Ewing Sarcoma

Pathology:  Following are some of the important pathological features:

Bones affected:

• It commonly occurs in long bones (in two-third cases), mainly in the femur and tibia.
• About one-third of cases occur in flat bones, usually in the pelvis and calcaneum.
• Occasionally, it is known to have a multicentric origin.

Site: The tumour may begin anywhere, but diaphysis of the long bone is the most common site.

Gross pathology:

• The tumour characteristically involves a large area, or even the entire medullary cavity.
• The tumour tissue is grey white.
• It is soft and may be thin, almost like pus.
• The bone may be expanded, and the periosteum elevated, with sub-periosteal new bone formation, often in layers.
• The tumour ruptures through the cortex early, and extends into the soft tissues.

Histopathology:

• The tumour comprises of sheets of quite uniform, small cells, resembling lymphocytes.
• Often, the tumour cells surround a central clear area, forming a pseudo-rosette.
• The tumour grows fast and metastasises through the blood stream to the lungs and to other bones.

1. In a typical case-

• Lytic lesion in the medullary zone of the midshaft of a long bone, with cortical destruction and new bone formation in layers – onion-peel appearance.

2. In atypical presentations, the tumour may be located in the metaphysis, and may be confused with osteomyelitis.

• Predominant soft tissue component with little cortical destruction,
• In flat bones, it is primarily a lytic lesion with hardly any new bone formation.