Aneurysmal Bone Cyst

Aneurysmal bone cyst (ABC) is a benign, locally aggressive bone lesion characterised by blood-filled spaces separated by fibrous septa containing multinucleated giant cells, reactive bone, and fibrous tissue. Despite its benign nature, ABC can grow rapidly and cause significant bone destruction. A key diagnostic consideration is that secondary ABC change can occur in association with other bone lesions — most importantly giant cell tumour, chondroblastoma, osteoblastoma, and telangiectatic osteosarcoma — and this co-existing pathology must not be missed.

• Incidence: approximately 0.14 per 100,000 per year; accounts for approximately 1–2% of primary bone tumours
• Age: predominantly children and young adults; 80% occur before age 20 years; peak incidence in second decade
• Male:female ratio: approximately equal (slight female preponderance in some series)
• Most common locations: posterior elements of spine (most common spinal lesion in children), metaphysis of long bones (femur, tibia, humerus); eccentric position; may expand cortex dramatically
• Primary ABC (70%): no identifiable underlying lesion — driven by USP6 gene rearrangement (17p13)
• Secondary ABC (30%): ABC change superimposed on a pre-existing lesion — GCT most commonly (see below); correct identification of the primary lesion is critical

• Histology: cavernous blood-filled spaces without endothelial lining (not true vascular spaces); fibrous septa containing fibroblasts, osteoclast-like giant cells, reactive woven bone, and haemosiderin; no atypia
• USP6 gene rearrangement (TRE17 oncogene; 17p13): present in approximately 70% of primary ABCs — highly specific molecular marker; absent in secondary ABCs; useful for confirming primary ABC diagnosis and excluding secondary lesions
• USP6 rearrangement detection: FISH or RT-PCR on biopsy material — now standard in many specialist centres
• Solid variant of ABC: less common; predominantly solid fibrous tissue with scattered blood-filled spaces; may be mistaken for osteosarcoma or other sarcomas; molecular testing particularly useful here
• Secondary ABC changes: the underlying primary lesion drives local haemorrhage and cyst formation; the giant cells and septa are reactive, not the primary pathology — always sample the solid component of the lesion thoroughly at biopsy

• Progressive, worsening pain over the affected bone; swelling if cortex expanded; tenderness on palpation
• Rapid growth is characteristic — a defining feature that distinguishes ABC from simple bone cyst (which is typically latent)
• Pathological fracture: occurs in approximately 10–20% — particularly common in proximal femur and humerus
• Spinal ABC: back pain, scoliosis, and neurological symptoms if posterior element expansion compresses the spinal canal
• Rapid growth + lytic eccentric metaphyseal lesion with blown-out cortex in a child = ABC until proven otherwise — but must exclude telangiectatic osteosarcoma
• Calcitonin-sensitive ABC: rare; responds to calcitonin injection — anecdotal evidence; not standard practice

• Plain radiographs: eccentric, lytic, expansile metaphyseal lesion; thinned but intact cortex with characteristic "blown-out" appearance; trabeculated internal architecture (soap-bubble); may have narrow or wide zone of transition
• CT scan: defines cortical integrity, extent of expansion, and relationship to adjacent structures; shows trabeculations and fluid-filled spaces; guides surgical approach
• MRI fluid-fluid levels: blood products of different ages layer in cystic spaces — hyperintense supernatant over hypointense sediment; pathognomonic appearance BUT also seen in GCT with secondary ABC change and telangiectatic osteosarcoma — not diagnostic alone
• Periosteal reaction: thin eggshell periosteal rim (periosteal new bone around expanded lesion); no aggressive periosteal reaction (Codman triangle absent in primary ABC)
• Bone scan: intense peripheral uptake with central photopenia ("doughnut sign") — reflects peripheral vascular reactive bone with central avascular cystic content
• MRI: solid component within cystic lesion raises suspicion for secondary ABC — biopsy the solid component specifically

• Telangiectatic osteosarcoma is the critical differential — it has fluid-fluid levels identical to ABC on MRI; it is high-grade malignant; ALWAYS biopsy any suspected ABC to exclude this diagnosis

Treatment of primary ABC aims to eradicate the lesion while preserving bone stock and joint function. Multiple effective treatment modalities exist.

• Radiation is contraindicated in ABC — risk of post-radiation sarcomatous transformation, especially in children
• Prophylactic fixation: for pathological or impending fractures — fix first, then treat lesion; long bone reinforcement may be needed after curettage
• Spinal ABC: embolisation is first-line for reducing intraoperative blood loss; surgery for neurological compromise or instability; intralesional curettage preferred over wide resection when possible in spine

• Biopsy the solid component: any ABC with a solid component on MRI must have that component targeted specifically at biopsy — the solid component is the diagnostic key; sampling only the cystic fluid will miss GCT, chondroblastoma, or telangiectatic osteosarcoma
• USP6 FISH testing on biopsy: positive = primary ABC confirmed; negative in a suspected ABC = high suspicion for secondary ABC — send full molecular panel to exclude malignant differential
• Denosumab for ABC: off-label but growing evidence; RANKL is expressed by stromal cells in ABC; denosumab induces ossification and reduces cyst activity; useful for unresectable spinal/pelvic lesions, recurrent ABC, and as a bridge before surgery; 120 mg subcutaneous monthly as for GCT protocol
• Recurrence after curettage: treat repeat recurrences with repeat curettage if technically feasible; consider change of modality (sclerotherapy, embolisation) for multiple recurrences; wide resection of expendable bones if repeated curettage fails
• Pathological fracture management: allow fracture to heal first with immobilisation or fixation; treat ABC once fracture is healed (approximately 6–8 weeks); fracture haematoma may cause spontaneous partial ABC resolution — re-image before deciding on intervention

• ABC: eccentric, lytic, expansile metaphyseal lesion; blown-out cortex; soap-bubble trabeculations; child/young adult
• Fluid-fluid levels on MRI: seen in ABC but also in GCT with secondary ABC change and telangiectatic osteosarcoma — NOT diagnostic alone; biopsy mandatory
• Telangiectatic osteosarcoma = most important differential — identical imaging to ABC; malignant; biopsy septa for atypical stromal cells
• Primary ABC: USP6 gene rearrangement (17p13) in 70%; molecular marker of primary ABC
• Secondary ABC (30%): underlying GCT, chondroblastoma, osteoblastoma — identify and treat the primary lesion
• Radiation contraindicated in ABC — post-radiation sarcoma risk, especially in children
• Doughnut sign on bone scan: peripheral uptake + central photopenia — reflects reactive peripheral bone and avascular central cyst
• Fallen fragment sign = simple bone cyst (UBC), not ABC — distinguish these two
• Spinal ABC: posterior elements of spine; most common spinal bone tumour in children; embolisation before surgery
• Denosumab: off-label for unresectable or recurrent ABC — induces ossification via anti-RANKL mechanism