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It starts on the inside of the bowel wall. The cancer cells grow by dividing themselves, and then start to invade nearby healthy cells, growing deeper into the bowel wall.
Eventually the tumour may grow through the bowel wall, and cancer cells from it may get in to the blood stream or the lymphatic system. The cells can then be carried and spread to other parts of the body - usually the liver. (This is because the blood supply from the intestines drains in to the liver.) Less often, cells may travel further, and settle in the lung or other parts of the body.
When cancer cells from a tumour settle in other parts of the body and start growing there, they are called secondary cancers (metastases).
