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Normal Bone Tissue

Bone is the supporting framework of the body. Most bones are hollow. The outer part of bones consists of a network of fibrous tissue called matrix onto which calcium salts are deposited. At each end of the bone is a zone of cartilage, a softer form of bone-like tissue. Cartilage is made of a fibrous tissue matrix mixed with a gel-like substance. Unlike bone, cartilage does not contain much calcium.

Cartilage acts as a cushion between bones and, together with ligaments and some other tissues, forms the joints between bones. The 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 a femur (thigh bone).

The outside of the bone is covered with a layer of fibrous tissue called periosteum. The bone itself contains 2 kinds of cells. The osteoblast is the cell responsible for forming bone, and the osteoclast is the cell responsible for dissolving bone. Although bone looks to be a very unchanging organ, the truth is that it is very active. New bone is constantly forming, and at the same time, old bone is dissolving.

Bone marrow is the soft tissue inside the hollow bones. The marrow of some bones consists only of fatty tissue. The marrow of other bones is a mixture of fat cells and blood-forming (hematopoietic) cells. These blood-forming cells produce red blood cells, white blood cells, and blood platelets. There are some other cells in the marrow such as plasma cells, fibroblasts, and reticuloendothelial cells.

All these tissues can develop into a cancer.

Types of Bone Cancers

Most of the time when someone with cancer is told they have bone cancer, the doctor is talking about a cancer that spread there from somewhere else. This is called metastatic cancer and can be seen in people with advanced breast cancer, prostate cancer, lung cancer as well as many others. When these cancers in the bone are examined under a microscope they resemble the tissue they came from. If someone has lung cancer spread to bone, the cells of the cancer look and act like lung cancer cells, not bone cancer cells, even after they have spread from the lungs to the bones. They are treated with the same kind of treatment (chemotherapy drugs, for example) that is used for lung cancer. For more information about metastatic bone cancer, please see the American Cancer Society document "Bone Metastases," as well as the document on the specific place where the cancer started ("Breast Cancer," "Lung Cancer," "Prostate Cancer," etc.).

Other kinds of cancers that are sometimes called one cancers?start in the bone marrow -- in the blood forming cells. The most common of these is multiple myeloma. Leukemia is generally considered a lood cancer?rather than a one cancer?but, like multiple myeloma, the malignant cells start in the bone marrow. 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. Instead, individual documents are available for these cancer types.

The main type of cancers that are true one?cancers are called sarcomas. This is a term that describes the type of tissue that the cancer started in. 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.

Primary Bone Tumors

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 called sarcomas.

Benign Bone Tumors

Benign bone tumors do not spread to other tissues and organs and are not life threatening. They are generally cured by surgery. Types of benign bone tumors include osteoid osteoma, osteoblastoma, osteochondroma, enchondroma, and chondromyxoid fibroma. These benign tumors are not discussed further in this document, which is limited to bone cancers.

Malignant Bone Tumors

Osteosarcoma: Osteosarcoma (also called osteogenic sarcoma) is a cancerous tumor of the bone itself, and it is the most common primary bone cancer. Although osteosarcoma most often occurs in young people between the age of 10 and 30, about 10% of cases develop in people in their 60s and 70s. This cancer is rare during middle age. More males than females get this cancer. These tumors develop most often in bones of the arms, legs, or pelvis. For more information, see the American Cancer Society document, steosarcoma.?

Chondrosarcoma: This is a cancer of cartilage cells and is the second most common primary bone cancer. This cancer is uncommon in people younger than 20. After age 20, the risk of developing chondrosarcoma continues to rise until reaching about 75 years. Men and women are equally likely to get this cancer.

Chondrosarcoma is a cancer that develops from the cartilage of bones. Cartilage is softer than bone but more firm than most tissues (ears are mainly cartilage). Cartilage is actually the tissue from which most bones form. First cartilage develops and then calcium compounds are deposited to form bone. Most of the cartilage in a developing bone eventually becomes bone. But cartilage will remain at the ends of the bone. There it acts as a cushion between bones. Cartilage is mainly found at the end of long bones and at the place in the chest where the ribs meet the sternum (breastbone). Cartilage is also part of the face. Other structures that contain cartilage are the trachea (windpipe) and larynx (voicebox). Chondrosarcomas can develop in any of these places. Most develop in bones such as the pelvis, leg bone or arm bone. Occasionally, chondrosarcoma will develop in the trachea, larynx, and chest wall. Other sites are the scapula, ribs, or skull.

Benign (non-cancerous) tumors of cartilage are more common than malignant ones. These are called enchondromas. Another type of benign tumor is the osteochondroma. This is a bony projection capped by cartilage. Both of these benign tumors rarely turn into cancer. There is a little higher chance of cancer developing in people who have many of these tumors, but this is still not common.

Chondrosarcomas are classified by their grades, a measure of how fast growing they are. This is determined by the pathologist (a doctor specially trained to examine and diagnose tissue samples under a microscope) who examines the chondrosarcoma under the microscope. Most chondrosarcomas are low grade (grade I), meaning they are not likely to spread, or intermediate grade (grade II). High grade (grade III) chondrosarcomas, which are the most likely to spread, are less common.

Some chondrosarcomas have distinctive features under a microscope. Some of these variants of chondrosarcoma tend to have a better prognosis (outlook for survival) than usual chondrosarcomas, and others tend to be more aggressive. Dedifferentiated chondrosacromas start out as typical chondrosarcomasbut then some parts of the tumor change into cells that closely resemble those of an osteosarcoma or fibrosarcoma. This variant of chondrosarcoma tends to occur in older patients and is more aggressive than usual chondrosarcomas. In contrast, clear cell chondrosarcoma is a rare variant that grows slowly and rarely spreads to other parts of the body unless it has already recurrent several times in the original location. Mesenchymal chondrosarcoma shares some similarities with Ewing tumor. Although these tumors can grow rapidly, they are sensitive to radiotherapy and chemotherapy.

Ewing tumor: This cancer (also called Ewing sarcoma) is named after Dr. James Ewing, who first described it in 1921. Ewing tumors usually develop in bones, and less than 10% arise in other tissues and organs. They most often arise in the long bones of the legs and arms but may also develop in the pelvis and other bones. Ewing tumor is the third most common primary bone cancer. Unlike osteosarcoma, Ewing tumors of bone form in the cavity of the bone. This cancer usually appears in children and teenagers and is uncommon in adults over age 30. Ewing tumors occur most often in white people and are rare among African Americans and Asian Americans. For more information, see the American Cancer Society document, ?a href="/docroot/CRI/CRI_2_3x.asp?dt=48">Ewing Family of Tumors.?

Malignant fibrous histiocytoma: This form of cancer occurs more often in "soft tissues" (types of connective tissues other than bone, such as ligaments, tendons, fat, and muscle) but can rarely start in bones. When it does develop in bones, it usually affects the legs (often around the knees) or arms. This cancer usually occurs in elderly and middle-aged adults and is rare among children. Malignant fibrous histiocytoma (often abbreviated as MFH) tends to grow quickly. It often spreads to other parts of the body, most often to lymph nodes and to the lungs.

Fibrosarcoma and malignant fibrous histiocytoma: This is another type of cancer that develops more often in "soft tissues" than it does from bones. Fibrosarcoma usually occurs in elderly and middle-aged adults. Bones most often affected include those of the legs, arms, and jaw.

Giant cell tumor of bone: This type of primary bone tumor has benign and malignant forms. The benign (non-cancerous) form is most common. These tumors typically affect the leg (usually, near the knees) or arm bones of young and middle-aged adults. Fewer than 10% of giant cell bone tumors are initially cancerous and spread to other parts of the body, but after surgery giant cell bone tumors often recur (come back) locally (in the same place where the cancer started). When giant cell bone tumors recur, they become more likely to spread to other parts of the body, especially if they recur several times.

Chordoma: This primary tumor of bone usually occurs in the base of the skull and bones of the spine. They usually develop in adults older than 30 years, and are about twice as common in men than in women. Chordomas tend to grow slowly and usually do not spread to other parts of the body, but they often come back in the same area if they are not removed completely. When they do spread, lymph nodes, the lungs, and the liver are the most common areas for secondary tumors.

Other Cancers That Develop in Bones

The following 2 cancers develop in bones but are not discussed further in this document.

Lymphoma: Non-Hodgkin lymphomas generally develop in lymph nodes but sometimes start in the bone. Primary non-Hodgkin lymphoma of bone is considered a widespread disease because multiple sites in the body are usually involved. The outlook and treatment are similar to other non-Hodgkin lymphomas of the same subtype and stage. Treatment is similar to lymphomas that start in lymph nodes but differs from primary bone sarcomas. For more information, see the American Cancer Society document, ?a href="/docroot/CRI/CRI_2_3x.asp?dt=32">Non-Hodgkin Lymphoma.?

Multiple myeloma: Although multiple myeloma almost always starts in bones, doctors do not consider this a bone cancer because it develops from the plasma cells of the bone marrow (the soft inner part of some bones). Although it causes bone destruction, it is no more a bone cancer than is leukemia. It is treated as a widespread disease. At times, myeloma can be first found as a single tumor in a single bone, but most of the time it will go on to spread to the marrow of other bones. For more information, see the American Cancer Society document, "Multiple Myeloma."