Aneurysmal Bone Cyst

Description

Pathogenesis

The exact pathogenesis of ABC is not entirely clear, but it is thought to involve a reactive process following trauma or a pre-existing lesion. Genetic factors also seem to play a role, with USP6 gene rearrangements often found in ABCs.

Subtypes

ABCs are typically classified based on their radiographic appearance, which corresponds to different stages of the disease:

• Active ABCs: Characterised by their aggressive, rapidly expansile behaviour.
• Latent ABCs: Slower growth with a tendency to self-heal.
• Classic ABCs: Presenting with typical clinical, radiographic, and histologic features.

Epidemiology, Risk Factors & Associations

• Most common in the first two decades of life, with a peak incidence between 10-20 years (70% of cases).
• Slight female predilection
• May be secondary to other lesions:
  • Most common: Giant cell tumour, osteoblastoma, chondroblastoma
  • Other: Fibrous dysplasia, chondromyxoid fibroma, fibroxanthoma, solitary bone cyst.

Clinical Features

Patients may present with pain and swelling in the affected bone. Pathologic fracture may occur in weight-bearing bones. In spinal ABCs, neural symptoms due to compression of the spinal cord or nerve roots can occur.

Complications

• Pathological fractures due to thinning of the bone cortex (15-30% of cases).
• Neurological complications in cases of vertebral ABCs.
• Local recurrence after treatment, especially if the lesion was not completely removed.
• No malignant degeneration.

Pathological Features

Histopathology

• Macroscopic: Characterised by blood-filled, multi-cystic spaces separated by septa.
• Microscopic: Septa contain spindle-shaped fibroblasts and areas of reactive woven bone and osteoclast-like giant cells.

Serology

There are no specific serological markers for ABC.

Biochemistry

No specific biochemical markers for ABC.

Molecular Pathology

• FISH: Ubiquitin specific protease 6 (USP6) gene rearrangement (70-75%)

Radiological Features

General Features

• Expansile lytic lesion often with a soap bubble or honeycomb appearance. Well-defined margins with a thin rim of reactive bone is characteristic. Usually eccentric. May appear central if large.
• Most frequently involves the long bones (femur, tibia, humerus), particularly the metaphysis. It can also occur in the posterior elements of the spine, pelvis, and flat bones.
• Fluid-fluid levels seen on cross-sectional imaging, indicative of blood sedimentation in the cystic spaces.
• No osteoid or chondroid matrix

XR

• Shows a sharply defined, lytic, expansile lesion with a blown-out appearance.
• Sclerotic margins.

CT

• Non-contrast: Shows the lytic lesion with internal septations and a thin, sclerotic rim.
• C+ Arterial: Not typically performed.
• C+ Venous: Not typically performed.

MRI

• Some ABCs contain small regions of solid tumour, isointense to skeletal muscle
• T1: Lesion is typically hypointense to isointense.
• T2: Multicystic lesion of varying signal intensities representing different stages of blood products, with internal septations. Fluid-fluid levels can be seen.
• Gad+: Enhancement of the septations and wall of the cyst can be seen.

NM

• PET FDG: Not typically performed.
• Bone scan may show doughnut sign – photopaenic centre with peripheral uptake, a non-specific finding

Grading and Staging

There is no standardised grading or staging system for ABCs.

Diagnosis

The diagnosis is usually made based on a combination of clinical, radiographic, and histopathologic findings. Solid/nodular areas should be biopsied.

Differential Diagnosis

• Telangiectatic osteosarcoma : Most important differential. Similar but more aggressive appearance, eccentric, metaphyseal with fluid-fluid levels (representing necrosis and haemorrhage). Suspect if areas of cortical breakthrough or soft tissue mass, or if nodularity along septations or periphery, or if patient is a young adult.
• Giant cell tumour (GCT): Typically in adults, epiphyseal location, may also demonstrate fluid-fluid levels.
• Unicameral (simple) bone cyst: Typically in the proximal humerus or femur in children, central location, lacks septations but demonstrates pseudotrabeculations on radiograph. Fluid levels may be present but less complex.
• Osteoblastoma: Primary differential for posterior vertebral expansile lesion. Usually demonstrate osseous matrix and typically has extensive perilesional oedema.

Other lesions demonstrating doughnut sign:

• Giant cell tumour
• Simple bone cyst

Prognosis

ABCs are benign and have a good prognosis with appropriate treatment. Recurrence is seen in about 20-30% of cases.

Management

The management of ABCs usually involves referral to an orthopaedic surgeon. Treatment options include curettage with or without bone grafting, selective arterial embolisation, and in some cases, complete resection. In recent years, denosumab, a RANKL inhibitor, has shown promise in the treatment of ABCs.