What is cancer?

Cancer is a disease of the body’s cells. Our bodies are always making new cells to replace worn-out cells or to heal damaged cells after an injury. This process is controlled by certain genes: the codes that tell our cells how to grow and behave. Cancers are caused by damage to these genes. Usually this damage happens during our lifetime. Some people inherit a damaged gene from a parent, which means that if they develop cancer it may be at an earlier than average age.

The beginnings of cancer

Normally, cells grow and multiply in an orderly way. However, damaged genes can cause them to behave abnormally. They may grow into a lump, which is called a tumour. Tumours can be benign (not cancerous) or malignant (cancerous). Benign tumours are surrounded by a capsule and do not spread to other parts of the body.

The beginnings of cancer

Diagram showing the beginnings of cancer

How cancer spreads

Diagram showing how cancer spreads

Malignant tumours invade into the surrounding tissues, and may form a secondary cancer or metastasis in another part of the body. For a cancer to grow bigger than the head of a pin, it must grow its own blood vessels. This is called angiogenesis.

The way cancer is treated

Cancer is treated by surgery, chemotherapy (drug treatment), immunotherapy — for example, monoclonal antibodies (see “Treatment for advanced cancer”) — hormone treatment or radiation treatment. Sometimes only one of these methods of treatment is used for a cancer. Sometimes more than one is used.