A 14-year-old girl from rural Maharashtra presents with a 4-month history of pain and swelling in the right tibia. She reports night sweats and has lost 3 kg in weight. On examination, there is a firm, tender swelling over the proximal tibia with warmth and restricted knee movement. Laboratory findings: hemoglobin 10.2 g/dL, ESR 92 mm/hr, alkaline phosphatase 180 IU/L. Plain radiographs show a mixed lytic and sclerotic lesion in the diaphysis with periosteal new bone formation. A biopsy is performed. Which of the following histological findings would be most consistent with the suspected diagnosis?

Explanation

## Histopathology of Ewing Sarcoma ### Microscopic Features **Key Point:** Ewing sarcoma is a small round cell malignancy characterized by uniform, densely packed cells with a high nuclear-to-cytoplasmic ratio and scant cytoplasm. The cells are typically arranged in sheets or nests with minimal intervening stroma. ### Diagnostic Histological Criteria | Feature | Ewing Sarcoma | Osteosarcoma | Giant Cell Tumor | |---------|---------------|--------------|------------------| | **Cell type** | Small round cells | Spindle cells, pleomorphic | Multinucleate giant cells | | **Nuclear-to-cytoplasmic ratio** | High (>1:1) | Variable | Low | | **Cytoplasm** | Scant, clear or eosinophilic | Abundant | Abundant | | **Stroma** | Minimal | Abundant osteoid/bone | Hemosiderin-laden macrophages | | **Necrosis** | Extensive | Variable | Minimal | | **Immunohistochemistry** | CD99+, FLI1+ | Alkaline phosphatase+, osteocalcin+ | CD68+, RANKL+ | ### Immunohistochemical Profile **High-Yield:** Ewing sarcoma is **CD99-positive** (membrane staining in >95% of cases) and **FLI1-positive** (nuclear staining). These markers are essential for diagnosis, especially in small round cell tumors where morphology alone may be ambiguous. ### Molecular Confirmation **Clinical Pearl:** The gold standard for diagnosis is demonstration of the **t(11;22)(q24;q12) translocation** resulting in the **EWSR1-FLI1 fusion gene**. This can be detected by: 1. Fluorescence in situ hybridization (FISH) 2. Reverse transcription polymerase chain reaction (RT-PCR) 3. Next-generation sequencing (NGS) About 85–90% of Ewing sarcomas carry this translocation; the remaining 10–15% have variant translocations (e.g., EWSR1-ERG, EWSR1-ETV1). ### Why This Is NOT the Other Options ```mermaid flowchart TD A[Small Round Cell Tumor of Bone]:::outcome --> B{Histology + Immunophenotype}:::decision B -->|Spindle cells + osteoid| C[Osteosarcoma]:::outcome B -->|Small round cells + CD99+| D[Ewing Sarcoma]:::outcome B -->|Multinucleate giant cells| E[Giant Cell Tumor]:::outcome B -->|Epithelioid + caseating granulomas| F[Tuberculosis/Granulomatous disease]:::outcome ``` **Mnemonic:** **EWSR1-CD99** = Ewing Sarcoma Recognizes CD99 (the immunohistochemical hallmark). **Warning:** Do not confuse Ewing sarcoma (small round cells, CD99+) with: - **Osteosarcoma** (spindle cells, osteoid production, CD99−) - **Rhabdomyosarcoma** (small round cells but desmin+ and myogenin+, not CD99+) - **Lymphoma** (small round cells but CD20+/CD45+, not CD99+)