The colon and rectum together form the lower part of the digestive tract. The colon is divided into four distinct parts. The first section, ascending colon, travels up the right side of the abdomen, turns left, and continues into the transverse colon. After traveling across the abdomen, the transverse colon turns right and continues downward, becoming the descending colon. The fourth and final section is the S-shaped sigmoid colon, which meets the rectum, and then joins the anus.
Cancer can invade any of the four sections of the colon, or the rectum.
Colorectal cancer starts with a single cell that mutates and grows into a visible polyp. A polyp is a mass of benign tissue found in the inner lining of the colon. If a polyp is allowed to remain in the colon, it can grow into a cancerous tumor that can invade other organs. Researchers don't really understand exactly how a polyp progresses to cancer, but most polyps take 3-7 years to become cancerous. Colorectal cancer is often curable and a highly treatable disease when it is diagnosed early and localized.