CT-Guided Radiofrequency Ablation of Osteoid Osteoma

Indication. A symptomatic osteoid osteoma — the classic night pain relieved by NSAIDs in a child or young adult (peak 5 to 25 years, male predominance around two to three times), with CT demonstrating a lucent nidus less than 1.5 cm surrounded by reactive sclerosis — that is unsuitable for, or has failed, a time-limited trial of medical management. RFA is the treatment of choice for an accessible appendicular nidus; it is also appropriate for many spinal lesions provided the thermal margin can be protected. Confirm the diagnosis before abating. The combination of a typical clinical picture and a characteristic CT nidus is diagnostic. Bone scintigraphy shows the double-density sign (intense nidus uptake within a halo of moderate uptake); MRI shows surrounding marrow and soft-tissue oedema but can miss the nidus itself, so CT is the planning modality. Biopsy is reserved for atypical or larger lesions where osteoblastoma or another lesion is in question. Cautions and relative contraindications. RFA is avoided or modified when:

• The nidus lies within about 1 cm of the spinal cord, cauda equina or a major nerve root and cannot be displaced or monitored safely — favour open precision excision.
• The nidus is in weight-bearing cortex (femoral neck, tibia) where a large thermal lesion risks fracture — protect weight-bearing or plan prophylactic fixation.
• The lesion is juxta-articular — thermal damage to cartilage is a concern.
• Pregnancy, an uncooperative patient who cannot stay still, or local infection at the entry site. The one decision that matters. Once the diagnosis is secure, the choice is the access strategy and the thermal plan, not whether to treat:

Consent specifically for a short post-procedural pain flare and low-grade fever (post-ablation syndrome), a small risk of skin burn or local infection, a fracture risk for weight-bearing sites, neural risk for spinal sites, and the possibility of persistence or recurrence requiring a second ablation. Setup. This is a CT-suite, image-guided procedure, not an open theatre operation. General or regional anaesthesia is used (general anaesthesia for nearly all children, since the patient must be completely motionless during drilling and ablation). Position the patient so the CT gantry has unobstructed access to the lesion, apply sterile prep and drape, and place grounding pads on clean, hairless thigh skin with full contact (check both pads to avoid a pad burn). A musculoskeletal radiologist or tumour surgeon trained in ablation performs it with CT fluoroscopy.

The goal: under CT guidance, advance a drill to the nidus, place the radiofrequency electrode active tip within the nidus, and deliver a thermal dose of about 90 °C for about 6 minutes that destroys the whole nidus while sparing everything beyond roughly 1 cm of it. The "exposure" here is percutaneous — there is no open dissection, so the steps below are the CT-guided access trajectory and then the ablation.

Rehabilitation | Phase | Timing | Activity and restrictions | Monitoring | |-------|--------|---------------------------|------------| | 1 | 0 to 48 hours | Rest, then mobilise; NSAIDs and analgesia; protected weight-bearing for a limb lesion | Post-ablation pain and fever | | 2 | 48 hours to 2 weeks | Resume normal activity; weight-bearing per site; puncture-site care | Confirm pain relief; no fevers | | 3 | 2 to 8 weeks | Graded return; protected weight-bearing for femoral neck or tibia | Symptom resolution | | 4 | 8 weeks onward | Full unrestricted activity | Re-image only if pain recurs | Most patients go home the same day, are back to school or desk work within days, and full activity within a few weeks (longer, protected, for weight-bearing sites). Pain relief is often immediate, though a transient flare for 24 to 72 hours is common. Clinical success is around 90 percent after a single session; persistent or recurrent pain is usually re-ablated successfully. Complications

Epidemiology. Osteoid osteoma is a common benign bone-forming tumour, accounting for roughly 10 to 12 percent of benign bone tumours. It affects males two to three times more often than females, peaks between 5 and 25 years, and most often arises in the cortex of the femur (especially the neck) and the tibia; around 10 percent occur in the spine, typically in the posterior elements (pedicle, lamina). It is usually solitary. Pathoanatomy. The lesion has two parts: a small nidus of highly vascular osteoid and woven bone (less than 1.5 cm in an osteoid osteoma), enclosed by a dense zone of reactive sclerotic bone that is the body's response, not the tumour itself. The characteristic pain is prostaglandin-mediated: the nidus produces remarkably high levels of prostaglandin E2, which both sensitises unmyelinated nerve fibres within it and explains the dramatic response to NSAIDs and the nocturnal pattern. Because only the nidus is the lesion, destroying it alone is curative — the rationale for percutaneous ablation.

Key evidence. Rosenthal pioneered percutaneous electrode ablation of osteoid osteoma in 1992 and, in a landmark comparison with operative treatment, showed RFA to give equivalent success at markedly lower morbidity (1998). Subsequent large CT-guided series confirmed a primary success rate around 90 percent with low recurrence and minimal complications, and refined thermal-protection techniques for spinal lesions. This consistent evidence — same cure, less morbidity, day-case recovery — is why percutaneous RFA displaced open excision as the standard of care and is a perennial viva favourite.