A 14-year-old girl with a 2-month history of pain in the proximal tibia is found to have a lytic lesion on plain radiographs. Core needle biopsy confirms Ewing sarcoma with t(11;22) translocation. Which investigation is most appropriate for assessing the local extent of the tumor and planning surgical margins?

## Imaging for Local Staging and Surgical Planning in Ewing Sarcoma **Key Point:** Magnetic resonance imaging (MRI) is the gold standard for assessing local tumor extent, soft tissue involvement, and marrow infiltration — all critical for surgical margin planning and limb-salvage decisions in Ewing sarcoma. ### Why MRI is Superior for Local Assessment 1. **Soft tissue definition** — MRI shows tumor extension into surrounding muscles, fat planes, and neurovascular structures with exquisite detail. 2. **Marrow involvement** — Detects intramedullary extension along the bone shaft, which directly affects surgical margins. 3. **Compartmental spread** — Identifies whether the tumor is confined to one compartment or crosses fascial planes. 4. **Relationship to vital structures** — Shows proximity to neurovascular bundles, joints, and growth plates — essential for limb-salvage planning. 5. **No radiation** — Important in pediatric patients. ### Comparison of Staging Investigations | Investigation | Purpose | Strength | Limitation | | --- | --- | --- | --- | | **MRI (local)** | Local extent, soft tissue, marrow | Best soft tissue contrast; defines surgical margins | Cannot assess distant metastases | | **Chest CT** | Pulmonary metastases | Detects lung mets (present in ~20% at diagnosis) | Does not assess local tumor | | **Bone scan (Tc-99m)** | Skeletal metastases | Detects bone mets; whole-body survey | Poor spatial resolution; low sensitivity for small lesions | | **PET-CT** | Metastatic disease + local metabolic activity | High sensitivity for mets; prognostic (SUV) | Radiation; not first-line for local staging | | **CT (local)** | Cortical bone detail, surgical planning | Good for cortical involvement | Poor soft tissue contrast; less useful than MRI | **Clinical Pearl:** The standard staging protocol for Ewing sarcoma is: (1) **MRI of the primary site** for local extent and surgical planning, (2) **Chest CT** to detect pulmonary metastases, and (3) **Bone scan or PET-CT** to detect skeletal metastases. MRI is performed FIRST and is essential before any surgical consultation. **High-Yield:** Intramedullary skip lesions (tumor nodules separated from the main mass by normal marrow) occur in ~5–10% of Ewing sarcomas and can only be reliably detected by MRI. Their presence affects surgical planning and prognosis. **Mnemonic: MRI for LOCAL, CT for METS** - **MRI** = Local extent, soft tissue, marrow, surgical margins - **CT chest** = Metastases (lungs) - **Bone scan or PET** = Metastases (skeleton)