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 improve with NSAIDs. On examination, there is a firm, warm, tender mass palpable over the distal femoral metaphysis. Plain radiographs show a mixed lytic and sclerotic lesion with cortical destruction and a "sunburst" pattern of periosteal reaction. Serum alkaline phosphatase is elevated at 180 IU/L. 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, typically arising in the metaphyseal regions of long bones during periods of rapid skeletal growth. ### Characteristic Features of This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Age** | 16 years | Peak incidence 10–25 years (growth spurt) | | **Location** | Distal femur metaphysis | Most common site (around knee in 50% of cases) | | **Presentation** | Progressive pain, night pain, swelling | Aggressive behavior; night pain suggests malignancy | | **Imaging** | Sunburst pattern, cortical destruction | Pathognomonic for osteosarcoma | | **Lab** | Elevated alkaline phosphatase | Reflects high osteoid production | ### Radiographic Hallmarks **High-Yield:** The "sunburst" (radiating spicules of new bone) and "Codman triangle" (periosteal reaction) are classic for osteosarcoma and reflect rapid tumor growth breaking through the cortex. **Clinical Pearl:** Osteosarcoma is a disease of **growing bone** — it arises in the metaphyseal region where bone growth is most active. The distal femur, proximal tibia, and proximal humerus account for >50% of cases. ### Pathophysiology 1. Arises from osteoblasts in the metaphyseal region 2. Produces malignant osteoid and bone (mixed lytic/sclerotic appearance) 3. Rapidly breaches the cortex → soft tissue extension 4. High mitotic rate → elevated alkaline phosphatase (osteoid production) 5. Early hematogenous spread (lungs most common) ### Diagnostic Workup - **Plain radiographs:** Sunburst, Codman triangle, cortical destruction - **MRI:** Best for soft tissue extent and surgical planning - **CT chest:** Staging (pulmonary metastases in ~20% at diagnosis) - **Biopsy:** Confirms diagnosis (shows malignant osteoid) - **Alkaline phosphatase & LDH:** Prognostic markers ### Management **Key Point:** Multimodal therapy (chemotherapy + surgery) has improved 5-year survival to ~70%. 1. **Neoadjuvant chemotherapy** (cisplatin, doxorubicin, methotrexate) 2. **Wide surgical resection** (limb-salvage or amputation) 3. **Adjuvant chemotherapy** 4. **Surveillance for metastases** **Mnemonic — COSS (Cooperative Osteosarcoma Study):** Cisplatin, dOxorubicin, methotrexate, Surgery — the backbone of modern osteosarcoma therapy. ### Why Osteosarcoma (Not Other Differentials) - ~~Ewing sarcoma~~ — typically diaphyseal (shaft), not metaphyseal; lacks sunburst pattern - ~~Giant cell tumor~~ — epiphyseal location, benign-appearing; occurs in skeletally mature patients - ~~Osteomyelitis~~ — acute infection; would show systemic signs (fever, elevated WBC); no night pain pattern [cite:Robbins 10e Ch 26]