A 14-year-old boy from rural India presents with a 3-month history of progressive pain and swelling over the mid-shaft of his left femur. He reports fever on and off for the past 6 weeks and has lost 4 kg of body weight. On examination, the swelling is warm, tender, and fixed to underlying bone. Laboratory investigations show hemoglobin 10.2 g/dL, WBC 14,500/μL with left shift, and ESR 68 mm/hr. Plain radiographs reveal a mixed lytic and sclerotic lesion in the diaphysis with an onion-skin periosteal reaction and soft tissue mass. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Key Features **Key Point:** Ewing sarcoma is the second most common primary malignant bone tumor in children and adolescents, typically presenting in the 10–20 year age group with pain, swelling, and systemic symptoms. ### Diagnostic Hallmarks in This Case | Feature | Ewing Sarcoma | Osteomyelitis | Osteosarcoma | |---------|---------------|---------------|---------------| | **Age** | 10–20 years (peak) | Any age | 10–25 years (around growth plate) | | **Location** | Diaphysis (shaft) | Metaphysis | Metaphysis (distal femur, proximal tibia) | | **Radiographic pattern** | Onion-skin (lamellated) periosteal reaction | Ill-defined lysis | Sunburst, mixed lytic-sclerotic | | **Systemic symptoms** | Fever, weight loss, malaise | Fever, chills | Rare systemic symptoms | | **ESR/Inflammatory markers** | Markedly elevated | Very elevated | Mildly elevated | | **Soft tissue involvement** | Common | Abscess/sinus tract | Common | **High-Yield:** The **onion-skin (lamellated) periosteal reaction** is pathognomonic for Ewing sarcoma. This represents repeated cycles of tumor growth and periosteal new bone formation. ### Pathophysiology 1. Arises from neural crest-derived mesenchymal cells in bone marrow 2. Characteristic translocation: **t(11;22)(q24;q12)** — EWSR1-FLI1 fusion gene (present in ~90% of cases) 3. Rapid growth with early soft tissue extension 4. Diaphyseal location is typical (unlike osteosarcoma which favors metaphysis) ### Imaging & Diagnosis **Clinical Pearl:** Ewing sarcoma often mimics infection clinically (fever, elevated inflammatory markers, pain). However, the **diaphyseal location** and **onion-skin periosteal reaction** distinguish it from osteomyelitis, which typically affects the metaphysis with a more localized lytic defect. - **Plain radiographs:** Mixed lytic-sclerotic lesion, onion-skin periosteal reaction, soft tissue mass - **MRI:** Gold standard for local staging; shows intramedullary and extraosseous extent - **CT chest:** Staging (pulmonary metastases in ~20% at presentation) - **Biopsy:** Small round blue cell tumor; immunohistochemistry positive for CD99 and FLI1 **Mnemonic: EWING** — **E**xtra-osseous soft tissue, **W**ide (diaphyseal), **I**nflammatory (fever, elevated ESR), **N**eural crest origin, **G**enetic translocation t(11;22) ### Why Diaphyseal Location Matters Ewing sarcoma arises in the **diaphysis (shaft)** because it originates from bone marrow mesenchymal cells. Osteosarcoma, by contrast, arises from osteoblasts at the metaphysis near the growth plate. ## Management Overview - **Neoadjuvant chemotherapy** (vincristine, doxorubicin, cyclophosphamide ± ifosfamide) - **Surgery** (wide local excision or amputation) - **Adjuvant chemotherapy** - **Radiation** (if inadequate surgical margins or unresectable) - **5-year survival:** ~70% with multimodal therapy (vs. <10% with surgery alone) [cite:Robbins 10e Ch 26]