= FIBROXANTHOMA = NONOSTEOGENIC FIBROMA = XANTHOMA = XANTHOGRANULOMA OF BONE= FIBROUS METAPHYSEAL-DIAPHYSEAL DEFECT = FIBROUS MEDULLARY DEFECT

Frequency: up to 40% of all children >2 years of age

Etiology: lesion resulting from proliferative activity of a fibrous cortical defect that has expanded into medullary cavity

Histo: interlacing whorled bundles of spindle-shaped fibroblasts + scattered multinucleated giant cells + foamy xanthomatous cells, variable degree of hemosiderin; usually cellular with only small amounts of collagen

Age: 8–20 years; 75% in 2^nd decade of life

• usually asymptomatic; pain if large

Location: shaft of long bone; mostly in bones of lower extremity, esp. about knee (posteromedial surface of distal femur (55%) + proximal tibia); distal tibia; fibula

Site: eccentric / cortical to subcortical metaphyseal region, several cm shaftward from epiphysis, mostly intramedullary, rarely purely diaphyseal

• well-circumscribed multiloculated bubbly oval osteolytic area
• alignment along long axis of bone, >2 cm in length
• scalloped sclerotic margin toward medulla; V- or U-shaped at one end
• mild expansile remodeling = endosteal scalloping + thinning ± overlying bulge
• no periosteal reaction
• migrates toward center of diaphysis
• resolves with age

NUC:

• minimal / mild uptake on bone scan

MR:

• 80% hypointense on T1WI + T2WI ← extensive hypocellular fibrous tissue, hemosiderin pigment
• 20% hypointense on T1WI + hyperintense on T2WI ← massive aggregation of foamy histiocytes
• peripheral hypointense rim + internal septation ← marginal reactive sclerosis + trabeculation
• adjacent marrow edema generally absent
• intense contrast enhancement (in 80%) / marginal septal enhancement (in 20%) on T1WI

CAVE: lesions >33 mm long involving >50% of the transverse bone diameter need observation

Prognosis: spontaneous healing in most cases

Cx:

1. Pathologic fracture (not uncommon)
2. Hypophosphatemic vitamin D–resistant rickets + osteomalacia (tumor may secrete substance that increases renal tubular resorption of phosphorus)

DDx:

1. Fibrous cortical defect (<2 cm in greatest diameter)
2. Adamantinoma (midshaft of tibia)
3. Chondromyxoid fibroma (bulging of cortex more striking, hyperintense on T2WI)
4. Fibrous dysplasia (internal septations rare)
5. Aneurysmal bone cyst (heterogeneously hyperintense with fluid-fluid levels)
6. Intraosseous ganglion (hyperintense on T2WI)