Diagnosis:
Aneurysmal bone cyst
Discussion:
Single axial section of a knee CT which reveals an aneurysmal bone cyst in the left
distal femur with a fluid-fluid level. The fluid-fluid level sign is by no means
pathognomonic, but is helpful in narrowing the differential. The sign is produced by a
cystic lesion with water density fluid layering atop blood. The fluid-fluid level is often
seen in aneurysmal bone cysts; but it may also be seen in giant cell tumor, telangiectatic
osteosarcoma, and chondroblastoma.

References:

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