Osteosarcoma is the most common malignant bone tumor in youth. The average age at diagnosis is 15. Boys and girls have a similar incidence of this tumor until late adolescence, at which time boys are more commonly affected. The cause is not known. In some cases, osteosarcoma runs in families and at least one gene has been linked to increased risk. This gene is also associated with familial retinoblastoma, a cancer of the eye which occurs in children. It appears to be a defective tumor inhibitor gene. Osteosarcoma tends to occur in the bones of the thigh (distal end, near the knee), upper arm (proximal end, near the shoulder), and shin (proximal end, near the knee). This cancer occurs most commonly in larger bones and in the area of bone with fastest growth rate. Osteosarcoma can occur in any bone, however. The most common symptom initially is pain. As the tumor grows, there may be visible swelling and limitation of motion. Tumors in the legs cause limping, while those in the arms cause pain on lifting. Swelling over the tumor may be warm and slightly reddened.
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