Cancer metastasis. Illustration of cancer cells (pink, green and yellow) 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, there are two primary cancers, one at centre right in the lungs (pink), and one in the intestines (yellow). The lung cancer has spread to the arms bones, one of the clavicles (collar bones) and to the brain. The intestinal cancer has spread to the liver (yellow), to the lungs (green) and then to the brain (green). Lymph nodes and vessels, one of the routes by which cancers spread, are also shown.