What is Osteosarcoma?

Osteosarcoma is a type of cancer that starts in the bones. To understand osteosarcoma, it helps to know something about the normal structure of bones.

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 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, 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 being dead. 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, that is, 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. Most osteosarcomas happen in children and young adults, but it can happen at any age. Like other cancers, osteosarcoma can spread beyond the bone into nearby tissues. Cancer cells can also break away from the main tumor and spread through the blood to other bones or to the lungs or other organs. This process is called metastasis.

When this cancer starts in the cells that make bone, the cancer-filled bone is not as strong as normal bones. Osteosarcoma often starts near the ends of the long bones in the legs, especially in the part of the thigh bone next to the knee, and the part of the lower leg bone next to the knee. The arm bone near the shoulder is the second most common place for this cancer to start. But it can start in other bones, too. Osteosarcoma that starts in places like the hip bone (pelvis), shoulder, or jaw (and other places where it’s hard to do surgery) tends to have a worse outcome.

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 much better outlook for survival than others.

Osteosarcomas can be grouped as low grade, intermediate grade, or high grade. If the tumor has few dividing cells and looks more like normal bone, it is called a low-grade osteosarcoma. If it has many dividing cells and is growing quickly, it is high grade. Most osteosarcomas in children and teens are high grade.

The grade is important because it tells the doctor how likely the cancer is to grow and spread to other parts of the body. It 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, “After the tests: Staging.”

Other types of bone tumors

Ewing tumors are the second most common kind of bone cancer 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.

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 called Bone Metastasis.

Many bone tumors are not cancer. These are called benign (be-nine) bone tumors. Benign bone tumors do not spread to other parts of the body and most of the time they are not dangerous.


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