A 16-year-old boy presents to the orthopedic clinic with a 3-month history of progressive pain and swelling around the left knee. The pain is worse at night and not relieved by NSAIDs. On examination, there is a firm, tender swelling over the distal femur with warmth and dilated veins over the mass. X-ray shows a mixed lytic and sclerotic lesion in the metaphyseal region of the distal femur with cortical destruction and a sunburst pattern of periosteal reaction. Alkaline phosphatase is elevated at 450 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 region 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; often during growth spurts | | Location | Distal femur metaphysis | Most common site (around knee in 50% of cases) | | Presentation | Progressive pain, worse at night | Typical; often mistaken for growing pains initially | | Physical findings | Firm swelling, warmth, dilated veins | Indicates aggressive, rapidly growing lesion | | X-ray pattern | Mixed lytic/sclerotic, sunburst, cortical destruction | Pathognomonic for osteosarcoma | | Alkaline phosphatase | Elevated (450 IU/L) | Reflects high osteoblastic activity | ### Pathological Hallmarks **High-Yield:** Osteosarcoma is characterized by: 1. Malignant spindle cells producing osteoid/bone directly (not through endochondral ossification) 2. High mitotic rate and marked cellular pleomorphism 3. Necrosis and hemorrhage within the tumor ### Imaging Pattern — Sunburst Appearance ```mermaid flowchart TD A[Aggressive bone tumor]:::outcome --> B{Location & age?}:::decision B -->|Metaphysis, age 10-25| C[Osteosarcoma likely]:::action B -->|Diaphysis, age 10-20| D[Ewing sarcoma likely]:::action C --> E[Mixed lytic/sclerotic pattern]:::outcome E --> F[Sunburst periosteal reaction]:::outcome F --> G[Cortical destruction]:::outcome G --> H[Elevated ALP]:::outcome ``` **Clinical Pearl:** The sunburst pattern results from aggressive tumor growth breaking through the cortex and lifting the periosteum, which then lays down bone perpendicular to the cortical surface. ### Diagnostic Confirmation - **MRI:** Best for assessing soft tissue extension and marrow involvement - **CT chest:** Mandatory to detect pulmonary metastases (present in ~20% at diagnosis) - **Biopsy:** Confirms diagnosis; shows malignant osteoid production - **Alkaline phosphatase & LDH:** Elevated; useful for monitoring response to therapy ### Treatment & Prognosis **Key Point:** Modern multimodal therapy (neoadjuvant chemotherapy + surgical resection + adjuvant chemotherapy) has improved 5-year survival to ~70%. **Mnemonic: COSS** — Chemotherapy, Operative resection, Staging (chest imaging), Surveillance [cite:Robbins 10e Ch 26] [cite:Orthopedic Surgery Essentials Ch Bone Tumors]