A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. He reports the pain is worse at night and not relieved by NSAIDs. On examination, there is a firm, warm swelling over the distal femur with restricted knee movement. Plain radiographs show a mixed lytic and sclerotic lesion with periosteal new bone formation and a "sunburst" pattern. Alkaline phosphatase is elevated at 420 U/L. MRI reveals a large intramedullary tumor with soft tissue extension. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults, typically arising in the metaphyseal regions of long bones during periods of rapid skeletal growth. ### Characteristic Features | Feature | Osteosarcoma | Ewing's Sarcoma | GCT | Osteomyelitis | |---------|--------------|-----------------|-----|---------------| | **Age** | 10–25 years (peak 15–20) | 10–30 years | 20–40 years | Any age | | **Site** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Variable | | **Pain pattern** | Insidious, progressive, night pain | Acute onset, systemic symptoms | Chronic, dull | Acute, fever, chills | | **Alkaline phosphatase** | Elevated (osteoid production) | Normal/low | Normal | Normal | | **Radiographic pattern** | Sunburst, mixed lytic/sclerotic, Codman's triangle | Permeative, "onion-skin" | Eccentric, lytic, soap-bubble | Sequestrum, involucrum | ### Radiographic Hallmarks **High-Yield:** The "sunburst" (radiating spicules of new bone perpendicular to cortex) and "Codman's triangle" (periosteal reaction) are pathognomonic for osteosarcoma. **Clinical Pearl:** Night pain unresponsive to NSAIDs in an adolescent with metaphyseal swelling is a red flag for osteosarcoma until proven otherwise. ### Pathophysiology 1. Arises from osteoblasts during periods of rapid bone growth 2. Produces osteoid and bone (hence elevated alkaline phosphatase) 3. Aggressive, rapidly invades soft tissues 4. High propensity for early pulmonary metastasis (20% at presentation) ### Diagnostic Workup **Key Point:** Diagnosis requires: - Plain radiographs (initial screening) - MRI (for local staging and soft tissue extent) - CT chest (metastatic screening) - Tissue biopsy (histological confirmation) - Alkaline phosphatase and LDH (prognostic markers) ### Management **High-Yield:** Neoadjuvant chemotherapy followed by surgical resection (limb-salvage or amputation) and adjuvant chemotherapy. Current 5-year survival with multimodal therapy is ~70%. **Mnemonic: COSS** — **C**hemotherapy, **O**perative resection, **S**urgical margins, **S**urvival depends on chemotherapy response.