Colorectal cancer is a type of cancer that starts in the large intestine (colon) or rectum. Sometimes it is called rectal cancer because the colon and rectum are connected by a passageway, the anus. Cancer in these organs is the growth of abnormal cells that can invade and destroy healthy tissue. This cancer can also spread to other parts of the body.

Most of the time, what is colorectal cancer cancer begins in polyps – small overgrowths of tissue in the lining of the colon and rectum. These may or may not produce symptoms. Most polyps are non-cancerous but some can turn into cancer over time. These pre-cancerous polyps are called adenomas. Some inherited conditions increase the risk of developing colorectal cancer, such as familial adenomatous polyposis and hereditary nonpolyposis colon cancer (Lynch syndrome).

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The most common sign of colorectal cancer is blood in stool or changes in the way bowel habits work. People with these signs should see their health care provider right away.

If the cancer is in an early stage, surgery may cure it. But doctors often use chemotherapy and radiation to reduce the chance that cancer will come back. Chemotherapy uses medicines to kill cancer cells or slow their growth. They may be given alone or with other treatments such as a colonoscopy or sigmoidoscopy, radiation therapy or immunotherapy.

Immunotherapy uses medicines to boost your own immune system to help fight the cancer or to prevent it from coming back after treatment. This is used for a small number of advanced colorectal cancers, such as those that have certain genetic mutations like KRAS or BRAF mutations, or have a pattern of mismatch repair deficiency.