Epidemiology

Incidence: peak at 10–20 years
- Second most common primary bone tumor in children and adolescents, after osteosarcoma

Etiology

Cell of Origin: Neural crest / neuroectodermal origin (part of the Ewing family of tumors / PNET spectrum).
- Therefore, it is not associated with new bone formation (ie, radiodense areas on X-ray) and is typically located in the diaphysis (vs the metaphysis) c

Location: Diaphysis and metadiaphysis of long bones (femur > tibia > humerus) and flat/axial bones (pelvis, ribs, spine).
Local Symptoms:
- Localized, deep, progressive bone pain (worse at night or with weight-bearing).
- Tender, palpable, warm soft-tissue mass overlying the bone.
Systemic Symptoms:
- Fever, malaise, weight loss, night sweats (mimics osteomyelitis or systemic infection). c

Initial Step: Plain Radiograph (X-ray) of the affected extremity.
- Findings: Permeative, osteolytic lesion with poorly defined (“moth-eaten”) margins; soft-tissue mass. c
- Periosteal reaction: Characteristic “onion-skin” (lamellated, multilayered subperiosteal bone formation); may also show Codman triangle.
Biopsy
- Sheets of uniform, small, round, blue cells. c
- Scant cytoplasm, prominent nuclei.
- Homer-Wright rosettes (circular grouping of tumor cells around a central fibrillary space) may be present, indicating neural differentiation.
Chromosomal translocation t(11;22)(q24;q12) which leads to expression of fusion protein EWS-FLI1

Osteomyelitis:
- Diff by: Positive blood/bone aspirate cultures, focal cortical destruction without large soft tissue mass, rapid clinical response to targeted Abx.
Osteosarcoma:
- Diff by: Metaphysis location (esp. around the knee: distal femur / proximal tibia), “sunburst” periosteal pattern, pleomorphic spindle cells forming osteoid matrix, rare systemic B-symptoms.
Osteoid Osteoma:
- Diff by: Pain strictly worse at night and rapidly relieved by NSAIDs; XR shows small radiolucent nidus (<1.5–2 cm) surrounded by dense sclerotic bone; no soft tissue mass.
Langerhans Cell Histiocytosis (Eosinophilic Granuloma):
- Diff by: Well-demarcated “punched-out” lytic bone lesions without significant periosteal reaction; biopsy shows CD1a/S100 (+) and Birbeck granules (tennis racket appearance on EM).
Neuroblastoma:
- Diff by: Typically presents in younger children (<2–5 yo); primary adrenal/retroperitoneal mass; ↑ urine catecholamines (VMA, HVA).

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