A 35-year-old woman undergoes imaging for a painless, slowly enlarging mass in the proximal humerus. CT shows a well-defined, non-expansile lesion with central calcification and cortical thickening. The lesion has been stable in size over 5 years. Which of the following imaging features is NOT consistent with a benign bone tumor (osteochondroma or enchondroma)?

Explanation

## Benign Bone Tumors: Imaging Characteristics ### Features of Benign Lesions (Osteochondroma, Enchondroma) **Key Point:** Benign bone tumors are characterized by slow growth, well-defined margins, and absence of aggressive periosteal reactions. | Feature | Benign Lesion | Malignant Lesion | |---------|---|---| | Zone of transition | Narrow (<1 cm) | Wide (>2 cm) | | Periosteal reaction | None or simple | Sunburst, Codman triangle | | Cortical involvement | Thickening, sclerosis | Cortical destruction | | Calcification | Central, organized | Scattered, disorganized | | Soft tissue mass | Absent or small | Large, heterogeneous | | Growth rate | Slow or static | Rapid | | Margin definition | Sharp, well-defined | Ill-defined | ### Why Sunburst Periosteal Reaction is NOT Benign **High-Yield:** A **sunburst periosteal pattern** is an **aggressive periosteal reaction** characteristic of **malignant tumors** (especially osteosarcoma). Benign lesions typically show **no periosteal reaction** or at most a simple, smooth periosteal response. **Clinical Pearl:** The presence of a sunburst pattern indicates rapid, aggressive tumor growth and is a **red flag for malignancy**. Benign lesions grow slowly and do not trigger aggressive periosteal reactions. **Mnemonic: SUNBURST = SINISTER** — Sunburst periosteal pattern indicates aggressive malignancy; benign lesions lack this finding. ### Imaging Hierarchy of Aggressiveness ```mermaid flowchart TD A[Bone Lesion]:::outcome --> B{Periosteal Reaction?}:::decision B -->|None/Smooth| C[Benign or indolent]:::outcome B -->|Sunburst/Codman| D[Aggressive malignancy]:::urgent C --> E[Narrow zone of transition]:::outcome C --> F[Well-defined margins]:::outcome C --> G[Cortical sclerosis]:::outcome D --> H[Wide zone of transition]:::outcome D --> I[Cortical destruction]:::outcome D --> J[Large soft tissue mass]:::outcome ``` ### Typical Benign Findings **Osteochondroma:** - Sessile or pedunculated exostosis - Cortical continuity with parent bone - No periosteal reaction - Cap of hyaline cartilage (not visible on plain radiographs) **Enchondroma:** - Central location in medullary cavity - Stippled or popcorn-like calcification - Well-defined, sclerotic margin - No periosteal reaction - Stable over years [cite:Robbins 10e Ch 24]