Although we may think of bone as hard or "dead," it really is a complex, living tissue. Like all other tissues of the body, it contains living cells. There are several types of cells in our bones.

• Osteoblasts are responsible for forming bone matrix (connective tissue and mineral that gives bone its strength).

• Osteoclasts prevent too much bone matrix from accumulating and help bones maintain their proper shape. By depositing or removing minerals from the bones, osteoclasts help control the amount of these minerals in the blood.

• Bone marrow (found in some bones) contains fat cells and, most importantly, blood-forming cells. All blood cells (red blood cells, white blood cells, and platelets) are made here.

There are 2 main types of bones – flat bones and long bones. The flat bones help to protect the brain and the organs of the chest, abdomen, and pelvis. The skull bones and sternum (breast bone), for example, are flat bones. The long bones support the legs and arms. Muscles that move the arms and legs are attached to these long bones.

Osteosarcoma (also called osteogenic sarcoma) is the most common type of cancer that develops in bone. Like osteoblasts of normal bone, the cells that form this cancer produce bone matrix. However, the "malignant bone" tissue of an osteosarcoma is not as strong as normal bones.

Like other cancers, osteosarcomas can spread beyond the bone into nearby tissues (such as muscle, tendons, and fat). Cancer cells from osteosarcoma can also sometimes break away from the main tumor and spread through the bloodstream to other bones, or to the lungs or other internal organs. This process is called metastasis.

Osteosarcoma develops most commonly near the ends of the long bones, especially around the knees. In children and adolescents about 80% of osteosarcomas develop in the bones around the knee – the distal femur (the part of the thigh bone next to the knee) and proximal tibia (the part of the lower leg bone next to the knee). The proximal humerus (the part of the upper arm bone close to the shoulder) is the second most common location for osteosarcoma (about 10%). These sites tend to have a good outlook for cure. However, osteosarcoma can develop in any bone, including the bones of the pelvis, shoulder, and jaw. Pelvic osteosarcomas, and osteosarcomas arising in bones where surgery is difficult tend to have a poorer outcome.

Not all malignant (cancerous) bone tumors are osteosarcomas. Ewing tumor is the second most common malignant bone tumor in children and is described in a separate American Cancer Society document. Most other types of bone cancers are usually found in adults and are rare in children. These include chondrosarcoma (cancer that develops from cartilage) and malignant fibrous histiocytoma of bone.

Many bone tumors are not malignant. Cells that form benign tumors do not have the ability to spread to other parts of the body and are generally not dangerous. There are many types of benign bone tumors. These tumors are formed by different types of cells. Benign tumors formed by bone cells are osteomas. Benign tumors formed by cartilage cells are chondromas. Benign tumors with both bone and cartilage cells are osteochondromas. Other benign bone tumors of children and young adults include eosinophilic granuloma of bone, non-ossifying fibromas, xanthomas, giant cell tumors, and lymphangiomas.

Some benign bone tumors can be diagnosed by physical examination and how they look on x-rays. In other cases, a biopsy (sample of tumor taken for examination under the microscope) is needed to see whether the tumor is benign or malignant.

Subtypes of Osteosarcoma

Several subtypes of osteosarcoma can be identified by how they look on x-rays and under the microscope. These subtypes are worth mentioning because some have a much better prognosis (outlook for survival) than others.

Based on how they look (how closely they resemble normal bone) under the microscope, osteosarcomas can be classified as low grade, intermediate, or high grade. If the tumor has few dividing cells, it is a low-grade osteosarcoma. If the tumor has many dividing cells it is a high-grade osteosarcoma. A high-grade osteosarcoma also has a lot of dead cells in the tumor. This usually means the cancer is growing so fast it outgrows its source of nutrition.

High-grade Osteosarcomas
(These have different names based on their appearance under the microscope.)

• Pagetoid – means coming from Paget disease.
• Extra skeletal – means starting in an non-bony area of the body
• Post-irradiation – osteosarcomas in bone that had once received radiotherapy
• osteoblastic
• chondroblastic
• fibroblastic
• mixed
• small cell
• telangiectatic
• high-grade surface

Intermediate-grade Osteosarcomas

• periosteal

Low-grade Osteosarcomas

• parosteal
• intramedullary or intraosseous well differentiated

The overall grade of the tumor is more important than the subtype. This tells doctors how likely it is that the cancer will spread to other parts of the body. Low-grade tumors should be completely removed, but the patients usually do not require chemotherapy. These tumors have an excellent prognosis. All patients with high-grade osteosarcoma require both chemotherapy and surgery. The outlook and treatment of intermediate-grade osteosarcoma varies. Most osteosarcomas that occur in children are high-grade. Last Revised: 03/19/2007