A 14-year-old girl with a 2-month history of pain in the right pelvis and constitutional symptoms (fever, weight loss) undergoes plain radiographs that show a mixed lytic-sclerotic lesion in the ilium. The clinical and radiological picture is highly suspicious for Ewing sarcoma. Which investigation is most appropriate as the next step to assess for metastatic disease?

Explanation

## Staging and Metastatic Workup in Ewing Sarcoma ### Role of PET-CT in Staging **Key Point:** PET-CT with 18F-FDG is the most sensitive investigation for detecting skeletal and soft tissue metastases in Ewing sarcoma and is now the standard of care for staging in most high-volume centers. - Detects both osteolytic and osteosclerotic metastases with high sensitivity (~90%) - Identifies pulmonary and extrapulmonary soft tissue metastases - Provides metabolic information (SUV) that correlates with tumor burden and prognosis - Replaces or supplements bone scan and chest radiography in modern practice - Essential for risk stratification: presence of metastases at diagnosis defines Stage IV disease (poor prognosis) ### Staging Workup Algorithm for Ewing Sarcoma ```mermaid flowchart TD A[Histologically confirmed<br/>Ewing Sarcoma]:::outcome --> B[Local staging:<br/>MRI of primary site]:::action B --> C[Assess for metastases]:::decision C --> D[PET-CT with 18F-FDG<br/>whole body]:::action D --> E{Metastases<br/>detected?}:::decision E -->|Yes| F[Stage IV<br/>Poor prognosis]:::urgent E -->|No| G[Chest CT<br/>High-resolution]:::action G --> H{Pulmonary<br/>lesions?}:::decision H -->|Yes| I[Stage IV]:::urgent H -->|No| J[Stage I-III<br/>Localized disease]:::outcome J --> K[Neoadjuvant chemotherapy<br/>+ surgery/RT]:::action ``` ### Comparison of Staging Investigations | Investigation | Sensitivity for Metastases | Specificity | Role | Limitation | |---|---|---|---|---| | **PET-CT (18F-FDG)** | ~90% (skeletal + soft tissue) | High | **Gold standard** for staging | Cost, availability in some centers | | **Bone scan (Tc-99m)** | ~70% (skeletal only) | Moderate | Supplementary if PET unavailable | Misses soft tissue; poor specificity | | **Chest CT** | ~85% (pulmonary) | High | Essential for lung metastases | Does not assess skeletal metastases | | **Skeletal survey** | ~50% (large lesions only) | Low | Obsolete for staging | Insensitive; replaced by PET-CT | | **Serum markers alone** | N/A | N/A | Prognostic only | Cannot detect metastases | **High-Yield:** Approximately 15–25% of patients with Ewing sarcoma present with metastatic disease at diagnosis. The most common sites are: 1. Lungs (85% of metastases) 2. Other skeletal sites (10–15%) 3. Bone marrow and soft tissues (rare) **Clinical Pearl:** Chest CT is mandatory even if PET-CT is negative, because small pulmonary nodules (<5 mm) may be missed on PET-CT but are clinically significant for prognosis and treatment planning. **Warning:** ~~Skeletal survey~~ is no longer recommended for staging Ewing sarcoma; it has been replaced by PET-CT. Bone scan is now reserved for centers without PET-CT access.