is the most common cancerous (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. In rare cases, osteosarcoma occurs in adults. The cause of osteosarcoma is not known. In some cases, it runs in families, and at least one gene has been linked to increased risk. Although osteosarcoma tends to occur in the larger bones, such as the shin (near the knee), thigh (near the knee) and upper arm (near the shoulder), it can occur in any bone. A number of variants of osteosarcoma exist, including conventional types (osteoblastic, chondroblastic, and fibroblastic), telangiectatic, multifocal, parosteal, and periosteal. Treatment usually starts after a biopsy of the tumor and includes chemotherapy followed by surgery. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.