When cells divide abnormally and uncontrollably, they can form a mass or lump of tissue. This lump is called a tumor. Bone tumors form in your bones. As the tumor grows, abnormal tissue can displace healthy tissue. Tumors can either be benign or malignant. Benign tumors aren’t cancerous. While benign bone tumors typically stay in place and are unlikely to be fatal, they’re still abnormal cells and may require treatment. Benign tumors can grow and could compress your healthy bone tissue and cause future issues. Malignant tumors are cancerous. Malignant bone tumors can cause cancer to spread throughout the body. Benign tumors are more common than malignant ones. According to the American Academy of Orthopaedic Surgeons (AAOS), the most common type of benign bone tumor is an osteochondroma. This type accounts for between 35 and 40 percent of all benign bone tumors. Osteochondromas develop in adolescents and teenagers. These tumors form near the actively growing ends of long bones, such as arm or leg bones. Specifically, these tumors tend to affect the lower end of the thighbone (femur), the upper end of the lower leg bone (tibia), and the upper end of the upper arm bone (humerus). These tumors are made of bone and cartilage. Osteochondromas have been considered to be an abnormality of growth. A child may develop a single osteochondroma or many of them.