Ewing sarcoma

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Basics - Blue Cell Tumour

  • Small round blue cell tumour (SRBCT) of bone and soft tissue.
  • Origin: Believed to be of neural crest cell / neuroectodermal origin.
  • Microscopy:
    • Sheets of uniform, small cells with round nuclei and scant, clear cytoplasm.
    • Homer-Wright pseudorosettes (cells around a fibrillary meshwork) may be seen.
    • Cytoplasm is PAS-positive due to glycogen, which is diastase-sensitive.
  • IHC Marker: CD99 (MIC2) is the most sensitive marker.

Genetic Hallmark: Translocation t(11;22)(q24;q12), resulting in the EWS-FLI1 fusion gene.

Ewing sarcoma histology: small round blue cells

Pathogenesis - Translocation Tango

  • Primary Driver: Specific chromosomal translocations.
  • Classic Translocation: t(11;22)(q24;q12) is the hallmark, found in ~85% of cases.
    • Fuses the EWSR1 gene on chromosome 22 with the FLI1 gene on chromosome 11.
    • Creates the EWSR1-FLI1 fusion oncogene.
  • Fusion Protein: The resulting chimeric protein acts as an aberrant transcription factor, driving oncogenesis.
  • 📌 Mnemonic: Think of basketballer Patrick Ewing (jersey #33): 11 + 22 = 33.

⭐ The EWSR1-FLI1 fusion is the most common type. Other partners for EWSR1 exist (e.g., ERG), but are less frequent.

image chromosomal translocation diagram)

Diagnosis - Onion Skin & Codman

  • X-ray Findings: The initial imaging modality.
    • Location: Most commonly affects the diaphysis of long bones (femur, tibia, humerus) and the pelvis.
    • Appearance: Poorly-defined, lytic lesion with a permeative or "moth-eaten" pattern of bone destruction.
  • Periosteal Reaction: Key diagnostic signs.
    • "Onion skin" appearance: A lamellated, layered reaction.
    • Codman triangle: A triangular area of new subperiosteal bone.
    • A "sunburst" or "hair-on-end" pattern can also occur.

Exam Favourite: The classic description is a destructive lesion in the diaphysis of a long bone with an "onion skin" periosteal reaction.

Multilayer periosteal reaction (onion skin) on X-ray

Management - Chemo Sandwich

  • Overall Strategy: A multi-modal approach combining chemotherapy and local control measures, structured like a sandwich.
  • Neoadjuvant (Induction) Chemotherapy:

    • Pre-operative treatment to shrink the primary tumor.
    • Regimen: VDC/IE (Vincristine, Doxorubicin, Cyclophosphamide alternating with Ifosfamide, Etoposide).
  • Local Control:

    • Surgery: Wide local excision / limb salvage is preferred.
    • Radiotherapy: For unresectable tumors or positive surgical margins.
  • Adjuvant Chemotherapy:

    • Post-local control to eradicate micrometastases.

⭐ The VDC/IE regimen, administered over 9-12 months, is the standard of care and a major factor in improved survival rates for Ewing sarcoma.

High‑Yield Points - ⚡ Biggest Takeaways

  • Second most common primary malignant bone tumor in children, after osteosarcoma.
  • A small round blue cell tumor of neuroectodermal origin.
  • Characteristic translocation is t(11;22), resulting in the EWS-FLI1 fusion gene.
  • Most commonly involves the diaphysis of long bones (like the femur) and the pelvis.
  • X-ray findings include lytic lesions with an "onion skin" periosteal reaction.
  • Histology may show Homer-Wright rosettes.

Practice Questions: Ewing sarcoma

Test your understanding with these related questions

A 55-year-old male presents to his primary care physician complaining of right hip pain for the past eight months. He also reports progressive loss of hearing over the same time period. Radiographic imaging reveals multiple areas of expanded bony cortices and coarsened trabeculae in his right hip and skull. Laboratory analysis reveals an isolated elevation in alkaline phosphatase with normal levels of serum calcium and phosphate. Which of the following histologic findings is most likely to be seen if one of the lesions were biopsied?

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Flashcards: Ewing sarcoma

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The rash of Roseola is said to have a red, "_____" appearance

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The rash of Roseola is said to have a red, "_____" appearance

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