# Bone Tumor Ablation and Cement Augmentation

## Overview

Percutaneous ablation of bone tumors encompasses both palliative treatment of painful osseous metastases and curative-intent treatment of benign tumors (osteoid osteoma). Ablation modalities: radiofrequency ablation (RFA), cryoablation, and microwave ablation (MWA). Cement augmentation (cementoplasty) often combined with ablation for structural reinforcement and additional pain relief. Image guidance: CT (primary), fluoroscopy (supplemental), cone-beam CT.

## Indications

### Palliative Ablation of Osseous Metastases

Painful bone metastases refractory to or unsuitable for radiation therapy. Limited number of painful lesions (typically 1–3 treated per session). Common primary tumors: lung, breast, renal, thyroid, prostate. Pain reduction the primary endpoint; tumor control is secondary. Can be combined with radiation therapy for synergistic effect.

### Osteoid Osteoma (Curative Intent)

RFA is the first-line treatment for symptomatic osteoid osteoma. Success rate >90% with single treatment session. Replaces surgical excision in most cases. Nidus identified on CT; RFA probe placed directly into the nidus. Typical treatment: 90°C for 4–6 minutes.

### Other Applications

Chondroblastoma (cryoablation preferred for subchondral lesions). Desmoid tumors (cryoablation). Local control of oligometastatic bone disease in curative-intent settings. Painful benign lesions (e.g., giant cell tumor in select cases).

<image>CT-guided radiofrequency ablation of an osteoid osteoma in the proximal femur showing the RFA probe tip positioned within the nidus</image>

## Ablation Modalities for Bone

| Feature | RFA | Cryoablation | MWA |
|---------|-----|-------------|-----|
| Primary bone application | Osteoid osteoma (curative) | Metastases near nerves | Large metastases |
| Protocol (osteoid osteoma) | 90°C for 4-6 min | Double freeze-thaw | Timed per device |
| Visualization | Limited | Excellent (ice ball on CT) | Limited |
| Neural safety | Moderate risk | Preferred near nerves | Moderate risk |
| Ablation speed | Moderate | Slow | Fastest |
| Hemorrhage risk | Low | Higher | Low |
| Evidence base | Strongest (osteoid osteoma) | Strong (metastases) | Growing |

### Radiofrequency Ablation (RFA)

Most established modality for osteoid osteoma. Electrode placed percutaneously through a bone biopsy needle or directly via drill. Bone cortex acts as thermal insulator, creating predictable ablation zones within medullary cavity. Standard protocol for osteoid osteoma: 90°C for 4–6 minutes. For metastases: larger ablation zones needed, may require multiple overlapping ablations.

### Cryoablation

Preferred when ice ball monitoring provides safety advantage near neural structures. Multiple probes can create large ablation volumes for metastatic lesions. Ice ball visible on CT — precise margin monitoring. Particularly useful for: Lesions near spinal cord or nerve roots (with thermal monitoring). Subchondral lesions near articular cartilage. Large soft tissue components extending beyond bone. Disadvantage: does not cauterize vessels; risk of post-procedural hemorrhage.

### Microwave Ablation (MWA)

Faster ablation times and larger zones than RFA. Less affected by bone's low electrical conductivity compared to RFA. Growing use in osseous metastases. Fewer published studies than RFA and cryoablation for bone applications.

## Cement Augmentation (Cementoplasty)

### Polymethylmethacrylate (PMMA)

Bone cement injected after ablation for structural reinforcement. Exothermic polymerization (~70°C) provides additional local tumor cytotoxicity. Stabilizes weakened bone and provides immediate pain relief through mechanical stabilization. Applied in weight-bearing bones (vertebral bodies, acetabulum, proximal femur, tibial plateau).

### Technique

After ablation, trocar or bone biopsy needle repositioned or left in place. PMMA mixed and injected under fluoroscopic or CT guidance during the "doughy" phase. Inject slowly with continuous imaging to monitor for extravasation. Stop injection if cement approaches posterior vertebral body wall, cortical breach, or vascular channels. Typical volumes: 2–6 mL for vertebral body, varies for extraspinal locations.

### Combined Ablation + Cementoplasty

Synergistic approach: ablation provides pain relief and local tumor control; cement provides structural support. Particularly effective for lytic metastases in weight-bearing bones. Ablation first to create cavity and reduce vascularity → then cement fills the ablated space. Studies show superior pain relief compared with either modality alone.

<image>Post-procedural CT showing PMMA cement filling a lytic metastasis in the acetabulum following cryoablation, providing structural reinforcement</image>

## Procedural Technique

### Pre-Procedural Planning

Review CT and MRI for lesion characterization, cortical integrity, and soft tissue extension. MRI essential for assessing relationship to spinal cord, nerve roots, and adjacent soft tissues. Nuclear medicine bone scan or PET-CT to confirm metabolically active lesion. Assess fracture risk using Mirels scoring system for long bones. Plan access trajectory avoiding neurovascular structures.

### Image Guidance

CT is the primary modality for bone ablation procedures. Fluoroscopy supplemental for needle advancement and cement injection monitoring. Cone-beam CT for intraprocedural assessment. For vertebral lesions: transpedicular or parapedicular approach. For appendicular skeleton: direct cortical puncture using bone biopsy needle system.

### Protective Measures

**Thermocouple monitoring**: temperature probes placed near neural structures (spinal cord, nerve roots). Threshold: ≤45°C at neural structures for heat-based ablation. For cryoablation: ice ball monitored to remain >1 cm from neural structures. **Hydrodissection/CO2 dissection**: separate tumor from adjacent soft tissue structures. **Epidural insulation**: saline or CO2 injection into epidural space for spinal lesions. **Motor-evoked potential monitoring**: for lesions adjacent to spinal cord (some centers).

### Osteoid Osteoma Protocol

Identify nidus on CT (typically <1.5 cm, cortical-based, with surrounding reactive sclerosis). Coaxial approach: 11-gauge bone biopsy needle advanced to nidus margin. RFA electrode advanced through needle into center of nidus. Biopsy can be performed prior to ablation (optional but may displace nidus). RFA at 90°C for 4–6 minutes. Post-ablation CT to confirm probe positioning. Outpatient procedure; most patients have immediate or near-immediate pain relief.

<image>Fluoroscopic image during cementoplasty of a lytic vertebral body metastasis showing controlled PMMA injection with monitoring for posterior extravasation</image>

## Outcomes

### Pain Palliation for Metastases

Significant pain reduction in 70–95% of patients. Mean pain score reduction: 3–5 points on visual analog scale (VAS) at 1–3 months. Durable pain relief at 6–12 months in most responders. Combined ablation + cementoplasty: >80% significant pain reduction. Reduced opioid use reported in most series.

### Osteoid Osteoma

Primary success rate: 90–96%. Recurrence rate: 5–10%, successfully retreated with repeat RFA. Immediate pain relief in >80% (often within 24 hours). Complication rate <2%. Preferred over surgical excision (less morbidity, shorter recovery, equivalent efficacy).

### Functional Outcomes

Improved mobility and weight-bearing after combined ablation + cementoplasty. Reduced pathologic fracture risk with cement stabilization. Improved quality of life scores in palliative series.

## Complications

### Common (Minor)

Post-procedural pain flare (24–72 hours). Local soft tissue swelling. Transient increase in pain at ablation site.

### Uncommon (Major)

Cement extravasation (asymptomatic in most cases). Venous embolization of cement (rare, usually asymptomatic). Epidural cement leak (can cause neural compression — surgical emergency). Foraminal cement leak (radiculopathy). Neural injury (nerve root, spinal cord) from thermal spread. Pathologic fracture through ablated bone (especially without cement augmentation). Skin burn from superficial lesion ablation. Infection (osteomyelitis, rare).

## Clinical Pearls

For osteoid osteoma, the diagnosis is often clinical + imaging; if the nidus is clearly identified, biopsy may not be necessary and risks displacing the target. Always place thermocouple probes when ablating near neural structures — "trust but verify" the ablation zone. When treating vertebral metastases, the transpedicular approach gives the most controlled access and protects the spinal canal. PMMA cement should be injected during the "toothpaste" consistency phase; too liquid risks venous embolization, too thick risks incomplete filling. Combined ablation + cementoplasty is more effective than either alone for painful lytic lesions in weight-bearing bone. Osteoblastic (sclerotic) metastases respond less well to ablation alone — consider drilling to create access for probe placement. For lesions near joints, cryoablation with ice ball monitoring can help preserve articular cartilage. Post-procedure pain flare is common — counsel patients and provide adequate analgesic prescriptions.

## References
- Rosenthal DI, et al. Osteoid Osteoma: Percutaneous Treatment with Radiofrequency Energy. Radiology. 1998;209(1):229-234.
- Goetz MP, et al. Percutaneous Image-Guided Radiofrequency Ablation of Painful Metastases Involving Bone. J Clin Oncol. 2004;22(2):300-306.
- Deschamps F, et al. Percutaneous Ablation of Bone Tumors. Diagn Interv Imaging. 2014;95(7-8):659-663.
- Wallace AN, et al. Combined Ablation and Cementoplasty for Osseous Metastases. AJNR Am J Neuroradiol. 2015;36(10):1806-1813.
- Tomasian A, Jennings JW. Percutaneous Ablation of Bone and Soft Tissue Tumors. Radiol Clin North Am. 2020;58(6):1033-1050.
- Cazzato RL, et al. Percutaneous Thermal Ablation of Bone Metastases: A Systematic Review. Cardiovasc Intervent Radiol. 2021;44(7):1003-1013.