The human body contains over 200 bones of different shapes and sizes.
The human skeleton
Bone is a living tissue made up of calcium and various proteins that make the bone strong and rigid. It also contains living cells which continuously break down and remove old bone, replacing it with new bone to maintain the bone's strength.
Each bone consists of a compact outer shell and a spongy inside. The inside contains the bone marrow, which produces blood cells.
The joints of the bones are covered in cartilage Ė a tough, flexible material, rather like gristle. Cartilage is more stretchy than bone, and it allows the bones to move freely at the joints. It also cushions the bones at the joint to stop them rubbing against each other.
Structure of a bone
The bones have several important functions.
The skeleton gives the body rigid support.
The joints act as levers so that the body can move.
The bones protect organs in the body; for example, the rib cage protects the heart and lungs, and the skull protects the brain.
The bones also store some of the body's essential minerals, especially calcium.
Although a secondary bone cancer can occur in any bone in the body, the most commonly affected bones are those of the spine, ribs, pelvis, skull, and the upper bones of the arms (humerus) and the legs (femur).
What is cancer?
The organs and tissues of the body are made up of tiny building blocks called cells. Cancer is a disease of these cells.
Cells in different parts of the body may look and work differently but most reproduce themselves in the same way. Cells are constantly becoming old and dying, and new cells are produced to replace them. Normally, cells divide in an orderly and controlled manner. If for some reason the process gets out of control, the cells carry on dividing, developing into a lump which is called a tumour.
Tumours can be either benign or malignant. Cancer is the name given to a malignant tumour. Doctors can tell if a tumour is benign or malignant by examining a small sample of cells under a microscope. This is called a biopsy.
In a benign tumour the cells do not spread to other parts of the body and so are not cancerous. However, if they continue to grow at the original site, they may cause a problem by pressing on the surrounding organs.
A malignant tumour consists of cancer cells that have the ability to spread beyond the original area. If the tumour is left untreated, it may spread into and destroy surrounding tissue. Sometimes cells break away from the original (primary) cancer. They may spread to other organs in the body through the bloodstream or lymphatic system.
The lymphatic system is part of the immune system - the body's natural defence against infection and disease. It is a complex system made up of organs, such as bone marrow, the thymus, the spleen, and lymph nodes. The lymph nodes (or glands) throughout the body are connected by a network of tiny lymphatic ducts.
When the cancer cells reach a new area they may go on dividing and form a new tumour. This is known as a secondary cancer or metastasis.
It is important to realise that cancer is not a single disease with a single type of treatment. There are more than 200 different kinds of cancer, each with its own name and treatment.
Types of cancer
The majority of cancers, about 85% (85 in a 100), are carcinomas. They start in the epithelium, which is the covering (or lining) of organs and of the body (the skin). The common forms of breast, lung, prostate and bowel cancer are all carcinomas.
Carcinomas are named after the type of epithelial cell that they started in and the part of the body that is affected. There are four different types of epithelial cells:
squamous cells - that line different parts of the body, such as the mouth, gullet (oesophagus), and the airways
adeno cells - form the lining of all the glands in the body and can be found in organs such as the stomach, ovaries, kidneys and prostate
transitional cells - are only found in the lining of the bladder and parts of the urinary system
basal cells - that are found in one of the layers of the skin.
A cancer that starts in squamous cells is called a squamous cell carcinoma. A cancer that starts in glandular cells is called an adenocarcinoma. Cancers that start in transitional cells are transitional cell carcinomas, and those that start in basal cells are basal cell carcinomas.
Leukaemias and lymphomas
These occur in the tissues where white blood cells (which fight infection in the body) are formed, i.e. the bone marrow and lymphatic system. Leukaemia and lymphoma are quite rare and make up about 6.5% (6.5 in 100) of all cancers.
Sarcomas are very rare. They are a group of cancers that form in the connective or supportive tissues of the body such as muscle, bone and fatty tissue. They account for less than 1% (1 in 100) of cancers.
Sarcomas are split into two main types:
bone sarcomas - that are found in the bones
soft tissue sarcomas - that develop in the other supportive tissues of the body.
Others forms of cancer
Brain tumours and other very rare forms of cancer make up the remainder of cancers.
Secondary cancer in the bone
When cancer occurs in the body, the place where it starts is known as the primary tumour. A malignant (cancerous) tumour is made up of millions of cancer cells. Some of these cells may break away from the primary cancer and be carried in the bloodstream to another part of the body. The cancer cells may settle in that part of the body and then form a new tumour. If this happens it is referred to as a secondary cancer or a metastasis.
Secondary bone cancer does not start in the bone, but is the result of cancer cells spreading to the bone from a primary tumour as described above. Sometimes only one area of bone is affected, but in some people a number of bone secondaries develop, often in different bones in the body. Not all the secondaries will cause symptoms or problems.
Although any type of cancer can spread to the bone, the most common types are cancers of
the breast, prostate, lung, thyroid and kidney. People who develop secondary bone cancer usually know that they have a primary cancer, although occasionally a secondary bone cancer is found before a primary cancer is diagnosed. If the primary cancer canít be found it is called an unknown primary tumour.
Secondary cancer in the bone is very different to primary bone cancer, where cancer begins in the bone itself. Primary bone cancer is a completely different type of cancer with very different treatments.