A 35-year-old woman undergoes MRI for a painless swelling in the proximal humerus. The lesion shows low signal intensity on T1 and T2-weighted sequences with a homogeneous appearance and a well-defined sclerotic border. Which imaging finding best distinguishes osteochondroma from osteoid osteoma?

## Distinguishing Osteochondroma from Osteoid Osteoma ### Key Imaging Features **Key Point:** The cartilage cap of osteochondroma shows bright T2 signal (hyaline cartilage), whereas osteoid osteoma presents with a characteristic nidus and surrounding reactive sclerosis without a cartilage cap. ### Comparative Table | Feature | Osteochondroma | Osteoid Osteoma | |---------|----------------|------------------| | **Age Group** | 10–30 years (growth plate closure) | 10–30 years (peak 20s) | | **Location** | Metaphysis (around knee 50%) | Diaphysis or metadiaphysis | | **Size** | Variable, often >2 cm | <1.5 cm (nidus) | | **Cartilage Cap** | Present, T2 hyperintense | Absent | | **Nidus** | Absent | Present, lucent center | | **Surrounding Sclerosis** | Minimal | Marked (reactive) | | **Pain** | Usually painless | Severe, nocturnal pain | | **MRI Signal** | Cap: T2 bright; base: low | Homogeneous low signal | | **CT Appearance** | Bony exostosis with cap | Nidus + sclerotic halo | ### Why Cartilage Cap with T2 Hyperintensity? **High-Yield:** The cartilage cap is pathognomonic for osteochondroma. On T2-weighted MRI, hyaline cartilage appears bright (high signal) due to its high water content. This cap is absent in osteoid osteoma, which instead shows a lucent nidus surrounded by dense sclerosis. **Clinical Pearl:** Osteochondroma is a benign cartilage-capped bony exostosis that arises from the growth plate. It grows away from the joint (exophytic growth) and typically regresses after skeletal maturity. Osteoid osteoma, by contrast, is a small benign osteoid-forming lesion that causes severe pain (often relieved by NSAIDs) due to prostaglandin production and surrounding inflammation. ### Mermaid Diagnostic Algorithm ```mermaid flowchart TD A[Benign Bone Lesion in Young Adult]:::outcome --> B{Cartilage cap on MRI?}:::decision B -->|Yes, T2 hyperintense| C[Osteochondroma]:::outcome B -->|No| D{Nidus + sclerotic halo?}:::decision D -->|Yes, on CT| E[Osteoid Osteoma]:::outcome D -->|No| F[Other benign lesion]:::outcome C --> G[Monitor for malignant transformation]:::action E --> H[NSAIDs, consider ablation if symptomatic]:::action ``` ### Why Other Options Are Misleading - **Nidus with surrounding sclerosis on CT:** This is the classic finding of osteoid osteoma, not osteochondroma. While it is a discriminatory feature, the question stem describes a homogeneous low-signal lesion on MRI, which better fits the cartilage cap of osteochondroma. - **Sessile or pedunculated bony exostosis:** While osteochondroma does present as an exostosis, this morphology is not unique to osteochondroma and does not highlight the key differentiating feature (cartilage cap). Osteoid osteoma does not form an exostosis. - **Homogeneous low signal on all sequences:** This describes osteoid osteoma (dense bone/sclerosis), not the distinguishing feature of osteochondroma, which has a bright cartilage cap on T2.