A 16-year-old boy presents with a 3-month history of progressive left knee pain and swelling. He is a competitive basketball player. On examination, there is a firm, warm swelling over the distal femur just above the knee joint. Plain radiographs show a mixed lytic and sclerotic lesion in the metaphysis of the distal femur with cortical destruction and a sunburst pattern of periosteal reaction. Serum alkaline phosphatase is elevated at 420 U/L. What is the most likely diagnosis?

Explanation

## Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults, typically arising in the metaphysis of long bones during periods of rapid skeletal growth. ### Epidemiology & Location - Peak incidence: 10–25 years (adolescence) - Most common sites: **distal femur** (40%), proximal tibia (15%), proximal humerus (10%) - Arises in areas of rapid bone growth (metaphyseal regions) - Male predominance (1.5:1) ### Pathological Features | Feature | Osteosarcoma | Ewing Sarcoma | GCT | |---------|--------------|---------------|-----| | **Age** | 10–25 years | 10–20 years | 20–40 years | | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (after fusion) | | **Radiographic pattern** | Sunburst, mixed lytic/sclerotic | Onion-skin (lamellated), permeative | Lytic, eccentric | | **Cortical involvement** | Cortical destruction common | Cortical destruction | Cortical erosion | | **Alkaline phosphatase** | Elevated (osteoid production) | Normal/mildly elevated | Normal | | **Histology** | Malignant osteoid production | Small round blue cells | Multinucleated giant cells | **High-Yield:** The **sunburst pattern** (radiating spicules of new bone perpendicular to cortex) and **mixed lytic/sclerotic lesion** in the **metaphysis** are pathognomonic for osteosarcoma. ### Clinical Pearl **Codman triangle** (periosteal reaction with lifting of periosteum and new bone formation) is seen in osteosarcoma and indicates aggressive tumor growth. ### Biochemical Marker - **Elevated serum alkaline phosphatase** reflects increased osteoid production by malignant osteoblasts - Prognostic value: levels correlate with tumor burden and response to chemotherapy ### Diagnosis & Staging 1. **Plain radiographs:** Sunburst pattern, cortical destruction, soft tissue mass 2. **MRI:** Best for assessing soft tissue extension and marrow involvement 3. **Biopsy:** Confirms diagnosis; shows malignant osteoid production 4. **Staging:** CT chest (lung metastases in ~20% at presentation) ### Management - **Neoadjuvant chemotherapy** (cisplatin, doxorubicin, methotrexate) followed by surgical resection - 5-year survival: ~70% with multimodal therapy (improved from <20% with surgery alone) **Mnemonic:** **OSTEOSARCOMA = Osteoid + Sarcoma** - **O**steoid production (elevated ALP) - **S**unburst pattern (radiographic hallmark) - **T**eenagers and young adults (age 10–25) - **E**xcited periosteum (Codman triangle) - **O**sseous metaphysis (distal femur, proximal tibia) - **S**arcoma (malignant) - **A**lkaline phosphatase elevated - **R**apid growth (aggressive) - **C**ortical destruction - **O**steoid in tumor (histology) - **M**alignant (requires chemotherapy + surgery) - **A**ggressive periosteal reaction [cite:Robbins 10e Ch 26]