What is cancer?

Cancer is not one but many different diseases. What all cancers have in common is that some of the cells in the body have become abnormal. These abnormal cells multiply and start to attack the body's healthy cells, and in many types of cancer a solid mass or tumour is formed. Cancers may spread from their original site to other parts of the body. The three main types of cancer are carcinomas, sarcomas, and leukaemias and lymphomas.

Can cancer be cured?

Many cancers can be cured if they are treated in time. It is estimated about one person in every three in New Zealand who gets cancer is cured. Even if the cancer cannot be cured, people often live for some time after treatment.

How cells work normally

Our bodies consist of billions of cells. Our bodies are constantly making new cells: to enable us to grow, to replace worn-out cells, or to heal damaged cells after an injury. All our organs, bones, muscles, skin, blood and other fluids are made up of specialised cells.

Although each group of cells has different functions they all reproduce by dividing. In childhood the cells divide rapidly. Once a person reaches adulthood, the division of cells slows down. In some parts of the body, such as the brain, the division of cells stops completely.

Some other cells also stop growing, but if they are damaged, they can grow to replace the lost ones. For example, kidney cells are able to grow when needed.  Other cells, such as blood, skin and hair are constantly being worn out and replaced.

When something goes wrong

Sometimes, for reasons not clearly understood, a cell is triggered into abnormal action. It starts dividing uncontrollably, forming a group of abnormal cells. These cells do not seem to know when to stop. As they multiply they form a mass, called a tumour, which can invade and destroy normal tissue. 

Sometimes cancer cells break away from the original (primary) cancer and travel to other parts of the body through the blood or lymph system. They may form new colonies of abnormal cells. In other parts of the body these new colonies of cells are called secondary growths or metastases.

Not all cancer is in the form of a solid tumour. For example, cancers of the blood (leukaemia) do not form tumours.

What is a tumour?

A lump formed by cells which have gone out of control and divided repeatedly is called a tumour. Not all tumours are cancers. There are two kinds of tumours:

• benign tumours (not cancerous), which may interfere with bodily functions and need to be surgically removed. They do not spread to other parts of the body and are usually not life-threatening,
• malignant tumours or cancers. A malignant tumour is made up of cancer cells. When it first develops, this malignant tumour may be confined to its original site, a cancer in situ (or carcinoma in situ). If these cells are not treated they may spread beyond their normal boundaries and into surrounding tissues (invasive cancer).

Types of cancer

There are more than 100 types of cancer. Cancers can be grouped into three main categories:

• carcinomas, which occur in the lining of the body's external and internal surfaces, eg, the skin, mouth and rectum,
• sarcomas, which form in connective tissues, eg, muscles and bones,
• leukaemias and lymphomas, which are cancers of the bone marrow and lymph glands.

Related topics

What starts cancer forming in the body? See What causes cancer?

What does that term mean? See Cancer glossary

Original material provided by the Cancer Society of New Zealand, May 2003. Reviewed by everybody, July 2010.