Introduction
Osteoid osteoma is a benign bone-forming tumor, usually <2 cm, that classically affects young males (10–30 years). It causes night pain relieved by NSAIDs (due to prostaglandin release).
Radiology is key for diagnosis, with X-ray, CT, MRI, and bone scan playing different roles.
Imaging Features
1. X-Ray Findings
- Nidus: Small, round/oval lucent lesion (<1.5–2 cm).
- May show central mineralization (calcified nidus).
- Surrounded by marked reactive cortical sclerosis and periosteal thickening.
- Best seen in long bone diaphysis (femur, tibia).
2. CT Scan (Modality of Choice)
- Best for identifying the nidus.
- Nidus appears as:
- Low attenuation center (lucent).
- Possible central calcification.
- Surrounded by sclerotic cortical thickening.
- Ideal for planning treatment (e.g., radiofrequency ablation).
3. MRI Findings
- Nidus may appear low-intermediate on T1, variable T2.
- Often shows intense surrounding marrow and soft tissue edema, which may obscure nidus.
- Gadolinium: intense enhancement of nidus.
- MRI can overestimate extent due to reactive edema.
4. Bone Scan (Scintigraphy)
- Double density sign:
- Central intense uptake (nidus).
- Surrounding less intense uptake (reactive sclerosis).
- Highly sensitive, useful for lesion localization.
Typical Locations
- Cortical diaphysis of long bones (femur, tibia) = most common.
- Posterior elements of spine → may cause painful scoliosis.
- Rarely intra-articular → minimal sclerosis, mimics arthritis.
Key Teaching Points
- Nidus = hallmark of osteoid osteoma.
- CT > MRI for diagnosis.
- Double density sign on bone scan = classic.
- Intra-articular osteoid osteoma is a diagnostic pitfall (little sclerosis, atypical).
Conclusion
Osteoid osteoma is a benign but painful bone tumor with distinct imaging features. CT is the gold standard for diagnosis, while bone scan and MRI provide supportive information. Recognizing the nidus and classic imaging signs is essential for accurate diagnosis and treatment planning.
