Cancer happens when cells in your body go through changes that make them grow out of control. Colorectal cancer is cancer that starts in either your colon or your rectum, which together make up the lower part of your digestive tract. It is the third most common cancer for both men and women. Cancer usually does not start in both the colon and rectum. But both types of cancer have a lot in common. So they are often referred to as colorectal cancer.
Most cancers in the colon or rectum begin in the inner lining as a polyp. A polyp is an abnormal growth of tissue and can also occur in other places besides your colon or rectum, such as inside your nose. Some polyps change into cancer, but not all do. The chance of changing into cancer depends on the type of polyp it is.
When a colon cancer is identified during a routine screening, it may have already been growing for several years. The original cancer is called the primary tumor. Your colon has several layers of tissue, and as the primary tumor grows, it extends outward through these layers. Once it has grown through all the layers, the cancer can spread to nearby tissue or organs. Sometimes, by the time colorectal cancer is found, some of the cells have already spread to other parts of the body, such as lymph nodes, liver, or lungs. This process is called metastasis, and it's what makes colorectal cancer especially life-threatening. That doesn't mean you can't treat colorectal cancer. You and your health care team have a lot of treatment options to choose from once you all understand the nature of your cancer.