## Ewing Sarcoma: Molecular and Diagnostic Foundations ### Molecular Biology and Cytogenetics **High-Yield:** Ewing sarcoma is defined by a **recurrent chromosomal translocation**: - **t(11;22)(q24;q12)** in ~85–90% of cases → **EWS-FLI1 fusion gene** - **t(21;22)** in ~5–10% of cases → **EWS-ERG fusion gene** - Rare variant translocations: t(7;22), t(17;22) These translocations create an **oncogenic fusion protein** that drives transcriptional dysregulation and malignant transformation. **Key Point:** The **EWS-FLI1 translocation is pathognomonic** for Ewing sarcoma and is used for diagnosis and prognosis (type 1 vs. type 2 fusion patterns correlate with outcome). ### Immunohistochemistry **Clinical Pearl:** CD99 (MIC2 antigen) is: - **Sensitive** (95–100% of Ewing sarcomas express it) - **Specific** (positive in Ewing sarcoma and synovial sarcoma; negative in osteosarcoma and chondrosarcoma) - A crucial diagnostic marker when combined with morphology and molecular testing Other positive markers: FLI1, NKX2.2, ERG (in EWS-ERG cases). ### Imaging and Staging **High-Yield:** FDG-PET/CT is essential for: 1. **Detecting skeletal metastases** (more sensitive than bone scan) 2. **Detecting pulmonary metastases** (combined with chest CT) 3. **Assessing response to neoadjuvant chemotherapy** (SUV reduction predicts prognosis) 4. **Surveillance for recurrence** ### Cell of Origin: The Critical Misconception **Warning:** Ewing sarcoma does **NOT** arise from differentiated osteoblasts or osteocytes (mature bone-forming cells). Instead, it arises from: - **Primitive mesenchymal cells** or **neural crest-derived cells** in the bone marrow or periosteum - The exact cell of origin remains debated, but it is **NOT** a tumor of differentiated osteoblasts This is a **common exam trap**: - **Osteosarcoma** → arises from osteoblasts (malignant osteoid production) - **Ewing sarcoma** → arises from primitive/undifferentiated mesenchymal cells ### Comparison: Bone Tumor Origins | Tumor | Cell of Origin | Key Feature | |-------|---|---| | **Osteosarcoma** | Differentiated osteoblast | Produces malignant osteoid | | **Ewing sarcoma** | Primitive mesenchymal cell | EWS-FLI1 translocation | | **Chondrosarcoma** | Differentiated chondrocyte | Produces malignant cartilage | | **Giant cell tumor** | Stromal cell (osteoclast precursor) | Hemosiderin, giant cells | ### Why Option 4 is Wrong Ewing sarcoma arises from **primitive, undifferentiated mesenchymal cells**, not from mature osteoblasts or osteocytes. This is a fundamental distinction from osteosarcoma and is a high-yield discriminator. [cite:Robbins 10e Ch 26]
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