Bone cancer starts in the bone. 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.
Normal bone tissue
To understand bone cancer, it helps to understand a little about normal bone tissue.
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.
Types of bone tumors
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 cancer that starts in the bone marrow and causes bone tumors 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.
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 bone tumors
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:
- Osteoid osteoma (OS-tee-oyd OS-tee-oh-ma)
- Osteoblastoma (OS-tee-oh-blas-TOE-muh)
- Osteochondroma (OS-tee-oh-kon-DROH-muh)
- Enchondroma (en-kon-DROH-muh)
- Chondromyxoid (kon-dro-MIX-oyd) 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 the most common primary bone cancer. This cancer starts in the bone cells. It most often occurs in young people between the ages of 10 and 30, but about 10% of osteosarcoma cases develop in people in their 60s and 70s. It is rare in middle-aged people, and is more common in males than females. These tumors develop most often in bones of the arms, legs, or pelvis.
Chondrosarcoma: Chondrosarcoma (KON-droh-sar-KOH-muh) is a cancer of cartilage cells. It is the second most common primary bone cancer. This cancer is rare in people younger than 20. After age 20, the risk of getting a chondrosarcoma goes up until about age 75. Women get this cancer as often as men.
Chondrosarcomas can develop anywhere there is cartilage. 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 (shoulder blade), ribs, or skull.
Benign (non-cancerous) tumors of cartilage are more common than malignant ones. These are called enchondromas. Another type of benign tumor that has cartilage is a bony projection capped by cartilage called an osteochondroma. These benign tumors rarely turn into cancer. There is a slightly higher chance of cancer developing in people who have many of these tumors, but this is still not common.
Chondrosarcomas are classified by grade, which measures how fast they grow. The grade is assigned by the pathologist (a doctor specially trained to examine and diagnose tissue samples under a microscope). The lower the grade, the slower the cancer grows. When a cancer is slow growing, the chance that it will spread is lower and so the outlook is better. Most chondrosarcomas are either low grade (grade I) 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. These variants of chondrosarcoma can have a different prognosis (outlook) than usual chondrosarcomas.
- Dedifferentiated(DEE- dih-feh-REN-shee-AY-ted) chondrosarcomas start out as typical chondrosarcomas but then some parts of the tumor change into cells like those of a high-grade sarcoma (such as high grade forms of malignant fibrous histiocytoma [HIS-tee-oh-sy-TOH-muh], osteosarcoma, or fibrosarcoma). This variant of chondrosarcoma tends to occur in older patients and is more aggressive than usual chondrosarcomas.
- Clear cell chondrosarcomasare rare and grow slowly. They rarely spread to other parts of the body unless they have already come back several times in the original location.
- Mesenchymal(meh-ZEN-kih-mul) chondrosarcomas can grow rapidly, but like Ewing tumor, are sensitive to treatment with radiation and chemotherapy.
Ewing tumor: Ewing tumor is the third most common primary bone cancer, and the second most common in children, adolescents, and young adults. This cancer (also called Ewing sarcoma) is named after the doctor who first described it in 1921, Dr. James Ewing. Most Ewing tumors develop in bones, but they can start in other tissues and organs. The most common sites for this cancer are the pelvis, the chest wall (such as the ribs or shoulder blades), and the long bones of the legs or arms. This cancer is most common in children and teenagers and is rare in adults older than 30. Ewing tumors occur most often in white people and are very rare among African Americans and Asian Americans.
Malignant fibrous histiocytoma: Malignant fibrous histiocytoma (MFH) more often starts in soft tissue (connective tissues such as ligaments, tendons, fat, and muscle) than in bones. This cancer is also known as pleomorphic undifferentiated sarcoma, especially when it starts in soft tissues. When MFH occurs in bones, it usually affects the legs (often around the knees) or arms. This cancer most often occurs in elderly and middle-aged adults and is rare among children. MFH mostly tends to grow locally, but it can spread to distant sites, like the lungs.
Fibrosarcoma: This is another type of cancer that develops more often in soft tissues than it does in bones. Fibrosarcoma usually occurs in elderly and middle-aged adults. Bones in the legs, arms, and jaw are most often affected.
Giant cell tumor of bone: This type of primary bone tumor has benign and malignant forms. The benign (non-cancerous) form is most common. Giant cell bone tumors typically affect the leg (usually near the knees) or arm bones of young and middle-aged adults. They don’t often spread to distant sites, but tend to come back where they started after surgery (this is called local recurrence). This can happen several times. With each recurrence, the tumor becomes more likely to spread to other parts of the body. Rarely, a malignant giant cell bone tumor spreads to other parts of the body without first recurring locally.
Chordoma: This primary tumor of bone usually occurs in the base of the skull and bones of the spine. It develops most often in adults older than 30, and is about twice as common in men as in women. Chordomas tend to grow slowly and often do not spread to other parts of the body, but they often come back in the same area if they are not removed completely. The lymph nodes, the lungs, and the liver are the most common areas for secondary tumor spread.
Other cancers that develop in bones
Non-Hodgkin lymphoma generally develops in lymph nodes but sometimes starts in the bone. Primary non-Hodgkin lymphoma of the bone is often a widespread disease because multiple sites in the body are usually involved. The outlook is similar to other non-Hodgkin lymphomas of the same subtype and stage. Primary lymphoma of the bone is given the same treatment as lymphomas that start in lymph nodes − it is not treated like a primary bone sarcoma.
Multiple myeloma almost always develops in bones, but doctors do not consider it a primary 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 (called a plasmacytoma) in a single bone, but most of the time it will spread to the marrow of other bones.