The defining feature of a cancerous tumour is its ability to spread not only locally but also to distant sites in the body by a process called metastasis. In metastasis, a cancerous cell detaches from a tumour and travels in the blood or lymph to a new location. The cell must overcome many obstacles if it is to settle in a new site and form a secondary tumour, also known as a metastasis. It must survive attacks from the immune system and stimulate the growth of blood vessels (angiogenesis) to provide oxygen and nutrients.
Cancerous cells may spread into the lymphatic system, which is a network of vessels that drains lymph fluid to nearby lymph nodes, where the fluid is filtered. A cell may become trapped in a lymph node and multiply to form a tumour. Immune cells in the lymph node attack the tumour and may halt the progression of the cancer.
As a tumour grows, it invades surrounding tissues and can enter nearby lymph vessels. If some of the cancerous cells from the tumour detach themselves in the lymph vessel, they may be carried along the vessel until they reach and settle in a lymph node.
A cancerous cell enters a local lymph node, where it begins dividing to form a tumour. The tumour usually remains within the node, and the cells of the immune system may temporarily stop it from spreading to other parts of the body.
Cancer often spreads to sites in the body that have a good blood supply, such as the liver, lungs, bone, and brain. The liver is a particularly common site since it receives blood from the heart and intestines. When cancerous cells reach very small blood vessels, they pass through the walls to invade tissues.
The growing tumour ruptures the walls of nearby blood vessels. Some cancerous cells detach from the tumour and pass through blood vessel walls into the blood circulation.
A cancerous cell travels in the bloodstream until it becomes lodged in a capillary (tiny blood vessel) at a distant site. The cell then starts dividing to form a secondary tumour.
A cancerous cell obtains oxygen and nutrients from surrounding blood vessels by diffusion across its outer membrane, in the same way as normal cells. As the tumour enlarges, its inner cells are starved of nutrients. Enlarging tumours use two methods of acquiring nutrients – by invading existing blood vessels and by angiogenesis, the process by which a tumour stimulates new blood vessels to form. To survive and grow, a tumour must be able to stimulate angiogenesis successfully.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.