A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur just above the knee. He reports the pain is worse at night and does not respond well to NSAIDs. On examination, there is a firm, warm, tender mass over the distal femur with restricted knee flexion. Plain radiographs show a mixed lytic and sclerotic lesion in the metaphyseal region with cortical destruction and a sunburst pattern of periosteal reaction. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in children and adolescents, with peak incidence in the second decade of life. - Age: 16 years (typical age 10–25 years) - Location: Distal femur (most common site; also proximal tibia, proximal humerus) - Duration: Progressive over weeks to months - Pain: Worse at night, not relieved by NSAIDs (red flag for malignancy) ### Radiological Features | Feature | Osteosarcoma | Ewing Sarcoma | GCT | Chondrosarcoma | |---------|--------------|---------------|-----|----------------| | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Metaphysis/diaphysis | | **Pattern** | Mixed lytic + sclerotic | Permeative, "onion-skin" | Lytic | Stippled calcification | | **Periosteal reaction** | Sunburst, Codman triangle | Onion-skin, lamellated | Minimal | Minimal | | **Age** | 10–25 years | 10–20 years | 20–40 years | 40–60 years | **High-Yield:** The **sunburst pattern** (radiating spicules of new bone formation perpendicular to cortex) and **Codman triangle** (periosteal elevation with reactive bone) are pathognomonic for osteosarcoma. ### Pathological Features - Malignant spindle cell tumor producing osteoid/bone - Marked cellular atypia and mitotic activity - Rapid growth with early metastasis (lungs in 80% at presentation) ### Diagnostic Workup 1. **Plain radiographs**: Sunburst, Codman triangle, cortical destruction 2. **MRI**: Best for soft tissue extension and surgical planning 3. **CT chest**: Mandatory to detect pulmonary metastases 4. **Biopsy**: Confirms diagnosis (spindle cells with osteoid production) 5. **Alkaline phosphatase & LDH**: Often elevated; prognostic markers ### Management - **Neoadjuvant chemotherapy** (cisplatin, doxorubicin, methotrexate) followed by wide surgical resection - Limb-sparing surgery preferred when feasible - 5-year survival: ~70% with modern chemotherapy + surgery **Clinical Pearl:** Osteosarcoma has a predilection for areas of rapid bone growth (metaphyses of long bones); hence it is common around the knee in adolescents. **Mnemonic: OSTEOSARCOMA** — **O**steoid production, **S**unburst pattern, **T**eenagers/adolescents, **E**arly metastasis, **O**ver metaphysis, **S**welling + pain, **A**lkaline phosphatase ↑, **R**apid growth, **C**hemotherapy + surgery, **O**ld age rare, **M**alignant spindle cells, **A**round the knee. [cite:Robbins 10e Ch 26]