Cancer metastasis. Illustration of cancer cells (green) spreading inside the human body. Some cancer cells are highly mobile and spread rapidly round the body, invading other organs and tissues. This is known as metastasis. The cells may clump to form tumours known as secondary cancers. Once a cancer has spread, it can be much harder to treat and the prognosis can be poor. Here, the primary cancer is at bottom centre in the rectum, and has spread to the liver and lungs. Lymph nodes and vessels, one of the routes by which cancers spread, are also shown.