Cancer is a disease of the body's cells. Our bodies are always making new cells: so we can grow, replace worn-out cells, or heal damaged cells after an injury. This process is controlled by certain genes. If the genes that control the growth of cells are damaged, these abnormal cells will keep on dividing and create a lump inside our body called a tumour. These lumps can be benign (not cancerous) or malignant (cancerous).

Benign lumps do not spread to other parts of the body.

A malignant lump (more commonly called a malignant tumour) is made up of cancer cells that have grown their own blood vessels. When it first develops, this malignant tumour is confined to its original site. If these cells are not treated they may spread, either through the local tissue fluid channels (lymphatics) or in the bloodstream, to other parts of the body.

When these cells reach a new site they may continue to grow and form another tumour at that site. This is called secondary cancer or metastasis.

Cases of cancer have been recorded in almost all parts of the human body. Each type of cancer generally has its own symptoms, requiring different methods of diagnosis and treatment. Almost all types of cancer are in the form of a solid tumour or massive lump. A notable exception is cancer of the blood.