Osteosarcoma is a type of bone cancer. It starts in immature bone cells that normally form new bone tissue. Osteosarcoma destroys local tissue and weakens the bone. Very rarely, it presents in the soft tissues, outside of a bone. Osteosarcoma most often begins in the thigh or shin bones. The second most common place for osteosarcoma to begin is in the upper arm bone, close to the shoulder, but osteosarcoma can develop in any bone in the body. Osteosarcoma affects about 400 children and adolescents under the age of 20 each year, and it most commonly affects adolescents during the growth spurt.