Cancer metastasis. Illustration of cancer cells (blue, yellow, pink and 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, there are four primary cancers, two in the intestines (blue and yellow) and two in the lungs (pink and green). The cancers have spread to the arms bones, one of the clavicles (collar bones), to the liver, to the lungs, and to the brain. The intestinal cancer has spread to the lungs and the brain.