A 16-year-old boy from Delhi presents with a 3-month history of progressive left knee pain and swelling. On examination, he has a firm, non-tender mass over the anterolateral aspect of the distal femur. Plain radiographs show a mixed lytic and sclerotic lesion at the metaphyseal region of the distal femur with a "sunburst" periosteal reaction and cortical breakthrough. MRI reveals a large soft tissue mass with heterogeneous signal intensity and areas of necrosis. What is the most likely diagnosis?

Explanation

## Clinical Presentation and Imaging Findings **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults, with peak incidence at 10–25 years of age. ### Characteristic Imaging Features | Feature | Osteosarcoma | Ewing Sarcoma | Giant Cell Tumor | Chondroblastoma | |---------|--------------|---------------|------------------|----------------| | **Age** | 10–25 years | 10–30 years | 20–40 years | 10–20 years | | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Epiphysis | | **Periosteal reaction** | Sunburst, Codman angle | Onion-skin (lamellated) | None typical | None | | **Matrix** | Osteoid/bone production | Permeative, homogeneous | Lytic | Chondroid calcification | | **Soft tissue mass** | Large, heterogeneous | Large, homogeneous | Small or absent | Small | **High-Yield:** The **sunburst periosteal reaction** (radiating spicules of new bone) is pathognomonic for osteosarcoma and reflects aggressive tumor-induced bone formation. ### Pathophysiology Osteosarcoma arises from malignant mesenchymal cells that produce osteoid and bone matrix. The metaphyseal location reflects the site of rapid bone growth in adolescents. The mixed lytic–sclerotic pattern results from simultaneous bone destruction and abnormal osteoid production. **Clinical Pearl:** Osteosarcoma most commonly arises in the distal femur (40%), followed by proximal tibia (16%) and proximal humerus (10%) — all areas of rapid skeletal growth. **Mnemonic: METS** — Metaphysis, Epiphysis (rare), Teenagers, Sunburst (periosteal reaction) ### MRI Findings MRI is superior for assessing soft tissue involvement and marrow edema. Heterogeneous signal with areas of necrosis, hemorrhage, and cystic change is typical. The wide zone of marrow edema extending beyond the visible tumor is characteristic. **Warning:** Do not confuse osteosarcoma with benign lesions like osteochondroma or fibroma — malignant tumors show cortical breakthrough, large soft tissue mass, and aggressive periosteal reaction.