Cancer is a disease of the cells, which are the body's basic building blocks. Cancer starts in our genes. Our bodies constantly make new cells to enable us to grow, to replace worn-out cells, or to heal damaged cells after an injury. Certain genes control this process.

All cancers are caused by damage to these genes. This damage usually happens during our lifetime, although 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 called a tumour.

Tumours can be benign (not cancer) or malignant (cancer). Benign tumours do not spread outside their normal boundary to other parts of the body.

A malignant tumour is made up of cancer cells. When it first develops, this malignant tumour may be confined to its original site. This is known as 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, thus becoming invasive cancer.

Some benign tumours are precancerous and may progress to cancer if left untreated. Other benign tumours do not develop into cancer.