Aneurysmal Bone Cyst - Pathology

summary
- Aneurysmal Bone Cysts are benign, non-neoplastic, reactive bone lesions that most commonly occur in the femur and tibia. The condition typically presents in patients less than 20 years of age with focal pain and swelling.
- Diagnosis is made with radiographs showing an expansile, eccentric, and lytic lesion with bony septae and a biopsy showing blood-filled spaces without endothelial lining.
- Treatment is usually nonoperative for lesions associated with a fracture. Aggressive curettage, with adjuvant treatment and bone grafting, is indicated for symptomatic lesions without associated fracture.

Epidemiology
- Demographics
  - 75% of patients are < 20 yrs.
- Anatomic location
  - 15% in spine
  - >60% in long bones (Femur and tibia being most common)
  - 51% occured in the lower extremities, 22.5% in upper extremities
  - usually in metaphysis
  - metatarsal and calcaneus are the most common locations in the foot
  - posterior elements of pelvis
  - may be found in similar location as telangiectatic osteosarcomas

Etiology
- Pathophysiology
  - primary and secondary forms
    
    primary ABC
    
    driven by upregulation of the ubiquitin-specific protease USP6 (Tre2) gene on 17p13 when combined by a translocation with a promoter pairing
    
    most commonly described translocation t(16;17)(q22;p13) leading to juxtaposition of promoter region CDH11 on 16q22
    
    secondary ABC
    
    not considered a neoplasm because no known translocation has been identified
- Associated conditions
  - associated with other tumors 30% of time
    
    giant cell tumor
    
    chondroblastoma
    
    fibrous dysplasia
    
    chondromyxoid fibroma
    
    NOF

Presentation
- Symptoms
  - pain and swelling
  - may present with pathologic fracture
- Physical exam
  - neurologic deficits possible with spine lesions

Imaging
- Radiographs
  - expansile, eccentric and lytic lesion with bony septae ("bubbly appearance")
  - usually in metaphyseal
  - classic cases have thin rim of periosteal new bone surrounding lesion
  - no matrix mineralization
- MRI or CT scan
  - will show multiple fluid lines
  - lesion can expand into soft tissue

Studies
- Histology
  - Characteristic findings
    
    cavernous space
    
    blood-filled spaces without endothelial lining
  - cavity lining
    
    numerous benign giant cells
    
    spindle cells
    
    thin strands of woven (new) bone present

                        Differential
                    
                        Radiographic differential includes
                    
                        UBC
                    
                        telangiectatic osteosarcoma
                    
                        Histologic differential includes
                    
                        telangiectatic osteosarcoma
                    
                        giant cell tumor
                    
                        Differential of Aneurysmal Bone Cyst
                    
                        "Bubbly" lytic lesion on xray
                    
                        "Lakes of Blood" on histology
                    
                        Treatment is curettage and bone grafting 
                    
                        Aneurysmal Bone cyst
                    
                        o
                    
                        o
                    
                        o
                    
                        UBC
                    
                        o
                    
                        NOF
                    
                        o
                    
                        Giant Cell Tumor
                    
                        o
                    
                        Chondroblastoma
                    
                        o
                    
                        Chondromyoid fibroma
                    
                        o
                    
                        Osteoblastoma
                    
                        o
                    
                        Telangiectatic osteosarcoma
                    
                        o

Treatment
- Nonoperative
  - nonoperative fracture management
    
    indications
    
    ABC with acute fracture
    
    indicated until fracture has healed. Once healed, treat as an ABC without fracture unless the fracture has led to spontaneous healing of the ABC
- Operative
  - aggressive curettage (+/- adjuvant) and bone grafting
    
    indications
    
    symptomatic ABC without acute fracture
    
    technique
    
    possible adjuvants
    
    phenol
    
    argon beam
    
    liquid nitrogen
    
    outcomes
    
    local recurrence in up to 25% and more common in children with open physes

Action	Numeric Key	Letter Key	Function Key
Show Bullets		S	Enter (frontside only)
20%	1	N
40%	2	H
60%	3	F	Enter (backside only)
80%	4	E
100%	5	M
Toss	0	T

Differential of Aneurysmal Bone Cyst
	"Bubbly" lytic lesion on xray	"Lakes of Blood" on histology	Treatment is curettage and bone grafting
Aneurysmal Bone cyst	o	o	o
UBC	o
NOF	o
Giant Cell Tumor			o
Chondroblastoma			o
Chondromyoid fibroma			o
Osteoblastoma			o
Telangiectatic osteosarcoma		o