Cancer that begins in the colon is called colon cancer, and the cancer that begins in the rectum is called rectal cancer. Cancers affecting either of these organs also may be called colorectal cancer. It is a disease which malignant (cancer) cells first form in the tissues of the colon. Colorectal cancer occurs in rectum or the large intestine. This type of cancer occurs when abnormal tissues grow on the inner walls of the colon and rectum. These abnormal tissues commonly present in the form of polyps. Polyps grow as a projection of tissue away from the colon wall, remaining connected to the colon wall of a thin stalk. Their shape is similar that of a mushroom. Polyps are fairly common, especially in older people. The vast majority of polyps are not cancerous. However, some polyps will eventually become cancerous. Unchecked, a cancerous polyp gives rise to a tumor, which grows in size until it penetrates to bowel wall and involves adjacent organs and lymph nodes through the process known as metastasis.
In general, colorectal cancers tend to be slow growing, gradually enlarging and eventually penetrating the bowel wall. When they do spread, it is usually through invasion of nearby lymph nodes. On the right side of the colon near the cecum, cancers usually grow into the space within the colon. They can become large enough to be painful and are likely to cause bleeding. In these cases anemia from chronic blood loss is often the first sign and is why a stool test for occult, or hidden, blood is important.