Osteosarcoma (also called osteogenic sarcoma) is a type of cancer that starts in the bones. To understand osteosarcoma, it helps to know something about the normal structure and function of bones.

About normal bones

Many people may think of bones as just being part of the skeleton, like the steel girders that support a building. But bones actually have a number of different functions.

Some bones help support and protect our vital organs. Examples include the skull bones, breast bone (sternum), and ribs. These types of bones are often referred to as flat bones.

Other bones, such as those in the arms and legs, make a framework for our muscles that helps us move. These are called long bones.

Bones also make new blood cells. This is done in the soft, inner part of some bones called the bone marrow, which contains blood-forming cells. New red blood cells, white blood cells, and platelets are made in bone marrow.

Bones also provide the body with a place to store minerals such as calcium.

Because bones are very hard and don't change shape -- at least once we reach adulthood -- we may think of bones as being dead, but they are complex, living tissues. Like all other tissues of the body, bones have many kinds of living cells. Two main types of cells in our bones help them stay strong and keep their shape.

• Osteoblasts help build up bones by forming the bone matrix (the connective tissue and minerals that give bone its strength).

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

Osteosarcoma

Osteosarcoma is the most common type of cancer that develops in bone. Like osteoblasts of 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.

Most osteosarcomas occur in children and young adults. Teenagers are the most commonly affected age group, but osteosarcoma can occur at any age.

In children and young adults, 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 distal femur (the lower part of the thigh bone) or the proximal tibia (the upper part of the large lower leg bone). The proximal humerus (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, shoulder, and jaw. This is especially true in older adults.

Subtypes of osteosarcoma

Several subtypes of osteosarcoma can be identified by how they look on x-rays and under the microscope. Some of these subtypes have a better prognosis (outlook) than others.

Based on how they look under the microscope, osteosarcomas can be classified as high grade, intermediate grade, or low grade.

High-grade osteosarcomas: These are the fastest growing types of osteosarcoma. When seen under a microscope, they do not look like normal bone and have many cells in the process of dividing. Most osteosarcomas that occur in children are high grade. There are several types of high-grade osteosarcomas.

• Osteoblastic
• Chondroblastic
• Fibroblastic
• Mixed
• Small cell
• Telangiectatic
• High-grade surface (juxtacortical high grade)

Other high-grade osteosarcomas include:

• Pagetoid: a tumor that develops in someone with Paget disease of the bone
• Extra-skeletal: a tumor that starts in a part of the body other than a bone
• Post-radiation: a tumor that starts in a bone that had once received radiotherapy

Intermediate-grade osteosarcomas: These uncommon tumors fall in between high-grade and low-grade osteosarcomas. (They are treated as if they are low-grade osteosarcomas.)

• Periosteal (juxtacortical intermediate grade)

Low-grade osteosarcomas: These are the slowest growing osteosarcomas. The tumors look more like normal bone and have few dividing cells when seen under a microscope.

• Parosteal (juxtacortical low grade)
• Intramedullary or intraosseous well differentiated (low-grade central)

The grade of the tumor tells doctors how likely it is that the cancer will grow and spread to other parts of the body. The grade plays a role in determining the stage of the tumor and the type of treatment used. For more on staging, see the section, "How is osteosarcoma staged?"

Other types of bone tumors

Several other types of tumors can start in the bones.

Malignant (cancerous) bone tumors

Ewing tumors are the second most common malignant bone tumor in children. They are described in the American Cancer Society document, Ewing Family of Tumors.

Most other types of bone cancers are usually found in adults and are rare in children. These include chondrosarcoma (cancer that develops from cartilage), malignant fibrous histiocytoma, fibrosarcoma, chordoma, and malignant giant cell tumor of bone. For more information on these cancers, see the American Cancer Society document, Bone Cancer.

Many types of cancer that start in other organs of the body can spread to the bones. These are sometimes referred to as metastatic bone cancers, but they are not true bone cancers. For example, prostate cancer that spreads to the bones is still prostate cancer and is treated like prostate cancer. For more information, see the document, Bone Metastasis.

Benign (non-cancerous) bone tumors

Not all bone tumors are malignant. Cells that form benign tumors do not spread to other parts of the body. They are usually not life threatening and can often be cured by surgery. There are many types of benign bone tumors.

• Osteomas are benign tumors formed by bone cells.
• Chondromas are benign tumors formed by cartilage cells.
• Osteochondromas are benign tumors with both bone and cartilage cells.

Other benign bone tumors include eosinophilic granuloma of bone, non-ossifying fibroma, enchondroma, xanthoma, giant cell tumor, and lymphangioma.

The rest of this document refers only to osteosarcoma.

Last Medical Review: 01/14/2009
Last Revised: 01/14/2009