Osteosarcoma is a type of bone cancer. It starts in immature bone cells that normally form new bone tissue, destroys local tissue and weakens the bone. Very rarely, it presents in the soft tissues, outside of a bone. This type of cancer most often begins in the thigh or shin bones. The second most common place for it to begin is in the upper arm bone, close to the shoulder, but it 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.