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 swelling over the distal femur with restricted knee flexion. 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 280 U/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 during the growth spurt (ages 10–25 years). ### Characteristic Features of This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Age** | 16 years | Peak incidence during adolescent growth spurt | | **Location** | Distal femur metaphysis | Most common site (around the knee in 50% of cases) | | **Presentation** | Progressive pain, worse at night | Classic presentation; pain often precedes swelling | | **Response to NSAIDs** | Poor response | Suggests malignancy rather than inflammatory cause | | **Radiographic pattern** | Mixed lytic/sclerotic with "sunburst" periosteal reaction | Pathognomonic for osteosarcoma | | **Alkaline phosphatase** | Elevated (280 U/L) | Reflects high osteoblastic activity | ### Pathological Hallmarks **High-Yield:** The "sunburst" (radiating spicules) and "Codman triangle" (elevated periosteum) are classic radiographic signs of aggressive bone tumor growth, particularly osteosarcoma. **Key Point:** Osteosarcoma is characterized by malignant osteoid or bone production by tumor cells. Histologically, you will see: - Pleomorphic spindle cells - Abnormal mitotic figures - Direct osteoid/bone formation by tumor cells (not just reactive bone) - High cellularity and marked atypia ### Why Alkaline Phosphatase is Elevated Osteosarcoma cells produce osteoid and bone, which requires alkaline phosphatase activity. Elevated serum ALP correlates with tumor burden and is used for prognostic staging and monitoring response to chemotherapy. ### Diagnostic Confirmation 1. **MRI** — best for assessing soft tissue extension and marrow involvement 2. **CT chest** — essential for staging (lung metastases in ~20% at presentation) 3. **Biopsy** — histological confirmation (core needle or open biopsy) 4. **Alkaline phosphatase & LDH** — prognostic markers ### Treatment Approach **Clinical Pearl:** Modern osteosarcoma management combines neoadjuvant chemotherapy (doxorubicin, cisplatin, methotrexate), surgical resection with wide margins, and adjuvant chemotherapy. 5-year survival has improved to ~70% with this multimodal approach. **Mnemonic for osteosarcoma risk factors: GROWTH** - **G**rowth spurt (adolescence) - **R**adiation exposure (prior therapy) - **O**steogenesis imperfecta, Paget disease - **W**eak bones (hereditary conditions) - **T**all stature - **H**igh alkaline phosphatase [cite:Robbins 10e Ch 26]