A 16-year-old boy presents with progressive pain and swelling around the distal femur for 3 months. Physical examination reveals a firm, warm mass over the distal femur with restricted knee movement. Plain radiographs show a mixed lytic-sclerotic lesion with cortical destruction and periosteal new bone formation (sunburst pattern). Which is the investigation of choice to confirm the diagnosis and assess local extent?

Explanation

## Investigation of Choice for Osteosarcoma Staging and Confirmation **Key Point:** MRI is the gold standard for local staging and surgical planning in osteosarcoma because it defines soft tissue extension, intramedullary spread, and neurovascular involvement with superior soft tissue contrast resolution. ### Why MRI is Optimal **High-Yield:** MRI provides: - Best delineation of intramedullary tumor extension - Assessment of soft tissue mass and skip lesions - Relationship to neurovascular structures - Guidance for biopsy site selection and surgical margins - T1, T2, and STIR sequences detect bone marrow edema and tumor infiltration ### Role of Other Investigations | Investigation | Purpose | Timing | |---|---|---| | **Plain X-ray** | Initial screening, pattern recognition (sunburst, Codman triangle) | First-line | | **MRI** | Local staging, surgical planning, soft tissue extent | **Before biopsy/treatment** | | **CT chest** | Pulmonary metastases (present in 20% at diagnosis) | Staging after diagnosis confirmed | | **Bone scan** | Skeletal metastases, multifocal disease | Staging workup | | **Biopsy** | Histological confirmation | After imaging complete | **Clinical Pearl:** Although CT chest is essential for staging (osteosarcoma has high propensity for pulmonary metastases), it is NOT the investigation of choice for confirming local disease and surgical planning — that role belongs to MRI. **Warning:** Serum alkaline phosphatase and LDH are prognostic markers, not diagnostic investigations. They may be elevated but do not confirm diagnosis. ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion: bone pain + swelling]:::outcome A --> B[Plain X-ray]:::action B --> C{Typical pattern?}:::decision C -->|Sunburst/Codman| D[MRI for local staging]:::action D --> E[Assess soft tissue, marrow, neurovascular]:::action E --> F[Biopsy for histology]:::action F --> G[Diagnosis confirmed]:::outcome G --> H[CT chest for metastases]:::action H --> I[Treatment planning]:::action ``` [cite:Robbins 10e Ch 26]