Osteoid osteoma and osteoblastoma
- Lesion
Two benign bone-forming tumours with the same tissue: a tiny painful one in long bones and a bigger one in the spine.
What it is
Osteoid osteoma and osteoblastoma are benign tumours of young people made of woven bone trabeculae rimmed by osteoblasts. They look the same under the microscope and are separated by size and site: osteoid osteoma is under about two centimetres, usually in the cortex of the femur or tibia, and causes intense night pain from prostaglandin E2 made by the tumour; osteoblastoma is larger, favours the posterior elements of the spine and causes a dull ache that NSAIDs do not relieve. Both often carry rearrangements of the FOS or FOSB genes.
Structure
- Nidus: interlacing trabeculae of woven bone lined by plump osteoblasts and occasional osteoclasts in a loose, vascular stroma.
- No atypia and no permeation of host bone, which separates them from osteosarcoma.
- Osteoid osteoma: dense ring of reactive cortical bone around the nidus, often larger than the lesion itself.
- Osteoblastoma: well circumscribed, may expand the bone, with only a thin shell of reactive bone.
Step by step
- A FOS or FOSB rearrangement drives osteoblast proliferation.
- Tumour osteoblasts lay down woven bone trabeculae in a vascular stroma.
- Tumour cells overexpress cyclooxygenase and produce prostaglandin E2.
- Prostaglandins sensitize nearby nerve fibres and trigger reactive bone formation around the nidus.
- NSAIDs block cyclooxygenase and relieve the pain of osteoid osteoma.
Clinical notes
- Teenager with pain in the thigh that is worse at night and gone after an NSAID: classic osteoid osteoma.
- Spinal lesions can cause painful scoliosis; CT shows the nidus best.
- Pain disappears after removal of the nidus.
Test yourself
Think of the answer, then tap the question to see it.
Why is the pain of osteoid osteoma relieved by NSAIDs?
The tumour produces prostaglandin E2; NSAIDs block its synthesis.
How are osteoid osteoma and osteoblastoma separated?
Mainly by size and site: osteoid osteoma is small and in long-bone cortex; osteoblastoma is larger and in the vertebrae.
What is the nidus?
The small central zone of tumour woven bone and osteoblasts in a vascular stroma.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Primary Bone Cancer - NCI (National Cancer Institute, 2025).
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Representative model, for educational purposes only.