Sphenoid and ethmoidal masses are uncommon in children.
Aneurysmal bone cysts (ABC) may be primary or secondary, this this case secondary to a ethmoidal ossifying fibroma.
ABC's are typically located in the long bones, with less than 1% occurring in the skull. Only a limited number have been described in the ethmoids.
ABC's are multi-cystic masses of non-endothelial line blood filled spaces, which gives rise to the characteristic appearances on imaging, in particular MRI, with a mass separated by thin septae, which contain fluid-fluid levels. The levels representing blood of variable age, thus given different signal characteristics.