Colorectal cancer develops due to the uncontrollable growth of abnormal cells to form tumors that can be either benign or malignant. Several colorectal cancers originate in polyps, which are tissue growths similar to small, flat bumps or tiny mushroom-like stalks growing on the colon’s inner lining. Most colorectal cancers typically develop over time from adenomatous (precancerous) polyps. Polyps also change due to mutations (abnormalities) arising in cellular DNA. Identifying and removing precancerous polyps can be helpful in the prevention of colorectal cancer. Colonoscopy diagnoses polyps as they bulge into the colon, forming a mound on the colon wall. Most rectal and colon cancers are a type of tumor called adenocarcinoma, cancer of the cells that line the tissue inside the colon and rectum. Other types of cancer that occur less often but can begin in the colon or rectum include a neuroendocrine tumor of the gastrointestinal tract, gastrointestinal stromal tumor (GIST), small cell carcinoma, and lymphoma.
What is Colorectal Cancer?
Colorectal cancer begins when the normal cells grow out of control to form tumors 1. Colorectal cancer can be benign or malignant.
A tumor is malignant when it starts spreading to distant parts of the body. A benign tumor remains confined to its original location and doesn’t spread to other body parts. These changes usually take years to develop.
However, changes can occur in months or years when a person has an uncommon inherited syndrome.
The colon and rectum
The large intestine is a part of the body’s digestive system or gastrointestinal (GI) tract. The colon and rectum form the large intestine, which plays an essential role in the body’s ability to process waste. The colon forms the first 5 to 6 feet of the large intestine, and the rectum forms the last 6 inches, ending at the anus.
The colon and rectum have five sections. The ascending colon is the part that extends from a pouch called the caecum to the part of the colon near the liver. The caecum starts the large intestine into which the small intestine empties; it’s present on the right side of the abdomen. The transverse colon crosses the top of the abdomen. The descending colon takes waste down the left side. Finally, the sigmoid colon at the bottom takes waste down to the rectum. Waste leaves the body through the anus.
Many colorectal cancers start as polyp, which grows on the colon’s inner lining. Some polyps may change into cancer over time, though not all polyps become cancer. Most colorectal cancers typically develop over time from adenomatous (precancerous) polyps 2. Polyps can change after mutations (abnormalities) arise in their cellular DNA. If not removed or treated, a polyp can become a potentially life-threatening cancer. Finding and removing precancerous polyps can be helpful in the prevention of colorectal cancer. Polyps are most easily seen during a colonoscopy because they usually bulge into the colon, forming a mound on the colon wall that the doctor can find.
Around 10% of colon polyps are flat and difficult to find with a colonoscopy unless a dye is highlighted. These flat polyps have a significant risk of becoming cancerous, regardless of their size.
Hyperplastic polyps may develop in the colon and rectum but are not considered precancerous.
Types of colorectal cancer
Colorectal cancer may start in either the rectum or the colon. Cancer that begins in the colon is known as colon cancer, while those which originates in the rectum are called rectal cancer.
Most rectal and colon cancers are a type of tumor called adenocarcinoma, cancer of the cells that line the tissue inside the colon and rectum 3. This section covers adenocarcinoma explicitly. Other types of cancer that occur less often but can begin in the colon or rectum include a neuroendocrine tumor of the gastrointestinal tract, gastrointestinal stromal tumor (GIST), small cell carcinoma, and lymphoma.
- 1.Kuipers EJ, Grady WM, Lieberman D, et al. Colorectal cancer. Nat Rev Dis Primers. Published online November 5, 2015. doi:10.1038/nrdp.2015.65
- 2.Shussman N, Wexner SD. Colorectal polyps and polyposis syndromes. Gastroenterology Report. Published online January 23, 2014:1-15. doi:10.1093/gastro/got041
- 3.Mármol I, Sánchez-de-Diego C, Pradilla Dieste A, Cerrada E, Rodriguez Yoldi M. Colorectal Carcinoma: A General Overview and Future Perspectives in Colorectal Cancer. IJMS. Published online January 19, 2017:197. doi:10.3390/ijms18010197