# Ewing Sarcoma

> Ewing sarcoma for NEET-PG Orthopaedics covers the t(11;22) translocation, onion-skin periosteal reaction, systemic features and its mimicry of osteomyelitis.

- Canonical URL: https://prepelephant.com/topics/neet-pg/orthopaedics/ewing-sarcoma
- Exam / course: NEET-PG · Subject: Orthopaedics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Apley's System of Orthopaedics and Fractures; Robbins and Cotran Pathologic Basis of Disease
- How to cite: "Ewing Sarcoma", PrepElephant, https://prepelephant.com/topics/neet-pg/orthopaedics/ewing-sarcoma

## Direct answer

Ewing sarcoma is a small round blue cell tumour of bone and soft tissue, mostly in children and young adults, defined by a translocation most often t(11;22) that fuses EWSR1 with FLI1. It typically arises in the diaphysis of a long bone or the pelvis, with lytic destruction and onion-skin periosteal reaction on radiograph. Fever, raised ESR and anaemia can mimic osteomyelitis, so biopsy is needed first.

## What you must remember

- **Age:** mostly children and adolescents, with a peak in the second decade.
- **Genetics:** t(11;22)(q24;q12) with the EWSR1-FLI1 fusion in most cases, confirmed by molecular testing on tissue.
- **Site:** the diaphysis of long bones, the pelvis, ribs and chest wall, rather than the epiphysis.
- **Radiograph:** moth-eaten lytic destruction with onion-skin periosteal reaction and a large soft-tissue mass.
- **Systemic features:** fever, raised ESR, anaemia and sometimes leucocytosis, which can mislead towards infection.
- **Markers:** CD99 positivity on immunohistochemistry, with PAS-positive glycogen on staining.
- **Treatment:** multi-agent chemotherapy in alternating blocks, with local control by surgery or radiotherapy.
- **Prognosis:** metastases at diagnosis, especially to lung or bone, carry the worst outlook.

## Ewing sarcoma and osteomyelitis side by side

Osteomyelitis is the mimic examiners most often pair with Ewing sarcoma, because both destroy bone, provoke a periosteal reaction and cause fever. The comparison below shows where they split and which feature should prompt a biopsy rather than another course of antibiotics. Once the diagnosis is confirmed, chest imaging and a bone scan complete the staging.

| Feature | Ewing sarcoma | Osteomyelitis |
|---|---|---|
| Age | Child or adolescent | Any age, commonly a child |
| Site | Diaphysis, pelvis or chest wall | Metaphysis, spreading to the diaphysis |
| Periosteal reaction | Onion-skin, with moth-eaten lysis | New bone with sequestrum and involucrum |
| Blood picture | Anaemia, raised ESR, fever | Raised CRP and neutrophilia |
| Confirmation | Biopsy with EWSR1 fusion testing | Culture from aspirate or biopsy |

Each feature alone is non-specific, but a lytic diaphyseal lesion that fails to respond to antibiotics is the signal to biopsy. Repeated antibiotic courses without a diagnosis delay the one test that settles the question. Imaging alone seldom separates the two with certainty, so tissue confirmation is always required.

## Where the exam tries to catch you

The traps lie in the look-alikes and in assuming that normal systemic signs exclude the tumour.

- **Trap:** labelling every lytic diaphyseal lesion in a child as osteomyelitis — **Correct:** Ewing sarcoma is the key mimic, and a lesion that does not respond to antibiotics needs biopsy.
- **Trap:** choosing surgery alone because the local tumour looks limited — **Correct:** Ewing sarcoma is chemosensitive and needs systemic chemotherapy even when the local disease appears small.
- **Trap:** assuming a normal temperature and ESR exclude Ewing sarcoma — **Correct:** systemic features are common but not invariable, so the diagnosis rests on the lesion and the biopsy.

## Frequently asked questions

### Which translocation is characteristic of Ewing sarcoma?

Ewing sarcoma is characteristically associated with the t(11;22)(q24;q12) translocation, which fuses EWSR1 with FLI1. The fusion is found in most cases and is confirmed by molecular testing on tissue.

### How does Ewing sarcoma look different from osteomyelitis on X-ray?

Ewing sarcoma shows permeative, moth-eaten lysis with onion-skin periosteal reaction, typically in the diaphysis. Osteomyelitis shows new periosteal bone, sequestrum and involucrum, usually starting in the metaphysis.

### How is Ewing sarcoma treated?

Ewing sarcoma is treated with multi-agent chemotherapy given before and after local control, combined with surgery or radiotherapy to the primary site. The tumour is sensitive to both chemotherapy and radiotherapy.

### What worsens the prognosis of Ewing sarcoma?

Metastases at diagnosis, especially to the lungs or bone, are the most important adverse prognostic factor in Ewing sarcoma. A poor response to induction chemotherapy also worsens the outlook.