How to differentiate osteosarcoma from Ewing's (Ewings) sarcoma on radiograph?

Radiographic Differentiation of Osteosarcoma from Ewing Sarcoma

The key radiographic features that distinguish these tumors are: osteosarcoma typically shows metaphyseal location with osteoblastic new bone formation (sunburst pattern, Codman's triangle) in patients aged 10-14 years, while Ewing sarcoma characteristically presents with diaphyseal location, mottled bone appearance, and classic "onion skin" periosteal reaction in adolescents and young adults. 1, 2

Primary Location Within the Bone

Osteosarcoma:

• Predominantly affects the metaphyseal region of long bones, particularly around the knee 2
• Most commonly involves the distal femur, proximal tibia, and proximal humerus 2

Ewing Sarcoma:

• Characteristically involves the diaphysis (shaft) when arising in long bones 1
• Can also present in metadiaphyseal locations 3
• Most common sites are pelvic bones, femur, and chest wall bones 1

Bone Matrix and Appearance

Osteosarcoma:

• Shows osteoblastic new bone formation with mixed areas of lysis and sclerosis 4, 2
• Produces tumor osteoid matrix visible on radiographs 5
• May present as predominantly sclerotic, lytic, or mixed-density lesion 6, 2
• Demonstrates permeative pattern of destruction extending into the shaft 5

Ewing Sarcoma:

• Bone appears mottled on imaging 1
• Lesions may be lytic, mixed lytic-sclerotic, or rarely predominantly sclerotic 3
• Shows permeative destruction pattern 3

Periosteal Reaction Patterns

Osteosarcoma:

• Classic "sunburst" appearance from radiating spicules of periosteal new bone 2
• Codman's triangle formation where periosteum is lifted from cortex 2
• New bone formation extending into soft tissues 2

Ewing Sarcoma:

• Classic "onion skin" periosteal reaction (lamellated pattern) 1
• This multilayered appearance is the hallmark radiographic feature 1

Soft Tissue Component

Osteosarcoma:

• Soft tissue mass with visible new bone formation within it 2
• Cortical destruction with extension beyond bone 4

Ewing Sarcoma:

• Often presents with large soft tissue component 7
• Soft tissue mass may be prominent relative to bone involvement 3

Age Distribution

Osteosarcoma:

• Peak incidence at 10-14 years (during adolescent growth spurt) 7, 2
• Predominantly affects patients younger than 20 years 2

Ewing Sarcoma:

• Median age 14-15 years 1, 7
• Occurs in adolescents and young adults (AYAs) 1

Critical Diagnostic Pitfalls

Small-cell osteosarcoma can mimic Ewing sarcoma histologically but will show osteoblastic features radiographically with permeative destruction extending into the shaft—the radiographic features become crucial when biopsy shows only round cells 5

Early osteosarcoma may present with subtle features including minimal sclerosis, intramedullary confinement, or benign-appearing periosteal reaction, making diagnosis challenging 4

Ewing sarcoma radiographic features are protean and not always distinctive—the diagnosis must be confirmed with biopsy showing small round cells that are CD99-positive and molecular testing demonstrating EWSR1 gene rearrangements 1

Advanced Imaging Considerations

While plain radiographs provide the initial differentiation, MRI is invaluable for delineating extent of disease not readily apparent on plain films for both tumor types 3, 2. However, the fundamental radiographic patterns described above remain the primary means of initial differentiation before biopsy 2.

Any patient with radiographic findings suggesting either tumor must be referred immediately to a specialized bone sarcoma center before biopsy to prevent tissue plane contamination and optimize outcomes 7, 8.