Fluid-filled cyst in FD. True fluid-filled cysts can occur in FD lesions. Shown is the CT scan of fluid-filled cyst that arose in a 12 year old boy with MAS who presented with facial nerve parasthesias and displacement of the orbit that occurred over approximately one week. The fluid-filled nature of the lesion is demonstrated by the fluid-fluid level (arrow). These lesions are more frequent in the craniofacial bones and can be aggressive. They usually require prompt surgical intervention.