord compression)
• occasional systemic toxicity (high WBC, fever)

Location: (rarely multifocal)

1. spine (32–46%): 62–94% in neural arch, secondary extension into vertebral body (28–42%); cervical spine (31%), thoracic spine (34%), lumbar spine (31%), sacrum (3%)
2. long bones (26–32%): femur (50%), tibia (19%), humerus (19%), radius (8%), fibula (4%); unusual in neck of femur
3. small bones of hand + feet (15–26%): dorsal talus neck (62%), calcaneus (4%), scaphoid (8%), metacarpals (8%), metatarsals (8%)
4. calvarium + mandible (= cementoblastoma)

Site: diaphyseal (58%), metaphyseal (42%); eccentric (46%), intracortical (42%), centric (12%), may be periosteal

• similar to osteoid osteoma:
  • radiolucent nidus >2 (range, 2–12) cm in size
  • well demarcated (83%)
  • ± stippled / ringlike small flecks of matrix calcification
  • reactive sclerosis (22–91%) / no sclerosis (9–56%)
• progressive expansile lesion that may rapidly increase in size (25%):
  • cortical expansion (75–94%) / destruction (20–22%)
  • tumor matrix radiolucent (25–64%) / ossified (36–72%)
  • sharply defined soft-tissue component
  • thin shell of periosteal new bone (58–77%) / no periosteal reaction
• scoliosis (35%)
• talar osteoporosis ← disuse + hyperemia
• rapid calcification after radiotherapy
• marked enhancement

CT:

• multifocal matrix mineralization + sclerosis
• expansile bone remodeling, thin osseous shell

NUC:

• intense focal accumulation of bone agent (100%)

Angio:

• tumor blush in capillary phase (50%)

MR (of limited value in characterization & staging):

• mainly lytic lesion by CT:
  • low to intermediate signal intensity on T1WI
  • mixed intermediate to high intensity on T2WI
• lesion with some mineralization by CT:
  • mixed low and high signal intensity on T2WI
  • mainly of low signal intensity on T2WI
• surrounding edema

Prognosis: 10–15% recurrence after excision; incomplete curettage can effect cure due to cartilage production + trapping of host lamellar bone

DDx:

1. Osteo- / chondrosarcoma (periosteal new bone)
2. Osteoid osteoma (dense calcification + halo of bone sclerosis, stable lesion size <2 cm due to limited growth potential)
3. Cartilaginous tumors (lumpy matrix calcification
4. Giant cell tumor (no calcification, epiphyseal involvement)
5. Aneurysmal bone cyst
6. Osteomyelitis
7. Hemangioma
8. Lipoma
9. Epidermoid
10. Fibrous dysplasia
11. Metastasis
12. Ewing sarcoma