Bone cancer is an uncontrolled growth of abnormal cells within the bone. Bone provides structure to the body, protects internal organs, and creates and stores blood cells. It also acts as levers and braces for muscles to produce movement.

Mature bones are composed of three types of tissue:

• Compact tissue (hard outer portion of most bones)
• Cancellous tissue (spongy layer inside the bones that contains bone marrow)
• Subchondral tissue (smooth bone inside the joints)

The hard outer component of bone consists of a network of tissue onto which calcium salts are deposited. The bone is also surrounded by a layer of periosteum, a membrane of connective tissue. Located at each end of a bone is an area of softer tissue known as cartilage. The cartilage serves as cushioning between bones, and in combination with ligaments and additional tissues forms the joints.  

Most bones are hollow. Located inside hollow bones is bone marrow, a type of soft tissue that consists of fat cells and blood-forming cells. Blood-forming cells generate red blood cells, white blood cells and platelets. Additional cells found in the bone marrow include plasma cells, fibroblasts and reticuloendothelial cells. 

The bone itself contains two types of cells:

• Osteoblasts, which form new bone.
• Osteoclasts, which dissolve old bone.

People may think of the bones as not growing in adulthood, but in a process called remodeling the osteoblasts keep replacing bone tissue destroyed by the osteoclasts. This process re-creates every bone over a decade.

Bone tumors can be either benign (noncancerous) or malignant (cancerous). Both types can grow and compress normal bone tissue and absorb or replace it with abnormal tissue. Benign bone tumors, which are more common, do not spread and are rarely life-threatening.

The two main forms of cancer found in bone are primary bone cancer and secondary bone cancer. Primary bone cancer is cancer that begins in the bone. Secondary bone cancer, or metastatic bone cancer, is cancer that does not begin in the bone but has spread from cancer in another site. Cancer of the prostate, lung and breast are the most common cancers that metastasize to the bone. 

Cancer that begins in the bone is not the same condition as cancer that spreads to the bone. These forms of cancer can be distinguished from one another when their cells are examined under a microscope. For example, cancer that has spread from the lungs to the bone will have cells that are similar to lung cancer, not bone cancer. Primary bone cancer is rare. It is much more common for cancer to spread to a bone than to originate there.

The American Cancer Society (ACS) predicts about 2,300 new cases of primary cancer of the bones and joints in the United States in 2007. Men will account for approximately 1,300 of the cases, while 1,000 women will develop this type of cancer. Primary bone cancers account for less than 0.2 percent of all cancers and will cause about 1,300 deaths in 2007. Secondary bone cancer (cancer that has spread from another site) is more common. According to the ACS, almost all people who die of cancer (approximately 560,000 people per year) will have bone metastasis at some point during their illness.