Aneurysmal Bone Cyst
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Direct answer
Aneurysmal bone cyst is a benign, expansile lesion made of blood-filled spaces separated by septa, and most patients are under twenty. Its MRI shows fluid-fluid levels, and it is typically eccentric and lytic in the metaphysis of a long bone or in the posterior elements of the spine. A USP6 gene rearrangement is found in many cases, and treatment is curettage with bone grafting, with sclerotherapy or embolisation as options.
What you must remember
- Imaging: an expansile, multiloculated lytic lesion with a thin cortical shell, often eccentric in the metaphysis.
- MRI hallmark: fluid-fluid levels from layering of blood products, typical but not specific.
- Site: the long-bone metaphysis, such as the femur and tibia, and the posterior elements of the spine, where neurological compromise can occur.
- Genetics: a USP6 rearrangement, often t(16;17), is found in many primary cases.
- Primary and secondary: a secondary cyst develops within another lesion such as giant cell tumour, osteoblastoma or chondroblastoma.
- Histology: blood-filled spaces lined by fibrous septa with osteoclast-type giant cells, and no true endothelial lining.
- Recurrence: higher after incomplete curettage, so the cavity must be cleared thoroughly.
From imaging to definitive treatment
An expansile lytic lesion in a young patient tempts a rushed diagnosis, but a fixed sequence of imaging usually narrows the list before any tissue is taken. The steps below follow that sequence from the first radiograph to definitive surgery.
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Plain radiographs: Look for an eccentric, expansile lytic lesion with a thin cortical shell in a metaphysis or the posterior spine.
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MRI characterisation: Fluid-fluid levels support the diagnosis, although they are not specific, so the search for an alternative continues.
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Look for a solid component: A solid area within the lesion suggests a primary tumour such as giant cell tumour, which can host a secondary cyst.
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Biopsy when unclear: An incisional biopsy is taken along a planned tract when a solid tumour cannot be excluded.
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Embolise vascular lesions: Selective embolisation before surgery reduces blood loss, particularly in the spine and pelvis.
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Definitive surgery: Curettage with bone graft or cement is standard, with sclerotherapy as an alternative in selected sites, followed by review for recurrence.
Where the exam tries to catch you
The imaging clue is distinctive, but the traps come from its look-alikes and from mislabelling the lesion as malignant.
- Trap: calling every fluid-fluid level diagnostic of aneurysmal bone cyst Correct: fluid-fluid levels are typical but not specific, and solid tumours such as giant cell tumour can show them too.
- Trap: treating aneurysmal bone cyst as a malignancy needing wide resection Correct: it is benign, and curettage with grafting is standard, with resection reserved for limited situations.
- Trap: missing the underlying lesion in a secondary cyst Correct: a secondary cyst arises within giant cell tumour, osteoblastoma or chondroblastoma, so the underlying lesion must be sought.
Frequently asked questions
What is the typical age for aneurysmal bone cyst?
Aneurysmal bone cyst usually presents in children and young adults, with most patients younger than twenty. It is an expansile lesion that often causes pain or swelling of the affected bone.
What MRI feature suggests aneurysmal bone cyst?
Fluid-fluid levels on MRI suggest aneurysmal bone cyst, reflecting layering of blood products in the blood-filled spaces. The finding is typical but not specific, so a biopsy may still be needed.
Which genetic change is associated with aneurysmal bone cyst?
A USP6 gene rearrangement is found in many primary aneurysmal bone cysts, most often as a t(16;17) translocation. The rearrangement drives USP6 expression and is a useful marker in difficult cases.
How is aneurysmal bone cyst treated?
Aneurysmal bone cyst is usually treated with curettage and bone grafting, often after embolisation for vascular lesions. Sclerotherapy is an alternative in selected sites, and recurrence is more likely after incomplete curettage.