Aneurysmal bone cyst of the ethmoid on fibrous dysplasia: A usual association within a rare location.

Aneurysmal bone cyst is a non-neoplastic lesion consisting of blood filled sinusoidal spaces that expand from the affected bone. The most common locations of aneurysmal bone cysts are the metaphysis of long bones, followed by flat bones. Only 2% of all are found in the head and neck area, with mandible and maxilla being the most frequent sites involved, involvement of the ethmoidal bone is extremely rare. Their occurrence on a pre-existing bone lesion is described. Imaging features may assist in diagnosis by demonstrating blood-fluid levels, which are a characteristic finding in these lesions. We present computed tomography and magnetic resonance imaging findings of aneurysmal bone cyst of the ethmoid sinus on fibrous dysplasia in a 12-year-old female.