A sarcoma is a rare kind of cancer. Sarcomas are different from the much more common carcinomas because they happen in a different kind of tissue. Sarcomas grow in connective tissue cells that connect or support other kinds of tissue in your body. These tumors are most common in the bones, muscles, tendons, cartilage, nerves, fat, and blood vessels of your arms and legs, but they can also happen in other areas of your body. Although there are more than 50 types of sarcoma, they can be grouped into two main kinds: soft tissue sarcoma and bone sarcoma, or osteosarcoma. About 12,750 cases of soft tissue sarcoma and 800-900 new cases of bone sarcomas will be diagnosed in the U.S. in 2019.Sarcomas can be treated, often by having surgery to remove the tumor.Children and young adults get osteosarcoma more often than adults. And because healthy, active children and teens often have pain and swelling in their arms and legs, osteosarcoma might be mistaken for growing pains or a sports injury.