The great majority of cancers start at a single site in the body. When a group of cells within a particular organ or tissue become cancerous they start to multiply in an uncontrolled way. After a period of time they will form a lump or swelling which is called the primary cancer. If untreated then this mass of cancerous tissue will continue to grow as the cancer cells continue to multiply. 

Spread of the cancer may occur in one of several ways. 

Firstly there is direct spread. As the primary cancer increases in size it will invade into and destroy the normal tissue surrounding it. 

Secondly there is lymphatic spread. Tiny clumps of cells can break away from the primary cancer and enter the nearby lymph vessels. These very fine channels run in networks throughout the body, draining lymph fluid from the organs and tissues to clusters of tiny glands, or lymph nodes. The cancer cells may then settle in the lymph nodes and grow there, causing the glands to become swollen. This process usually occurs in a stepwise fashion with the lymph glands closest to the primary cancer being the first to be affected before further spread to progressively more distant glands takes place. 

Thirdly there is spread via the blood stream. Once again tiny clumps of cells break off from the primary cancer but enter the surrounding blood vessels and are carried to other parts of the body, often quite distant from the primary cancer. The clumps of cancerous cells finally lodge in other organs and begin to grow there. 

Fourthly there is spread within the body cavities. For example, within the abdomen the organs are bathed in fluid and cancer cells may break away from the primary growth and float through the fluid to settle on other organs or tissues.

When a cancer spreads to the lymph glands, through the blood stream or across body cavities the new cancers which are formed are called secondary cancers. 

Secondary cancers continue to resemble the primary cancer and do not take on the characteristics of the cells in their new location. So if cells break away from a primary cancer in the breast and spread through the blood stream to lodge in the lungs the resulting tumour is a secondary breast cancer in the lung, which is completely different from a primary cancer beginning in the lung. 

The likelihood of spread varies enormously with different cancers. Some primary cancers may shed cells into the lymph or blood vessels when they are still too small to be seen by the naked eye whereas others may grow to a substantial size and still show no signs of having spread. 

Those cancers which do spread into the blood or lymph vessels will usually send off multiple clumps of cells over a period of time and so may form a number of secondary cancers which may affect several different sites in the body. 

