Aneurysmal bone cysts
The aneurysmal bone cyst is a unique tumor that happens typically in children below the age of sixteen. The bone is expanded by these lesions and the walls of the lesion are line by blood vessel cells (endothelium). They themselves contain blood and hence are like aneurysms or dilated vessels. These lesions may exist in the secondary form where they are part of another lesion in as high as 20% of cases. These other forms are not believed to be primary tumors.
They are aggressive tumors causing invasion of bone. They usually need to be removed and filled with bone graft as they risk the bone to fracture and dissemination. In addition, in the spine especially the can cause compression of vital structures like the spinal cord.
Figure. This lady had a giant cell tumor of the distal femur presenting at the age of 30 years. Further evaluation showed there was an aneurysmal bone cyst residing in the giant cell tumor. A currettage and cryotherapy with liquid nitrogen was done saving her near-destroyed knee.