Ewing's sarcoma is a malignant tumor of the bone. It may occur in any bone, however it most commonly affects the pelvis, tibia (shin), fibula (shin), and femur (thigh). Occasionally, Ewing's sarcoma may occur in soft tissues, not bone. This is called "non-osseous Ewing's sarcoma." Ewing's sarcoma is a rare malignancy, which usually occurs in children and young adults less than age 30. It is not known what causes the tumor but it is not hereditary nor is it contagious. Nothing you have done or not done is responsible for your child's tumor.