## Periosteal Reactions in Bone Tumors **Key Point:** The sunburst (or radiating spicules) pattern is the hallmark aggressive periosteal reaction of osteosarcoma, representing rapid tumor growth breaking through cortex with radiating tumor vessels and new bone formation along Haversian and Volkmann canals. ### Periosteal Reaction Patterns — Comparison Table | Pattern | Appearance | Associated Lesion | Mechanism | | --- | --- | --- | --- | | **Sunburst (Radiating spicules)** | Radiating spicules perpendicular to cortex | Osteosarcoma (most common) | Aggressive tumor lifting periosteum with radiating vessels | | **Codman triangle** | Triangular area of new bone at lesion margin | Osteosarcoma, Ewing sarcoma, infection | Periosteum lifted at edge, new bone formation incomplete | | **Onion skin (Lamellated)** | Concentric rings of periosteal new bone | Ewing sarcoma, infection | Intermittent periosteal lifting and healing | | **Solid periosteal reaction** | Continuous dense bone layer | Benign lesions, hemangioma | Slow, benign process | **High-Yield:** Sunburst is virtually pathognomonic for osteosarcoma when seen on plain radiographs; it indicates an aggressive, rapidly growing malignancy. **Clinical Pearl:** Codman triangle, though often taught as "classic" for osteosarcoma, is actually a non-specific sign of any aggressive periosteal process and is less specific than sunburst for osteosarcoma diagnosis. **Mnemonic:** **SCOOP** — **S**unburst = osteosarcoma; **C**odman = any aggressive lesion; **O**nion skin = Ewing; **O**steoid = benign; **P**eriosteal = aggressive. 
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