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What Is Colorectal Cancer?

Colorectal cancer starts in the colon or the rectum. These cancers can also be called colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer are often grouped together because they have many features in common.

The colon and rectum

To understand colorectal cancer, it helps to know about the normal structure and function of the colon and rectum.

The colon and rectum make up the large intestine (or large bowel), which is part of the digestive system, also called the gastrointestinal (GI) system (see illustration below).

Most of the large intestine is made up of the colon, a muscular tube about 5 feet (1.5 meters) long. The parts of the colon are named by which way the food is traveling through them.

  • The first section is called the ascending colon. It starts with a pouch called the cecum, where undigested food comes in from the small intestine. It continues upward on the right side of the abdomen (belly).
  • The second section is called the transverse colon. It goes across the body from the right to the left side.
  • The third section is called the descending colon because it descends (travels down) on the left side.
  • The fourth section is called the sigmoid colon because of its “S” shape. The sigmoid colon joins the rectum, which then connects to the anus.

The ascending and transverse sections together are called the proximal colon. The descending and sigmoid colon are called the distal colon.

color illustration of the digestive system which shows the location of the esophagus, stomach, pancreas, transverse colon, descending colon, sigmoid colon, anus, rectum, appendix, cecum, ascending colon, small intestine, gallbladder and liver

How do the colon and rectum work?

The colon absorbs water and salt from the remaining food matter after it goes through the small intestine (small bowel). The waste matter that’s left after going through the colon goes into the rectum, the final 6 inches (15cm) of the digestive system. It’s stored there until it passes through the anus. Ring-shaped muscles (also called sphincters) around the anus keep stool from coming out until they relax during a bowel movement.

How does colorectal cancer start?

Polyps in the colon or rectum

Most colorectal cancers start as a growth on the inner lining of the colon or rectum. These growths are called polyps.

Polyps are quite common, especially as you get older. Most polyps are benign, or noncancerous. Some types of polyps can change into cancer over time (usually over many years). The chance of a polyp turning into cancer depends on the type of polyp it is. There are different types of polyps.

  • Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because of this, adenomas are called a precancerous condition. The 3 types of adenomas are tubular, villous, and tubulovillous. Tubular adenomas are the most common type of adenomatous polyps. Villous adenomas are the least common type of adenomatous polyps, but are more likely to change into cancer.
  • Hyperplastic polyps and inflammatory polyps: These polyps are more common, but in general they are not precancerous. Some people with large (more than 1cm) hyperplastic polyps might need colorectal cancer screening with colonoscopy more often.
  • Sessile serrated polyps (SSP) and traditional serrated adenomas (TSA): These polyps are often treated like adenomas because they have a higher risk of changing into cancer.

Other factors that can make a polyp more likely to contain cancer or increase someone’s risk of developing colorectal cancer include:

  • Size: If a polyp larger than 1 cm
  • Number: If more than 3 polyps are found
  • Histology: If dysplasia is seen in the polyp. Dysplasia means that the cells look abnormal, but they haven’t yet become cancer.

    For more details on the types of polyps and conditions that can lead to colorectal cancer, see Your Colon or Rectal Pathology Report: Polyps.

    How colorectal cancer spreads

    If cancer forms in a polyp, it can grow into the wall of the colon or rectum over time. The wall of the colon and rectum is made up of many layers. Colorectal cancer starts in the innermost layer (the mucosa) and can grow outward through some or all of the other layers (see picture below).

    When cancer cells are in the wall, they can then grow into blood vessels or lymph vessels (tiny channels that carry away waste and fluid). From there, they can travel to nearby lymph nodes or to distant parts of the body.

    The stage (extent of spread) of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum. For more on staging, see Colorectal Cancer Stages.

    illustration showing a cross section of the digestive tract (normal intestinal tissue) and details of the layers of the colon wall (including the mucosa (epithelium, connective tissue, thin muscle layer), submucosa, thick muscle layers, subserosa and serosa)

    Types of cancer in the colon and rectum

    Most colorectal cancers are adenocarcinomas. These cancers start in cells that make mucus to lubricate the inside of the colon and rectum. When doctors talk about colorectal cancer, they’re almost always talking about this type. Some subtypes of adenocarcinoma, such as signet ring and mucinous, may have a worse prognosis (outlook) than other subtypes of adenocarcinoma.

    Other, much less common types of tumors can also start in the colon and rectum. These include:

    • Carcinoid tumors. These start from special hormone-making cells in the intestine. See Gastrointestinal Carcinoid Tumors.
    • Gastrointestinal stromal tumors (GISTs) start from nerve cells in the wall of the gastrointestinal tract. Some are benign (not cancer). These tumors are most commonly found in the stomach and small intestine. They are not commonly found in the colon or rectum.  See Gastrointestinal Stromal Tumor (GIST).
    • Lymphomas are cancers of immune system cells. They mostly start in lymph nodes, but they can also start in the colon, rectum, or other organs. Information on lymphomas of the digestive system can be found in Non-Hodgkin Lymphoma.
    • Sarcomas can start in blood vessels, muscle layers, or other connective tissues in the wall of the colon and rectum. Sarcomas of the colon or rectum are rare. See Soft Tissue Sarcoma.

    The American Cancer Society medical and editorial content team

    Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.

    American Cancer Society. Colorectal Cancer Facts & Figures 2023-2025. Atlanta, Ga.

    Dekker E, Tanis PJ, Vleugels JLA, Kasi PM, Wallace MB. Colorectal cancer. Lancet. 2019 Oct 19;394(10207):1467-1480. doi: 10.1016/S0140-6736(19)32319-0. PMID: 31631858.

    Lawler M, Johnston B, Van Schaeybroeck S, Salto-Tellez M, Wilson R, Dunlop M, and Johnston PG. Chapter 74 – Colorectal Cancer. In: Niederhuber JE, Armitage JO, Dorshow JH, Kastan MB, Tepper JE, eds. Abeloff’s Clinical Oncology. 6th ed. Philadelphia, Pa. Elsevier: 2020.

    Li J, Ma X, Chakravarti D, Shalapour S, DePinho RA. Genetic and biological hallmarks of colorectal cancer. Genes Dev. 2021 Jun;35(11-12):787-820. doi: 10.1101/gad.348226.120. PMID: 34074695; PMCID: PMC8168558.

    Libutti SK, Saltz LB, Willett CG, and Levine RA. Ch 62 - Cancer of the Colon. In: DeVita VT, Hellman S, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg’s Cancer: Principles and Practice of Oncology. 11th ed. Philadelphia, Pa: Lippincott-Williams & Wilkins; 2019.

    Libutti SK, Willett CG, Saltz LB, and Levine RA. Ch 63 - Cancer of the Rectum. In: DeVita VT, Hellman S, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg’s Cancer: Principles and Practice of Oncology. 11th ed. Philadelphia, Pa: Lippincott-Williams & Wilkins; 2019.

    National Comprehensive Cancer Network (NCCN). NCCN Clinical Practice Guidelines in Oncology: Colorectal Cancer Screening. V.1.2023. Accessed at https://www.nccn.org/professionals/physician_gls/pdf/colorectal_screening.pdf on Jan 29, 2024.

    Last Revised: January 29, 2024

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