Cancer is a disease of the cells in the body. There are many different types of cell in the body, and many different types of cancer which arise from different types of cell. What all types of cancer have in common is that the cancer cells are abnormal and multiply out of control. However, there are often great differences between different types of cancer. For example:

  • Some grow and spread more quickly than others.
  • Some are more easy to treat than others, particularly if diagnosed at an early stage.
  • Some respond much better than others to chemotherapy, radiotherapy, or other treatments.
  • Some have a better outlook (prognosis) than others. For some types of cancer there is a very good chance of being cured. For some types of cancer, the outlook is poor.

So, cancer is not just one condition. In each case it is important to know exactly what type of cancer has developed, how large it has become, whether it has spread, and how well the particular type of cancer responds to various treatments. This will enable you to get reliable information on treatment options and outlook.

Normal body cells

The body is made up from millions of tiny cells. Different parts of the body, such as organs, bones, muscles, skin, and blood, are made up from different specialized cells. All cells have a center called a nucleus. The nucleus in each cell contains thousands of genes which are made up from a chemical called DNA.

The genes are like codes which control the functions of the cell. For example, different genes control how the cell makes proteins, or how and when to make hormones or other chemicals. Certain genes control when the cell should divide and multiply, and certain genes even control when the cell should die.

Most types of cell in the body divide and multiply from time to time. As old cells wear out or become damaged, new cells are formed to replace them. Some cells normally multiply quickly. For example, you make millions of red blood cells each day as old ones become worn out and are broken down. Some cells do not multiply at all once they are mature - for example, brain cells.

Normally, your body only makes the right number of cells that are needed.

Abnormal cells

Sometimes a cell becomes abnormal. This occurs because one or more of the genes in the cell has become damaged or altered. The abnormal cell may then divide into two, then four, then eight, and so on. Lots of abnormal cells may then develop from the original abnormal cell. These cells do not know when to stop multiplying. A group of abnormal cells may then form. If this group of cells gets bigger, it becomes a large clump of abnormal cells called a tumor.

A tumor is a lump or growth of tissue made up from abnormal cells. Tumors are divided into two types - benign and malignant.

Non-cancerous (benign) tumors

These may form in various parts of the body. Benign tumors grow slowly, and do not spread or invade other tissues. They are not cancerous and are not usually life-threatening. They often do no harm if they are left alone. However, some benign tumors can cause problems. For example, some grow quite large and may cause local pressure symptoms, or look unsightly. Also, some benign tumors that arise from cells in hormone glands can make too much hormone which can cause unwanted effects.

Cancerous (malignant) tumors

Malignant tumors tend to grow quite quickly, and invade into nearby tissues and organs, which can cause damage. The original site where a tumor first develops is called a primary tumor. Malignant tumors may also spread to other parts of the body to form secondary tumors (metastases). These secondary tumors may then grow, invade and damage nearby tissues, and spread again.

 Note: not all cancers form solid tumors. For example, in cancer of the blood cells (leukemia) many abnormal blood cells are made in the bone marrow and circulate in the bloodstream.

How do cancerous (malignant) tumors grow and spread?

Local growth and damage to nearby tissues

Malignant cells multiply quickly. However, to get larger, a tumor has to develop a blood supply to obtain oxygen and nourishment for the new and dividing cells. In fact, a tumor would not grow bigger than the size of a pinhead if it did not also develop a blood supply. Cancer cells make chemicals that stimulate tiny blood vessels to grow around them which branch off from the existing blood vessels. This ability for cancer cells to stimulate blood vessels to grow is called angiogenesis. Malignant cells have the ability to push through or between normal cells. So, as they divide and multiply, malignant cells invade and damage the local surrounding tissue.

Spread to lymph channels and lymph nodes

Some malignant cells may get into local lymph channels. (The body contains a network of lymph channels which drains the fluid called lymph which bathes and surrounds the body's cells.) The lymph channels drain lymph into lymph glands (called lymph nodes). There are many lymph nodes all over the body. A malignant cell may be carried to a lymph node and there it may become trapped. However, it may multiply and develop into a tumor. This is why lymph nodes that are near to a tumor may enlarge and contain cancerous cells.

Spread to other areas of the body

Some malignant cells may get into a local small blood vessel (capillary). They may then get carried in the bloodstream to other parts of the body. The cells may then multiply to form secondary tumors (metastases) in one or more parts of the body. These secondary tumors may then grow, invade and damage nearby tissues, and spread again.

Cells that make up benign tumors are different to cancerous (malignant) cells. Cells in benign tumors tend to be quite similar to normal cells. They do not invade local tissues. A benign tumor often grows slowly within a capsule or within normal cells which surround the tumor. A benign tumor tends to look and feel smooth and regular and have a well-defined edge. This is unlike a malignant tumor which may look craggy and irregular, and its edges tend to be mixed up with the nearby normal cells and tissue.

There are more than 100 different types of cancer. Each type is classified by the type of cell the cancer originates from - for example, a breast cell, a lung cell, etc. Each type of cancer generally falls into one of three categories:

  • Carcinomas are cancers that arise from cells which line a body surface, or the lining of a gland - for example, the skin, or the lining of the gut, mouth, cervix, airways, etc.
  • Sarcomas are cancers that arise from cells which make up the connective tissues such as bones or muscles. For example, an osteosarcoma is a cancer of bone tissue.
  • Leukemia's and lymphomas are cancers of cells in bone marrow and lymph glands (lymph nodes). For example, leukemia is a cancer of cells that make white blood cells.
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