Giant Bone Island

General Considerations

• Bone islands (enostoses) are areas of mature compact bone in cancellous bone in the medullary cavity
• Usually oriented with its long axis parallel to the cortex
• Most common in pelvis, proximal femurs and ribs
• May change in size over time, but change usually takes years

Clinical Findings

• Usually asymptomatic

Imaging Findings

• Round or ovoid
• Dense
• Greater than 2 cm in size (have been reported up to 10 cm in size)
• Spiculated, feathered or brush-like margin (also called “thorny radiation” “pseudophilia” “cumulus cloud appearance”)
• Thickening of trabecula
• No bone destruction
• No soft tissue mass
• Radionuclide bone scan usually shows no increased uptake
• MR: Loss of signal on all sequences, somewhat heterogeneous the larger they become

Differential Diagnosis

• Osteoblastic metastatic disease
• Osteosarcoma

Treatment

• None required
• If unsure of nature of lesion, follow-up in 3-6 months

Complications

• None

Giant Bone Island. Frontal and lateral knee radiographs show a large, well-circumscribed, ovoid, sclerotic lesion in the distal femur (white arrow) with its long-axis oriented parallel to the cortex and a spiculated (feathery) margin (yellow arrow).
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