Aneurysmal bone cyst
Pathogenesis:
Benign, expansile, blood-filled bone lesion that can be primary or secondary.
- Primary ABC: Neoplastic, associated with USP6 gene rearrangement.
- Secondary ABC: Cystic, haemorrhagic change developing within a pre-existing bone lesion (e.g. GCT, Chondroblastoma, Osteoblastoma, Fibrous dysplasia).
Epidemiology
- Age: Mostly under 20 years old
- Slight female predominance
Typical location
- Metaphysis of long bones
Can extend into the epiphysis in skeletally mature patients (after physeal closure).- Proximal tibia
- Distal femur
- Posterior elements of spine
Histology
- Blood-filled spaces separated by septa containing fibroblasts, osteoclast-type giant cells and woven bone.
- No malignant cells in primary ABC. If secondary, the underlying lesion may be benign or malignant.
Imaging features
Radiograph / CT
- Often described as an eccentric, expansile, lytic ‘blow-out’ lesion with a thin shell of cortex.
- Internal septations common
- Periosteal reaction may be present in aggressive cases
MRI
- Multiloculated lesion with numerous fluid-fluid levels
- T1: Variable depending on blood products; often low to intermediate
- T2: High signal intensity with layering of blood-fluid levels
- Enhancing septa post-contrast
- Marrow and adjacent soft-tissue oedema are common and not a red flag by themselves.
Pearl: marrow/soft-tissue oedema on CT/MRI doesn’t automatically mean malignancy in ABC.
Differential Diagnosis
| Diagnosis | Key Differences |
|---|---|
| Telangiectatic Osteosarcoma | - Similar imaging, but malignant - Look for nodular, enhancing soft-tissue components and permeative bone destruction. |
| Giant cell tumour (with ABC change) | - Older age (very rare in skeletally immature patients) - Extends to subchondral bone (due to epiphyseal involvement). - Usually has a substantial solid component; fluid–fluid levels are only part of the lesion. |
| Chondroblastoma (with ABC change) | - Epiphyseal location in adolescents - Calcifications present (stippled, chicken-wire) |
| Simple Bone Cyst | - Typical in proximal humerus/femur of children - Central, unilocular - May show ‘fallen fragment sign’ after fracture. |
| Osteoblastoma | - May have ABC changes, especially in spine - Solid component with mineralization |
Management
- Surgical curettage with bone grafting
- Selective arterial embolisation or percutaneous sclerotherapy for lesions that are surgically difficult or high-morbidity.
- Recurrence rate: 10–20%, highest recurrence risk after simple curettage in very young children.
Exam tips
- Classic exam clue: expansile metaphyseal lesion in a teenager with multiple fluid-fluid levels on MRI
- Secondary ABC component may appear in other tumors: GCT, chondroblastoma, osteoblastoma
- Fluid-fluid levels are not pathognomonic for ABC → Ddx: GCT, telangiectatic Osteosarcoma
- Telangiectatic osteosarcoma is the key malignant mimic → tends to be more permeative, with cortical destruction and a soft-tissue mass containing nodular enhancing tissue, not just fluid–fluid levels.
Real-world pearls
- Always check for solid nodular component or aggressive features to exclude malignancy (secondary ABC arising from primary malignancy)
- Recommend biopsy or further imaging if lesion is atypical, in an unusual location, or in a clearly skeletally mature patient.”
