A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the mid-shaft of the right femur. On examination, there is a firm, warm swelling with restricted hip movements. Plain radiographs show a permeative lesion in the diaphysis with an onion-skin periosteal reaction and cortical thickening. Which is the investigation of choice for confirming the diagnosis?

Explanation

## Investigation of Choice for Ewing Sarcoma Diagnosis **Key Point:** Bone biopsy with histopathological examination is the gold standard and investigation of choice for definitive diagnosis of Ewing sarcoma. Only histology can confirm the characteristic small round cell morphology and establish the diagnosis with certainty. ### Why Biopsy is Essential 1. **Definitive diagnosis** — Ewing sarcoma is a small round cell tumor; histology shows uniform cells with scant cytoplasm, round nuclei, and high mitotic activity. 2. **Immunohistochemistry confirmation** — CD99 (MIC2) positivity is a hallmark; also FLI1 and ERG positivity support diagnosis. 3. **Molecular testing** — FISH or RT-PCR for t(11;22) translocation (EWS-FLI1 fusion) is highly specific and can be performed on biopsy material. 4. **Excludes mimics** — Biopsy rules out osteomyelitis, lymphoma, and other small round cell tumors (rhabdomyosarcoma, neuroblastoma metastases). ### Role of Other Investigations | Investigation | Role | Limitations | | --- | --- | --- | | **Plain radiographs** | Initial imaging; shows permeative lesion, onion-skin periosteum | Non-specific; cannot diagnose | | **MRI** | Best for soft tissue extension, marrow involvement, surgical planning | Excellent for staging but NOT diagnostic | | **CT** | Chest CT for metastases; helps with treatment planning | Does not diagnose; used for staging | | **PET-CT** | Detects distant metastases; prognostic value | Not diagnostic; used for staging | **Clinical Pearl:** The classic radiographic triad of Ewing sarcoma is (1) diaphyseal location, (2) permeative lesion with ill-defined margins, and (3) onion-skin (lamellated) periosteal reaction. However, imaging alone cannot diagnose — biopsy is mandatory. **High-Yield:** t(11;22) EWS-FLI1 fusion is present in ~85–90% of Ewing sarcomas and is the most specific molecular marker. Biopsy material allows FISH or RT-PCR confirmation, which is critical for diagnosis and prognosis.