Your diagnosis? Aneurysmal bone cyst.

The etiology of aneurysmal bone cysts is uncertain, but they may originate as a localized arteriovenous malformation. These benign lesions can be primary or occur secondary to an underlying lesion. The majority of patients who present with aneurysmal bone cysts are younger than age 20 years. One half of lesions occur within the long bones and one third involve the spine. Most flat bone lesions, approximately 10%, occur in the pelvis. Fluid-fluid levels are common on CT and MRI but are not pathognomonic. Although aneurysmal bone cyst is benign, there may be aggressive clinical and imaging features. Treatment for aneurysmal bone cyst is surgical curettage, intraoperative adjuvant therapy, and bone grafting of the lesion. The prognosis following treatment is very good, although 10% to 20% of cases are reported to recur.