= developmental abnormality classified as benign bone tumor

Prevalence: 30–35% of children; M÷F = 2÷1

Peak age: 7–8 (range, 2–20) years; mostly before epiphyseal closure

Histo: fibrous tissue from periosteum invading underlying cortex

• asymptomatic, usually incidental finding

In 5% associated with: neurofibromatosis

Location: metaphyseal cortex of long bone (near physis); multiple locations bilateral + symmetric in 50%

1. femur (40%): posterior medial aspect of distal femur, proximal femur
2. tibia (40%): proximal tibia
3. fibula (10%)
4. others: proximal humerus, ribs, ilium

Size:<2 cm in largest diameter

Early stage:

• circular / oval eccentrically located radiolucent area extending parallel to long axis of host bone
• smooth lobulated / scalloped margins with well-defined thin rim of sclerosis
• no periosteal reaction
• high SI on T2WI (cystic component early stage) + low SI on T1WI
• little / no uptake on bone scintigraphy

Late stage:

• low SI on T2WI (progressive sclerosis + ossification) and on T1WI
• sclerotic bone island (may be residue of incompletely involuted cortical defect in the adult)

Prognosis:

1. involution over 2–4 years
2. potential to grow and encroach on the medullary cavity leading to nonossifying fibroma (defined as >2 cm in size)

DDx: Nonossifying fibroma