Ewing's sarcoma is a cancerous bone tumor that may occur in any bone in the body. When it occurs in soft tissue, it is called extraosseous sarcoma. The most commonly affected areas include the pelvis, thigh, lower leg, upper arm, and chest wall.

Leg and Pelvic Bones—Common Sarcoma Sites
Leg bones
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The causes of Ewing's sarcoma are not fully understood. Chromosomal rearrangement that alters the genes may be a factor.