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. This damage usually happens during our lifetime, particularly as we get older. A small number of people inherit a damaged gene from a parent.

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 do not spread to other parts of the body.

The beginnings of cancer

Diagram showing the beginnings of cancer


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.

Sometimes, cells break away from the original (primary) cancer and spread to other organs. When these cells reach a new site they may form a new tumour. This is called secondary cancer or metastasis. So, for example, if breast cancer spreads to the bones, it is called a breast cancer secondary in the bone. It is not considered to be bone cancer, which is a separate disease.

How cancer spreads

Diagram showing how cancer spreads