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Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Osteosarcoma is a type of cancer that starts in the bones. To understand osteosarcoma, it helps to know about the bones and what they do.

Normal bones

Many people think of bones as just being part of the skeleton, like the steel girders that support a building. But bones do a number of different things.

  • 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, like 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.
  • 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 in adults) we may think of bones as not being alive. But bones are really living tissue made up of living cells.

There are 2 main types of cells in our bones:

  • Osteoblasts help build up bones by forming the matrix of the bones – the connective tissue and minerals that give the bones strength.
  • Osteoclasts break down bone matrix to keep it from building up. These cells help bones keep their shape. They also help control the amount of minerals in the blood.


Osteosarcoma is the most common type of cancer that starts in the bone. Even though this cancer starts in the cells that make bone, the cancer-filled bone is not as strong as normal bones.

Most osteosarcomas occur in children and teens, but they can occur at any age.

Osteosarcoma often starts near the ends of the long bones in the legs, especially around the knee. The arm bone near the shoulder is the next most common place. But it can start in other bones, too, like the hip bone (pelvis), shoulder, or jaw.

Subtypes of osteosarcoma

There are several subtypes of osteosarcoma. The subtype is based on how the cancer looks on x-rays and under the microscope. Some subtypes have a better outlook for survival than others.

Osteosarcomas can be grouped as low grade, intermediate grade, or high grade.

  • Low-grade tumors have few dividing cells and look more like normal bone. They tend to grow slowly.
  • Intermediate-grade tumors are in between low- and high-grade tumors, although they are usually treated like low-grade tumors.
  • High-grade tumors have many dividing cells and look very abnormal. These tumors tend to grow quickly. Most osteosarcomas in children and teens are high grade.

The grade helps the doctor figure out the stage of the tumor and the best type of treatment to use. For more on staging, see the section “Staging of osteosarcoma.”

Other types of bone tumors

Ewing tumors are the second most common kind of bone cancer in children. They are described in our document Ewing Family of Tumors. Most other types of bone cancers are usually found in adults and are rare in children. For more information on other bone cancers, see our document Bone Cancer.

Many types of cancer that start in other places can spread to the bones, but they are not true bone cancers. They are sometimes called metastatic bone cancers. For instance, prostate cancer that spreads to the bones is still prostate cancer and is treated like prostate cancer. For more information, see our document Bone Metastasis.

Some bone tumors are not cancer. These are called benign bone tumors. Benign bone tumors do not spread to other parts of the body. Most of the time they are not a threat to life and can be cured by surgery.

The rest of this document covers only osteosarcoma.

Last Medical Review: 06/13/2014
Last Revised: 01/27/2016