Colorectal cancer starts in the colon or the rectum. These cancers can also be referred to separately as colon cancer or rectal cancer, depending on where they start.
Colon cancer and rectal cancer have many features in common. Most colorectal cancers develop slowly over several years. Before a cancer develops, a growth of tissue or tumor usually begins as a non-cancerous polyp on the inner lining of the colon or rectum. A polyp is a benign, non-cancerous tumor. Some polyps can change into cancer but not all do. If cancer forms into a polyp, it can eventually begin to grow into the wall of the colon or rectum. When cancer cells are in the wall, they can then grow into blood vessels or lymph vessels. Lymph vessels are thin, tiny channels that carry away waste and fluid. They first drain into nearby lymph nodes, which are bean-shaped structures containing immune cells that help fight against infections. Once cancer cells spread into blood or lymph vessels, they can travel to nearby lymph nodes or to distant parts of the body, such as the liver. If spread to distant parts of the body, it is called metastasis.