A 16-year-old boy presents with a 3-month history of progressive pain and swelling in the left femur. Clinical examination reveals a firm, tender mass with overlying skin erythema and warmth. Plain radiographs show a lytic lesion in the diaphysis with a 'onion-skin' appearance and periosteal reaction. Which investigation is most appropriate to confirm the diagnosis?

## Diagnostic Approach to Ewing Sarcoma ### Role of Core Needle Biopsy **Key Point:** Core needle biopsy under imaging guidance (CT or ultrasound) is the gold standard for tissue diagnosis of Ewing sarcoma and must precede any definitive treatment. - Provides histological confirmation with minimal morbidity - Allows immunohistochemical staining for CD99 and EWSR1-FLI1 fusion protein - Enables molecular testing for t(11;22) translocation (present in ~90% of cases) - Should be performed by the treating orthopedic oncologist or in consultation with them to avoid contamination of tissue planes ### Why Other Investigations Are Insufficient | Investigation | Role | Limitation | |---|---|---| | **Serum markers (ALP, LDH)** | Prognostic value, monitoring response | Non-specific; elevated in many bone tumors; cannot diagnose | | **MRI** | Staging, surgical planning, soft tissue extent | Excellent for local staging but cannot diagnose; must follow biopsy | | **Bone scan** | Detection of skeletal metastases | Insensitive for primary lesion characterization; used for staging | **High-Yield:** The classic imaging triad of Ewing sarcoma is: 1. Diaphyseal or metadiaphyseal location (femur > pelvis > tibia) 2. Onion-skin (lamellated) periosteal reaction 3. Sunburst pattern with soft tissue mass However, imaging alone cannot differentiate Ewing sarcoma from osteomyelitis, lymphoma, or other round-cell tumors — **tissue diagnosis is mandatory**. **Clinical Pearl:** Biopsy should be performed before MRI staging because the biopsy tract must be excised en bloc with the tumor; therefore, the biopsy site must be placed in a location that does not compromise the surgical approach. ### Diagnostic Confirmation Pathway ```mermaid flowchart TD A[Suspected Ewing Sarcoma<br/>on plain radiographs]:::outcome --> B[MRI for local staging<br/>and soft tissue extent]:::action B --> C{Imaging consistent<br/>with diagnosis?}:::decision C -->|Yes| D[Core needle biopsy<br/>under CT/US guidance]:::action D --> E[Histology: Small round cells<br/>CD99+, EWSR1-FLI1+]:::outcome E --> F[Molecular confirmation<br/>t(11;22) translocation]:::action F --> G[Staging: Chest CT,<br/>bone scan, PET-CT]:::action G --> H[Treatment planning<br/>Neoadjuvant chemotherapy]:::action ```