Where are atypical round cell neoplasms, such as Ewing's sarcoma, typically found in pediatric patients?

Anatomic Location of Atypical Round Cell Neoplasms

Atypical round cell neoplasms, particularly Ewing's sarcoma, are predominantly found in the metaphysis of long bones around the knee (distal femur and proximal tibia), with 50% occurring in extremities and 20% in the pelvis. 1

Primary Skeletal Sites

The most common skeletal locations include:

• Long bones of extremities - particularly the metaphyseal regions, with the area around the knee being the most frequent site 1, 2
• Pelvis - accounts for approximately 20% of cases and carries a worse prognosis 1
• Chest wall and ribs - common sites in the axial skeleton 3, 4
• Paravertebral regions - frequently involved in axial presentations 4

The median age at diagnosis is 15 years, with 90% of patients being under 20 years old. 1

Less Common Skeletal Sites

Atypical skeletal locations where these tumors can arise include:

• Metatarsal bones - rare presentation in the foot 4
• Scapula - uncommon shoulder girdle involvement 4
• Craniofacial bones and paranasal sinuses - particularly in adult presentations 1, 4

Extraskeletal Presentations

While Ewing's sarcoma classically arises in bone, extraskeletal presentations occur in deep soft tissues:

• Soft tissue masses - can occur without bone involvement, particularly in paravertebral and chest wall regions 4, 5
• Visceral locations - extremely rare cases have been reported in the gastric region 6
• Intracranial soft tissue - very atypical presentation 4
• Breast tissue - exceptionally rare location 4

Metastatic Sites at Diagnosis

Approximately 20% of patients present with detectable metastases, most commonly in: 1

• Lungs - the most common metastatic site, detected by chest CT 1, 3
• Bone and bone marrow - detected by bone scintigraphy and bone marrow aspirates 1, 3

Lung metastases alone carry a better prognosis (30% 5-year survival) compared to skeletal metastases (10% 5-year survival). 1

Clinical Pitfalls

A critical caveat is that Ewing's sarcoma often presents with a large soft tissue component extending beyond the bone of origin, which can mislead clinicians into thinking the primary site is in soft tissue when it actually originated in bone. 1 This emphasizes the importance of complete MRI imaging of the entire affected bone before biopsy to accurately determine the true extent and origin of disease. 3

The tumor spreads hematogenously rather than lymphatically, making pulmonary metastases the primary concern for staging. 2