Osteosarcoma is the most common histological form of primary bone cancer and is most prevalent in children and young adults. Like the osteoblasts in normal bone, the cells that form this cancer make bone matrix. But the bone matrix of an osteosarcoma is not as strong as that of normal bones.
Osteosarcoma usually develops in areas where the bone is growing quickly, such as near the ends of the long bones. Most tumors develop in the bones around the knee, either in the lower part of the thigh bone or the upper part of the shinbone. The part of the upper arm bone close to the shoulder is the next most common site. However, osteosarcoma can develop in any bone, including the bones of the pelvis (hips), shoulder, and jaw. This is especially true in older adults.