MRI of a 14-year-old girl with acute arm pain shows an expansile lytic lesion in the proximal humeral metaphysis containing multiple locules with internal fluid-fluid levels. There is no soft tissue component. The most likely diagnosis is:
- A Unicameral bone cyst
- B Aneurysmal bone cyst ✓
- C Telangiectatic osteosarcoma
- D Fibrous dysplasia
Explanation
Multiple blood-filled locules with fluid-fluid levels on MRI in an expansile metaphyseal lesion of a young patient are characteristic of aneurysmal bone cyst. Fluid-fluid levels represent layering of blood products of different ages within cavernous spaces. Telangiectatic osteosarcoma can mimic this appearance but shows cortical destruction, an aggressive periosteal reaction, and a malignant soft tissue mass, which are absent here. Unicameral bone cysts are unilocular and central rather than multiloculated and expansile, and fibrous dysplasia shows ground-glass matrix rather than blood-filled cavities.
Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.