Aneurysmal bone cyst

Introduction and etiology

• Vascular lesions consisting of widely dilated vascular channels; not lined by identifiable endothelium
• Etiology uncertain
  • Primary lesion in most cases: Appears de novo, possibly when an osseous AV fistula creates secondary erosive bone lesion
  • Occasionally secondary to or associated with other lesions; result of cystic changes in a chondroblastoma, osteoblastoma, giant cell tumor, or fibrous dysplasia

Tumor biology and incidence

• Not known to metastasize
• Locally aggressive
• Approximately 5% of all benign bone tumors
  • Number is relative: Aneurysmal bone cyst often found as a secondary lesion (as described above), and incidence of primary versus secondary not well described

Age

Most patients less than 30 years of age; 80% younger than 20 years

Gender

M:F = 1:1

Presentation

• Pain and occasionally a tender mass
• Patient usually complains of a mild, dull pain of several weeks' or months' duration

Physical findings

• Tender, warm swelling over area of involvement may be present
• Clinically apparent pathologic fracture rare
• Neurologic deficit may result from cord compression with spine lesion (uncommon)

Plain Films

Site

• Vertebra, femur, and tibia most commonly affected, but lesion may arise in any bone
• Spine and metaphysis of long bones most common site (can extend to end of the bone after closure of the physes)
• In children, rarely will extend across open physis to involve epiphysis
• Usually eccentric
• Can be seen in diaphysis of long bones as well as in flat bones (scapula, pelvis, vertebrae)
• May be cortically based, arising on the periosteal surface (rare)

Size

Variable, but usually >5cm.

Tumor effect on bone

Lytic and expansile

Bone response to tumor

• Marginal sclerotic rim common
• Periosteal new bone formation generally present
• Typically, thin layer of periosteal new bone outlines aneurysmal expansion into soft tissue ("neo-corticalization")

Matrix

None

Cortex

Thinned but intact

Soft tissue mass

May be present

CT scan

Useful in demonstrating regions of intact and damaged bone to plan approach to biopsy and treatment

MRI

• Bright T2-weighted signal intensity secondary to central blood/fluid within the lesion (also occurs in telangiectatic osteosarcoma and other benign tumors where there is secondary aneurysmal bone cyst component)
• Look for multiple fluid/fluid levels within tumor

Pathology

• Gross: Hemorrhagic cystic tissue
• Microscopic: Cavernous blood-filled spaces lacking an endothelial cell lining
• Walls of cysts consist of fibrous tissue and giant cells
• Bone may be present.
• Possible co-existence of other lesions such as giant cell tumor or osteoblastoma

Summary of radiographic and histologic features

Typical radiographic and histologic features of aneurysmal bone cysts compared with unicameral bone cysts

Differential diagnosis

• Unicameral bone cyst
  • Generally central in bone
• Giant cell tumor
  • Aneurysmal bone cyst and giant cell tumor may co-exist in same lesion
  • Aneurysmal bone cyst and giant cell tumor of spine usually located in vertebral body
• Osteoblastoma
  • Aneurysmal bone cyst changes may be evident in this lesion as well
  • Osteoblastoma a posterior spine tumor
• Telangiectatic osteosarcoma
  • Can have a similar appearance, with lytic expansile pattern and fluid-fluid levels
  • Telangiectatic osteosarcoma usually larger, more destructive

Natural history

Rapid growth is usual on serial x-rays

Diagnosis and treatment

• Confirm diagnosis through biopsy and frozen section
  • Open biopsy often necessary; needle biopsy usually gets only blood
  • Biopsy may be done in same surgical setting as definitive surgical procedure
• Surgical management: Curettage, thermal coagulation with cautery, thorough excision of lining with high-speed burr, and, occasionally, use of adjuvant therapy (such as hydrogen peroxide or phenol) for some aggressive or recurrent aneurysmal bone cysts
• Large lesions may be bone grafted (autograft, allograft, bone substitutes, or a combination of these)
  • Additional stabilization with hardware may be needed if large lesions is located in high-stress area (eg, proximal femur) and/or associated with unstable pathologic fracture
• En bloc resection appropriate for lesions in non-essential bones (rib or fibula)
  • Resection also appropriate for recurrent aggressive lesions
• Avoid radiation therapy
  • Associated with later development of sarcoma
  • May also damage reproductive organs and active growth areas of long bones
  • Can cause other complications