CIM Case: Aneurysmal Bone Cyst

Clinical Scenario

Patient: 13-year-old female Presentation: Gradual onset pain in the right knee over 8 weeks, worsening with activity, no history of trauma, no night sweats or weight loss Relevant history: Previously healthy, active in netball, no previous fractures, no family history of bone tumours Examination findings:

• Tender fullness over the proximal tibial metaphysis, medial aspect
• No warmth or erythema
• Full range of motion at knee (slight discomfort at end-range)
• No effusion
• No lymphadenopathy
• Normal neurovascular examination
• Antalgic gait

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP and Lateral Knee X-rays

Radiological features:

• Eccentric, expansile lytic lesion in the proximal tibial metaphysis
• Size approximately 4 x 3 cm
• Well-defined margins with thin cortical shell ("blown out" appearance)
• Multiple thin internal septations
• No periosteal reaction
• No soft tissue mass
• No matrix mineralisation
• Adjacent physis appears intact

Image 2: MRI Knee with Contrast

MRI findings:

• Well-defined multiloculated cystic lesion
• Multiple fluid-fluid levels (pathognomonic)
• T1: Hypointense with variable signal (blood products)
• T2: Hyperintense with characteristic layering
• Thin peripheral enhancement
• No solid enhancing component (important)
• Intact overlying cortex
• No adjacent marrow oedema
• No soft tissue extension

Image 3: CT Scan

CT findings:

• Thinned but intact cortical shell
• Internal septations visible
• No cortical destruction
• No matrix mineralisation
• Confirms geographic bone destruction

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Aneurysmal Bone Cyst:

• Adolescent (10-20 years)
• Eccentric metaphyseal location
• Expansile lytic lesion with "blown out" cortex
• Fluid-fluid levels on MRI
• No solid enhancing component
• No matrix mineralisation

Distinguish from Telangiectatic Osteosarcoma:

Critical Management Points

1. Biopsy is ESSENTIAL - exclude telangiectatic osteosarcoma
2. Sample the wall - secondary ABC is found in rim, not cyst
3. Adjuvant therapy reduces recurrence - phenol, cryotherapy
4. 35% are secondary - look for underlying lesion
5. Fluid-fluid levels are NOT pathognomonic - other lesions can have them
6. Most recurrences within 2 years - close follow-up essential

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Common Examiner Follow-ups

Q: "What is the USP6 gene rearrangement?"

USP6 (ubiquitin-specific peptidase 6) gene on chromosome 17p13:

• Rearranged in 65-70% of primary ABC
• Most common partner: CDH11 (t(16;17))
• Results in overexpression of USP6
• Promotes osteoclast activation and bone destruction
• NOT present in secondary ABC
• Can help distinguish primary from secondary lesions

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Q: "When would you use selective arterial embolisation?"

Agents used: Polyvinyl alcohol (PVA), Gelfoam, coils

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Q: "Can you use radiation for ABC?"

Radiation is generally AVOIDED due to:

• Risk of secondary sarcoma (1-3% in irradiated bone tumours)
• Most ABCs can be managed with surgery/embolisation
• Young patient population (long life expectancy)

Only considered for:

• Spine lesions inaccessible to surgery
• Multiple failed surgical treatments
• Life-threatening location (skull base)
• Dose: 20-30 Gy (low dose)

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Related Topics

• Unicameral Bone Cyst
• Giant Cell Tumour
• Telangiectatic Osteosarcoma
• Chondroblastoma
• Fibrous Dysplasia
• Spinal Bone Tumours