A 16-year-old boy presents with a 3-month history of progressive knee pain and swelling. Plain radiographs show a metaphyseal lesion in the distal femur with a wide zone of transition, cortical destruction, and a mixed lytic-sclerotic pattern. MRI demonstrates a heterogeneous mass with areas of necrosis and hemorrhage. Regarding the imaging characteristics of osteosarcoma, all of the following are true EXCEPT:

Explanation

## Osteosarcoma: Imaging Pearls ### Correct Answer Rationale **Key Point:** Osteosarcoma arises in the **metaphysis** (not diaphysis) of long bones, typically in **skeletally immature patients** (peak age 10–25 years), not mature adults. This is a classic trap in bone tumor imaging. ### Classic Imaging Features of Osteosarcoma | Feature | Characteristic | |---------|----------------| | **Location** | Metaphysis of long bones (distal femur, proximal tibia, proximal humerus) | | **Patient age** | 10–25 years (peak); rare in skeletally mature patients | | **Zone of transition** | Wide (aggressive lesion) | | **Cortical destruction** | Yes, with breakthrough | | **Periosteal reaction** | Sunburst (radiating spicules), Codman triangle, onion-skin | | **Matrix mineralization** | Mixed lytic-sclerotic, often with osteoid/chondroid matrix | | **Soft tissue mass** | Common; best seen on MRI and CT | ### Periosteal Reactions (High-Yield) **Key Point:** Both **Sunburst** and **Codman triangle** are aggressive periosteal reactions characteristic of osteosarcoma: - **Sunburst:** Radiating spicules of new bone perpendicular to cortex (highly suggestive of malignancy) - **Codman triangle:** Elevated periosteum with a triangular area of new bone at the margin (non-specific but indicates rapid growth) ### MRI Advantages in Osteosarcoma **Clinical Pearl:** MRI is superior for: 1. Assessing soft tissue extension (critical for surgical planning) 2. Detecting intramedullary marrow involvement 3. Evaluating neurovascular involvement 4. Identifying skip lesions (separate tumor foci in same bone) ### Why Option 1 is Correct Osteosarcoma arises in the **metaphysis** of skeletally **immature** patients. Stating it arises in the diaphysis of mature patients is anatomically and epidemiologically incorrect — this is a classic distractor that tests whether students confuse osteosarcoma (young, metaphyseal) with other bone tumors like fibrosarcoma or chondrosarcoma (older patients, diaphyseal).