Ewing sarcoma is a type of cancer that occurs in the bone or soft tissue. Areas that are commonly affected include the pelvis, thigh, lower leg, upper arm, and chest wall. Prognosis depends on the location of the tumor and whether it has spread to other parts of the body.

Leg and Pelvic Bones—Common Sarcoma Sites
Leg bones
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It is thought that Ewing sarcoma is caused by a genetic problem.