Osteosarcoma is a type of bone cancer that arises from the cells that form bones. It is the most common primary bone cancer, primarily affecting children and young adults, although it can occur at any age. 

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Osteosarcoma usually develops in the long bones, such as the arms or legs, particularly near the areas where bone growth occurs. However, it can also occur in other bones, such as the pelvis or skull. The exact cause of osteosarcoma is unknown, but several factors may increase the risk, including inherited genetic conditions, prior radiation therapy, and certain bone diseases.