A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. On examination, there is a firm, warm swelling around the knee with restricted 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 450 U/L. What is the most likely diagnosis?

## Diagnosis: Osteosarcoma ### Clinical Presentation The patient presents with classic features of osteosarcoma: - **Age:** 16 years old (peak incidence 10–25 years, with a second peak in elderly patients with Paget disease) - **Location:** Distal femur metaphysis (most common site; 50% of cases occur around the knee) - **Duration:** Subacute presentation (weeks to months) - **Exam findings:** Firm, warm swelling with restricted joint motion (periosteal inflammation) ### Radiological Findings **Key Point:** The sunburst (radiating spicules) and Codman triangle patterns are pathognomonic for osteosarcoma. - Mixed lytic and sclerotic lesion = aggressive, rapidly growing tumor - Cortical destruction = breaks through cortex - Sunburst periosteal reaction = tumor lifting periosteum and laying down bone perpendicular to cortex - Metaphyseal location = hallmark (not diaphyseal like Ewing sarcoma) ### Laboratory Findings **High-Yield:** Elevated alkaline phosphatase (ALP) reflects osteoid production by malignant osteoblasts. ALP and LDH are prognostic markers and used for monitoring response to chemotherapy. ### Pathology Osteosarcoma is a malignant tumor of mesenchymal origin that produces osteoid/bone. It is the most common primary malignant bone tumor in children and adolescents. ### Management Overview 1. **Staging:** MRI (local extent), CT chest (pulmonary metastases—present in 20% at diagnosis) 2. **Treatment:** Neoadjuvant chemotherapy (doxorubicin, cisplatin, methotrexate) followed by wide surgical resection ± adjuvant chemotherapy 3. **Prognosis:** 5-year survival ~70% with modern multimodal therapy; worse if metastatic at presentation **Clinical Pearl:** Osteosarcoma is highly radiosensitive on imaging but radiotherapy is NOT first-line treatment—chemotherapy and surgery are the cornerstone. Pulmonary metastases should always be excluded before surgery. **Mnemonic — OSTEOSARCOMA SITES:** **FEMUR TIBIA** (distal femur and proximal tibia account for >50% of cases; also humerus, pelvis, jaw)