# 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 --- ## Investigations Provided ### Laboratory Results | Test | Result | Normal Range | Interpretation | |------|--------|--------------|----------------| | Haemoglobin | 132 g/L | 115-145 | Normal | | WCC | 7.5 x 10⁹/L | 5-13 | Normal | | Platelets | 290 x 10⁹/L | 150-400 | Normal | | ESR | 12 mm/hr | 0-15 | Normal | | CRP | 3 mg/L | <5 | Normal | | ALP | 180 U/L | 150-400 | Normal (child) | | Calcium | 2.35 mmol/L | 2.15-2.55 | Normal | | Phosphate | 1.4 mmol/L | 1.0-1.8 | Normal | | LDH | 165 U/L | 120-300 | Normal | ### 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 **Diagnosis: Aneurysmal Bone Cyst (ABC)** **Classic Radiological Features:** | Feature | Description | This Case | |---------|-------------|-----------| | **Location** | Metaphysis of long bones | Proximal tibia ✓ | | **Appearance** | Eccentric, expansile, lytic | Yes ✓ | | **Margins** | Well-defined with thin cortical rim | Yes ✓ | | **Septations** | Internal trabeculations | Yes ✓ | | **Matrix** | None (no mineralisation) | Yes ✓ | **Fluid-Fluid Levels on MRI:** | Significance | Notes | |--------------|-------| | **Pathognomonic** | Highly characteristic of ABC | | **Mechanism** | Blood products settling by density | | **Other causes** | Telangiectatic osteosarcoma, GCT, simple cyst | | **Timing** | Patient must be still for 10+ minutes | **Common Locations:** | Site | Frequency | |------|-----------| | Long bone metaphysis | 50-60% | | Spine (posterior elements) | 15-20% | | Pelvis | 10-15% | | Flat bones | 10% | **Age Distribution:** - Peak age: 10-20 years - 80% occur before age 20 - Slight female predominance **Key Imaging Pearl:** Fluid-fluid levels + eccentric metaphyseal location + expansile lesion = ABC until proven otherwise. > **Consultant Tip:** Fluid-fluid levels are highly suggestive of ABC but NOT pathognomonic. Telangiectatic osteosarcoma can also have fluid-fluid levels - look for solid enhancing components. **Primary vs Secondary Aneurysmal Bone Cyst:** **Primary ABC (65%):** | Feature | Description | |---------|-------------| | **Definition** | De novo lesion, no underlying pathology | | **Genetics** | USP6 gene rearrangement on chromosome 17p13 | | **Translocations** | t(16;17), t(1;17), t(3;17) most common | | **Behaviour** | Locally aggressive but benign | **Secondary ABC (35%):** | Feature | Description | |---------|-------------| | **Definition** | ABC arising within pre-existing lesion | | **Mechanism** | Haemorrhagic change in another tumour | | **Genetics** | No USP6 rearrangement | **Lesions Commonly Associated with Secondary ABC:** | Lesion | Frequency | Key Distinguishing Feature | |--------|-----------|---------------------------| | **Giant Cell Tumour (GCT)** | Most common | Epiphyseal location, >20 years | | **Chondroblastoma** | Common | Epiphysis, "chicken-wire" calcification | | **Osteoblastoma** | Common | Spine, dense osteoid | | **Fibrous Dysplasia** | Common | Ground-glass matrix | | **Osteosarcoma** | Important | Aggressive features, solid component | | **Chondromyxoid fibroma** | Less common | Eccentric metaphysis | **Clinical Importance:** | Factor | Primary ABC | Secondary ABC | |--------|-------------|---------------| | **Treatment** | Curettage usually sufficient | Treat underlying lesion | | **Recurrence** | 15-25% | Depends on primary lesion | | **Biopsy** | Sample wall thoroughly | Look for primary tumour | **This Patient:** - Young age (13 years) - Typical metaphyseal location - No solid components on MRI - Likely PRIMARY ABC - Biopsy will confirm and exclude secondary lesion > **Consultant Tip:** Always sample the solid rim of an ABC at biopsy - a secondary lesion will be found in the wall, not the cyst contents. **Differential Diagnosis for Eccentric Lytic Metaphyseal Lesion:** **1. Telangiectatic Osteosarcoma (Most Important to Exclude):** | Feature | ABC | Telangiectatic Osteosarcoma | |---------|-----|----------------------------| | **Fluid-fluid levels** | Yes | Yes | | **Age** | 10-20 | 10-30 | | **Solid component** | Thin rim only | Thick nodular enhancement | | **Cortex** | Thin but intact | Destroyed | | **Periosteal reaction** | None | Aggressive (sunburst) | | **Matrix** | None | May have osteoid | **2. Giant Cell Tumour (GCT):** | Feature | ABC | GCT | |---------|-----|-----| | **Age** | <20 years | >20 years (closed physis) | | **Location** | Metaphysis | Epiphysis/subchondral | | **Fluid levels** | Yes | May have (secondary ABC) | | **Margin** | Well-defined | Non-sclerotic margin | **3. Unicameral Bone Cyst (UBC):** | Feature | ABC | UBC | |---------|-----|-----| | **Location** | Eccentric | Central | | **Septations** | Multiple | None (unilocular) | | **Expansion** | Blows out cortex | Minimal expansion | | **Fluid levels** | Yes | No | **4. Other Differentials:** | Lesion | Distinguishing Feature | |--------|----------------------| | **Chondromyxoid fibroma** | Matrix calcification | | **Non-ossifying fibroma** | Sclerotic margin, cortical-based | | **Osteoblastoma** | Osteoid production, spine | | **Infection (Brodie's)** | Penumbra sign, clinical context | **Critical Investigation:** - **Biopsy is ESSENTIAL** before treatment - Core needle or open biopsy - Sample the wall, not just cyst contents > **Consultant Tip:** Telangiectatic osteosarcoma is the great mimicker. Look carefully for any solid nodular enhancement - if present, treat as high-grade osteosarcoma until proven otherwise. **Management of Aneurysmal Bone Cyst:** **Preoperative Workup:** | Step | Purpose | |------|---------| | **Biopsy** | Confirm diagnosis, exclude malignancy | | **Staging studies** | Chest X-ray (metastases rare but rule out) | | **Angiography** | Consider if highly vascular (embolisation) | **Treatment Options:** | Option | Indication | Notes | |--------|------------|-------| | **Curettage + bone graft** | Standard treatment | 80-90% success | | **Curettage + adjuvant** | Reduce recurrence | Phenol, cryotherapy, argon beam | | **En bloc resection** | Expendable bone, recurrence | Fibula, rib, clavicle | | **Selective arterial embolisation** | Large/inaccessible lesions | May be primary or adjunct | | **Sclerotherapy** | Smaller lesions, poor access | Polidocanol, STS | **Extended Curettage Technique:** | Step | Detail | |------|--------| | **1. Approach** | Adequate exposure via cortical window | | **2. Curettage** | Remove all visible cyst lining | | **3. High-speed burr** | Extend margins by 1-2mm | | **4. Pulsatile lavage** | Remove debris | | **5. Adjuvant** | Phenol (90 seconds) OR cryotherapy (2-3 cycles) | | **6. Neutralise** | Alcohol or saline lavage | | **7. Graft** | Bone graft ± cement | **Adjuvant Options:** | Adjuvant | Mechanism | Considerations | |----------|-----------|----------------| | **Phenol** | Chemical cauterisation | Rinse thoroughly, skin burn risk | | **Liquid nitrogen** | Cryotherapy | 2-3 freeze-thaw cycles | | **Argon beam** | Thermal coagulation | Less collateral damage | | **Cement (PMMA)** | Heat + fills defect | Not in children near physis | **Bone Graft Options:** | Type | Indication | |------|------------| | **Autograft** | Smaller defects, optimal healing | | **Allograft** | Large defects | | **Synthetic** | When allograft unavailable | **This Patient:** - Extended curettage + adjuvant (phenol or cryotherapy) - Bone graft (allograft or autograft) - Protect weight-bearing until healed (6-8 weeks) > **Consultant Tip:** The recurrence rate is 15-25% with curettage alone. Adding adjuvant therapy reduces this to 5-10%. **Recurrence Risk Factors:** | Factor | Higher Risk | Lower Risk | |--------|-------------|------------| | **Age** | <10 years | >15 years | | **Location** | Spine, pelvis | Long bones | | **Size** | >5cm | <5cm | | **Treatment** | Curettage alone | Curettage + adjuvant | | **Open physis** | Yes | No | | **Incomplete excision** | Yes | Complete removal | **Recurrence Rates by Treatment:** | Treatment | Recurrence Rate | |-----------|-----------------| | Curettage alone | 20-30% | | Curettage + phenol | 10-15% | | Curettage + cryotherapy | 5-10% | | En bloc resection | <5% | | Selective embolisation only | 20-40% | **Timing of Recurrence:** - 90% occur within 2 years - Most within first 12 months - Rare after 3 years **Follow-up Protocol:** | Time | Assessment | |------|------------| | 6 weeks | X-ray (healing, graft incorporation) | | 3 months | X-ray (recurrence screening) | | 6 months | X-ray | | 12 months | X-ray | | 18 months | X-ray | | 24 months | X-ray | | Then annually | Until 5 years post-surgery | **Signs of Recurrence:** | Imaging Finding | Clinical Finding | |-----------------|------------------| | New lytic area within graft | Return of pain | | Expanding lesion | Swelling | | Loss of graft incorporation | Pathological fracture | **Management of Recurrence:** | Scenario | Approach | |----------|----------| | **Small recurrence** | Repeat curettage + adjuvant | | **Large recurrence** | Consider en bloc if expendable bone | | **Multiple recurrences** | Wide resection, reconstruction | | **Spine recurrence** | May need decompression + fusion | > **Consultant Tip:** Most recurrences present within 2 years. If X-rays are stable at 2 years, prognosis is excellent. **Spinal Aneurysmal Bone Cyst:** **Key Features:** | Feature | Description | |---------|-------------| | **Frequency** | 15-20% of all ABC | | **Location** | Posterior elements > vertebral body | | **Age** | Same as appendicular (10-20 years) | | **Spine level** | Thoracic > lumbar > cervical | **Presentation:** | Symptom | Cause | |---------|-------| | **Back pain** | Bone destruction | | **Neurological deficit** | Cord/root compression | | **Deformity** | Vertebral collapse, instability | | **Incidental** | Found on imaging for other reasons | **Imaging Considerations:** | Modality | Findings | |----------|----------| | **X-ray** | Expansile lytic lesion, may miss early | | **CT** | Defines bony involvement | | **MRI** | Fluid-fluid levels, cord compression | **Treatment Algorithm:** ``` Spinal ABC ↓ Assess stability + neurology ↓ ├── Stable, no neurology → Selective arterial embolisation │ May be definitive or preoperative │ ├── Unstable OR neurology → Surgery: │ Intralesional excision ± adjuvant │ Fusion if unstable │ └── Inaccessible/extensive → Serial embolisation ± radiation (rare, last resort) ``` **Surgical Considerations:** | Issue | Management | |-------|------------| | **Access** | Often requires circumferential approach | | **Bleeding** | Consider preoperative embolisation | | **Instability** | May need instrumentation | | **Neurology** | Decompression priority | | **Reconstruction** | Cage/graft if body involved | **Role of Embolisation:** | Scenario | Indication | |----------|------------| | **Primary treatment** | Stable, no neurology, small lesion | | **Preoperative** | Reduce surgical bleeding | | **Palliation** | If surgery not feasible | **Prognosis:** - Recurrence higher in spine (30-40%) than long bones - May require multiple treatments - Deformity may persist despite tumour control > **Consultant Tip:** Spinal ABC often requires multidisciplinary management - orthopaedic spine, interventional radiology, and sometimes radiation oncology. --- ## 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:** | Feature | ABC | Telangiectatic Osteosarcoma | |---------|-----|----------------------------| | Solid component | None | Thick nodular enhancement | | Cortex | Thin but intact | Destroyed | | Periosteal reaction | None | Aggressive | | LDH/ALP | Normal | Often elevated | ### 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 --- ## 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 --- **Q: "When would you use selective arterial embolisation?"** | Indication | Notes | |------------|-------| | **Large/vascular lesions** | Reduce surgical bleeding | | **Spinal ABC** | May be definitive or preoperative | | **Pelvic ABC** | Difficult surgical access | | **Recurrent lesions** | Before repeat surgery | | **Patient unfit for surgery** | Palliative/temporising | Agents used: Polyvinyl alcohol (PVA), Gelfoam, coils --- **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) --- ## Related Topics - Unicameral Bone Cyst - Giant Cell Tumour - Telangiectatic Osteosarcoma - Chondroblastoma - Fibrous Dysplasia - Spinal Bone Tumours