Bone is the supporting framework of your body. Most bones are hollow. The outer part of bones is a network of fibrous tissue called matrix onto which calcium salts are laid down.
The hard outer layer of bones is made of compact (cortical) bone, which covers the lighter spongy (trabecular) bone inside. The outside of the bone is covered with a layer of fibrous tissue called periosteum. Some bones are hollow and have a space called the medullary cavity which contains the soft tissue called bone marrow (discussed below). The tissue lining the medullary cavity is called endosteum. At each end of the bone is a zone of a softer form of bone-like tissue called cartilage.
Cartilage is softer than bone but more firm than most tissues. It is made of a fibrous tissue matrix mixed with a gel-like substance that does not contain much calcium.
Most bones start out as cartilage. The body then lays calcium down onto the cartilage to form bone. After the bone is formed, some cartilage may remain at the ends to act as a cushion between bones. This cartilage, along with ligaments and some other tissues connect bones to form a joint. In adults, cartilage is mainly found at the end of some bones as part of a joint. It is also seen at the place in the chest where the ribs meet the sternum (breastbone) and in parts of the face. The trachea (windpipe), larynx (voice box), and the outer part of the ear are other structures that contain cartilage.
Bone itself is very hard and strong. Some bone is able to support as much as 12,000 pounds per square inch. It takes as much as 1,200 to 1,800 pounds of pressure to break the femur (thigh bone). The bone itself contains 2 kinds of cells. The osteoblast is the cell that lays down new bone, and the osteoclast is the cell that dissolves old bone. Bone often looks as if it doesn't change much, but the truth is that it is very active. Throughout our bodies, new bone is always forming while old bone is dissolving.
In some bones the marrow is only fatty tissue. The marrow in other bones is a mixture of fat cells and blood-forming cells. The blood-forming cells produce red blood cells, white blood cells, and blood platelets. Other cells in the marrow include plasma cells, fibroblasts, and reticuloendothelial cells.
Cells from any of these tissues can develop into a cancer.
Most of the time when someone with cancer is told they have cancer in the bones, the doctor is talking about a cancer that has spread to the bones from somewhere else. This is called metastatic cancer. It can be seen in many different types of advanced cancer, like breast cancer, prostate cancer, and lung cancer. When these cancers in the bone are looked at under a microscope, they look like the tissue they came from. For example, if someone has lung cancer that has spread to bone, the cells of the cancer in the bone still look and act like lung cancer cells. They do not look or act like bone cancer cells, even though they are in the bones. Since these cancer cells still act like lung cancer cells, they still need to be treated with drugs that are used for lung cancer.
Other kinds of cancers that are sometimes called "bone cancers" start in the blood forming cells of the bone marrow − not in the bone itself. The most common bone cancer is called multiple myeloma. Another cancer that starts in the bone marrow is leukemia, but it is generally considered a blood cancer rather than a bone cancer. Sometimes lymphomas, which more often start in lymph nodes, can start in bone marrow. Multiple myeloma, lymphoma, and leukemia are not discussed in this document. For more information on these cancers, refer to the individual document for each.
A primary bone tumor starts in the bone itself. True (or primary) bone cancers are called sarcomas. Sarcomas are cancers that start in bone, muscle, fibrous tissue, blood vessels, fat tissue, as well as some other tissues. They can develop anywhere in the body.
There are several different types of bone tumors. Their names are based on the area of bone or surrounding tissue that is affected and the kind of cells forming the tumor. Some primary bone tumors are benign (not cancerous), and others are malignant (cancerous). Most bone cancers are sarcomas.
Benign tumors do not spread to other tissues and organs and so are not usually life threatening. They are generally cured by surgery. Types of benign bone tumors include: