## Investigation of Choice for Osteosarcoma Staging and Local Assessment ### Role of MRI in Osteosarcoma **Key Point:** MRI is the gold standard for assessing local tumor extent, soft tissue involvement, and intramedullary extension in osteosarcoma. It provides superior contrast resolution for surgical planning and detection of skip lesions. **High-Yield:** MRI must be performed BEFORE biopsy to avoid artifact and edema that can obscure true tumor margins. The T2-weighted sequences best delineate the tumor-muscle interface and neurovascular involvement. ### Staging Investigations in Osteosarcoma | Investigation | Purpose | Timing | |---|---|---| | **MRI (local)** | Local extent, soft tissue, skip lesions | First-line | | **CT chest** | Pulmonary metastases (present in 15–20% at diagnosis) | After MRI | | **Bone scan / PET-CT** | Skeletal metastases | Supplementary | | **Biopsy** | Histologic confirmation | After imaging | **Clinical Pearl:** In a teenager with a metaphyseal lesion near the knee and sunburst pattern on plain film, osteosarcoma is the leading diagnosis. MRI must be the next step to define surgical margins and rule out soft tissue extension before biopsy. ### Why MRI Is Superior for Local Staging 1. **Soft tissue planes:** Clearly shows tumor crossing cortex and involvement of surrounding muscles, tendons, and neurovascular structures. 2. **Intramedullary extension:** Detects proximal marrow involvement and skip lesions (present in ~5% of cases). 3. **Surgical planning:** Orthopedic surgeons use MRI to determine limb-salvage vs. amputation candidacy. 4. **No artifact from bone:** Unlike CT, MRI is not degraded by dense bone and cortical destruction. **Mnemonic:** **LEMON** — Local extent, Extension into soft tissue, Marrow involvement, Osseous skip lesions, Neurovascular assessment — all best shown by **MRI**. [cite:Robbins 10e Ch 26] 
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