Colorectal Cancer Basic Facts What is colorectal cancer? Cancer is a disease characterized by the unchecked division of abnormal cells. When this type of growth occurs in the colon or rectum, it is called colorectal cancer (CRC). The colon and rectum (colorectum), along with the anus, make up the large intestine, the final segment of the gastrointestinal (GI) system. The large intestine is sometimes called the large bowel, which is why CRC is sometimes referred to as bowel cancer. The function of the large intestine is to absorb water and electrolytes from food matter and eliminate feces. As depicted in Figure 1, the first part of the large intestine is the colon, a muscular tube about 1.5 meters (5 feet) long and 5 centimeters (2 inches) in diameter that is divided into 4 sections: • The ascending colon begins with the cecum (a pouch where undigested food is received from the small intestine) and extends upward on the right side of the abdomen. • The transverse colon crosses the body from right to left, and is referred to collectively with the ascending colon as the proximal, or right, colon. • The descending colon descends on the left side. • The sigmoid colon, named for its “S” shape, is the final portion of the colon and is referred to collectively with the descending colon as the distal, or left, colon. Waste passes from the sigmoid colon into the rectum – the final 15 centimeters (6 inches) of the large intestine – and is then expelled through the anus (2-3 centimeters or 1 inch). Despite their anatomic proximity, cancers in the anus are classified separately from those in the rectum because they usually originate from different cell types, and thus have different characteristics. However, tumors within the colorectum also vary in their molecular, biological, and clinical features, and in their association with risk factors.1, 2 For example, physical inactivity is associated with increased risk of cancer in the colon, but not in the rectum. In addition, patients are more likely to be diagnosed with tumors in the proximal colon if they are older (versus younger), black (versus white), or female (versus male).3, 4 What is a colorectal polyp? CRC almost always begins as a polyp, which is a noncancerous growth that develops in the mucosal layer (inner lining) of the colon or rectum. Polyps are common, detected in about half (including serrated polyps) of average-risk individuals 50 years of age or older undergoing colonoscopy, with higher prevalence in older age groups and among men compared to women.5 However, fewer than 10% of polyps are estimated to progress to invasive cancer,6, 7 a process that usually occurs slowly over 10 to 20 years and is more likely as polyps increase in size.8-10 Polyps are classified based on their growth pattern as adenomatous (i.e., adenoma), which is the most common cancer precursor, or serrated, so-called because of its saw-toothed appearance under a microscope.11 Serrated Figure 1. Anatomy of the Gastrointestinal System Esophagus Stomach Transverse colon Descending colon Sigmoid colon Ascending colon Anus Liver Small intestine Gallbladder Rectum Cecum Appendix ©2017, American Cancer Society, Inc., Surveillance Research 2 Colorectal Cancer Facts & Figures 2020-2022 polyps are further subdivided based on biological characteristics into sessile serrated polyps (SSPs), traditional serrated adenomas (TSAs), and hyperplastic polyps (HPs). Similar to adenomas, SSPs, TSAs, and large HPs are associated with an increased risk for CRC. SSPs are the most difficult to detect during colonoscopy because they are usually flat, covered with a mucous cap, and colored like the surrounding tissue. These features likely contribute to their role as precursors for a large proportion of cancers diagnosed prior to the next recommended colonoscopy (interval or post colonoscopy cancers).12 What are the stages of colorectal cancer? Once a polyp progresses to cancer, it can grow into the wall of the