A 16-year-old boy presents to the orthopaedic clinic with a 3-month history of progressive pain and swelling around the left knee. On examination, there is a firm, tender mass over the distal femur with warmth and overlying skin erythema. Plain radiographs show a mixed lytic and sclerotic lesion at the metaphysis of the distal femur with cortical destruction and a "sunburst" pattern of periosteal reaction. Serum alkaline phosphatase is elevated at 450 IU/L. MRI confirms a large soft tissue component. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation This case presents the classic triad of osteosarcoma: **Key Point:** Osteosarcoma is the most common primary malignant bone tumour in children and adolescents, typically arising in the metaphyseal regions of long bones during periods of rapid skeletal growth. ### Epidemiological & Location Features - **Age:** Peak incidence 10–25 years (coincides with growth spurts) - **Location:** Metaphysis of long bones, especially around the knee (distal femur 40%, proximal tibia 16%) - **Sex:** Slight male predominance ### Radiological Hallmarks | Feature | Osteosarcoma | Ewing Sarcoma | GCT | |---------|--------------|---------------|-----| | **Location** | Metaphysis | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | | **Pattern** | Mixed lytic + sclerotic | Purely lytic | Lytic | | **Periosteal reaction** | Sunburst, codman triangle | Onion-skin | Absent | | **Cortical involvement** | Early, aggressive | Variable | Late | | **Soft tissue mass** | Large, common | Smaller | Rare | **High-Yield:** The "sunburst" pattern (radiating spicules of new bone formation perpendicular to cortex) and **Codman triangle** (periosteal new bone at tumour margin) are pathognomonic for osteosarcoma. ### Biochemical Markers - **Alkaline phosphatase:** Elevated in 50–80% of cases (reflects osteoblastic activity) - **LDH:** Often elevated; prognostic significance ### Pathological Features - Malignant spindle cell tumour with osteoid/bone formation by tumour cells - High mitotic rate and necrosis - Aggressive local invasion and early metastatic potential (lungs in 20% at presentation) **Clinical Pearl:** Any adolescent with metaphyseal bone pain, swelling, and a mixed lytic-sclerotic lesion with sunburst periosteal reaction should be presumed to have osteosarcoma until proven otherwise. Urgent staging (chest CT, bone scan) and biopsy are mandatory. ### Management Principles 1. Staging: Chest CT (pulmonary metastases), bone scan, MRI for local extent 2. Biopsy: Core needle or open biopsy for histological confirmation 3. Neoadjuvant chemotherapy followed by wide surgical excision 4. Adjuvant chemotherapy (cisplatin, doxorubicin, methotrexate) 5. Prognosis: 5-year survival ~70% with modern multimodal therapy **Mnemonic: OSTEOSARCOMA = Osteoid-producing, Spindle cells, Teenage/young adult, Epiphyseal/metaphyseal, Osteoblastic, Sunburst/Codman, Aggressive, Rapid growth, Chemosensitive, Osteoclasts absent, Metaphysis, Alkaline phosphatase elevated**