= rare benign cartilaginous tumor; initially arising in cortex

Frequency:<1% of all bone tumors

Histo: chondroid + fibrous + myxoid tissue (related to chondroblastoma); may be mistaken for chondrosarcoma

Age: peak 2^nd–3^rd decade (range, 5–79 years); M÷F = 1÷1

• slowly progressive local pain, swelling, restriction of motion

Location:

1. long bones (60%): about knee (50%), proximal tibia (82% of tibial lesions), distal femur (71% of femoral lesions), fibula
2. short tubular bones of hand + feet (20%)
3. flat bones: pelvis, ribs (classic but uncommon)

Site: metaphyseal (47–53%), metadiaphyseal (20–43%), metaepiphyseal (26%), diaphyseal (1–10%), epiphyseal (3%); eccentric

Size: 1–10 cm in length; 4–7 cm in width

• expansile ovoid lesion with radiolucent center + oval shape at each end of lesion
• long axis parallel to long axis of host bone
• geographic bone destruction (100%)
• well-defined sclerotic margin (86%)
• expanded shell = bulged + thinned overlying cortex (68%)
• partial cortical erosion (68%)
• scalloped margin (58%)
• septations (57%) may mimic trabeculations
• stippled calcifications within tumor in advanced lesions (7%)
• NO periosteal reaction (unless fractured)

Prognosis: 25% recurrence rate following curettage

Cx: malignant degeneration distinctly unusual

DDx:

1. Aneurysmal bone cyst
2. Simple bone cyst
3. Nonossifying fibroma
4. Fibrous dysplasia
5. Enchondroma
6. Chondroblastoma
7. Eosinophilic granuloma
8. Fibrous cortical defect
9. Giant cell tumor