A 16-year-old boy presents to the orthopaedic clinic with a 3-month history of progressive pain and swelling around the right knee. He reports the pain is worse at night and does not respond well to NSAIDs. On examination, there is a firm, fixed mass around the distal femur with warmth and dilated veins over the area. X-ray shows a mixed lytic and sclerotic lesion with a "sunburst" pattern and cortical destruction. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumour in adolescents and young adults, typically arising around the metaphysis of long bones, especially around the knee (distal femur and proximal tibia). ### Characteristic Features | Feature | Osteosarcoma | Ewing Sarcoma | GCT | Osteomyelitis | |---------|--------------|---------------|-----|---------------| | **Age** | 10–25 years | 10–30 years | 20–40 years | Any age | | **Site** | Metaphysis (around knee) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Variable | | **Pain character** | Severe, night pain, progressive | Severe, systemic symptoms | Dull, chronic | Acute, fever | | **X-ray pattern** | Sunburst, mixed lytic-sclerotic | Permeative, "onion-skin" | Lytic, eccentric | Sequestrum, involucrum | | **Cortical destruction** | Yes, aggressive | Yes | No | Yes | **High-Yield:** The "sunburst" (radiating spicules of new bone formation perpendicular to cortex) and "Codman triangle" (periosteal reaction) are pathognomonic for osteosarcoma. ### Pathology 1. Malignant mesenchymal tumour producing osteoid/bone 2. Arises in areas of rapid bone growth (metaphysis) 3. Highly aggressive with early haematogenous spread (lungs in 20% at diagnosis) 4. Associated with prior radiotherapy, Paget disease, and hereditary retinoblastoma (RB1 mutation) ### Investigations - **Plain radiography:** Sunburst pattern, cortical destruction, soft tissue mass - **MRI:** Best for soft tissue extent and marrow involvement - **CT chest:** Staging (pulmonary metastases in 20%) - **Alkaline phosphatase & LDH:** Elevated, prognostic markers - **Biopsy:** Confirms diagnosis (spindle cells with osteoid production) **Clinical Pearl:** Night pain that is not relieved by NSAIDs is a red flag for malignancy in a young patient with bone pain. ### Management ```mermaid flowchart TD A[Osteosarcoma diagnosed]:::outcome --> B[Neoadjuvant chemotherapy]:::action B --> C[Wide surgical excision]:::action C --> D[Adjuvant chemotherapy]:::action D --> E[Surveillance for metastases]:::action E --> F{5-year survival}:::decision F -->|Good chemotherapy response| G[~70% survival]:::outcome F -->|Poor response| H[~40% survival]:::outcome ``` **Key Point:** Neoadjuvant chemotherapy followed by wide surgical resection and adjuvant chemotherapy is the standard of care. Prognosis depends on chemotherapy response and presence of metastases.