Colorectal cancer is cancer that develops in the last part of your digestive tract — your colon and rectum. Your colon is the last five or six feet of your large intestine. Your rectum is the last six inches.

Colorectal cancer occurs when cells of the colon or rectum start to grow out of control and form a tumor. Often, the cancer starts in a non-cancerous (benign) growth called an adenomatous polyp. Once cells become cancer, however, the polyp becomes a malignant tumor. A tumor can grow and crowd out normal cells. Cancer cells can also spread away from the tumor to other parts of the body, like the lungs or liver. This is called metastasis.

Almost all colorectal cancers start in cells that make mucous to lubricate the inside of the colon and rectum. This type of cancer is called adenocarcinoma.

Colorectal cancer is the fourth most common cancer in the United States. More than 135,000 Americans are diagnosed with this cancer each year. You may be at higher risk for colorectal cancer if you:

  • Are older than 50

  • Are male

  • Have a family history of colorectal cancer

  • Have inflammatory bowel disease (ulcerative colitis or Crohn’s disease)

  • Have adenomatous polyps

  • Are African American

  • Are inactive or overweight

  • Have a diet high in red or processed meats

  • Smoke cigarettes

Doctors try to test for colorectal cancer before any sign or symptoms develop. That's known as screening. Screening can find polyps before they become cancer. It can take 10 to 15 years for a polyp to become cancer so removing polyps can help prevent colorectal cancer. For most people, screening for colorectal cancer starts at age 50, though those at higher risk may start at age 45 or younger. Common ways of screening include:

  • Having a sigmoidoscopy or colonoscopy every 5 to 10 years. These tests involve looking into the rectum and colon with a flexible telescope.

  • Having a stool sample tested for blood and DNA changes in colorectal cells

To diagnose colorectal cancer, a sample of cancer cells (biopsy) is usually needed. The biopsy may be taken at the time of a screening colonoscopy or sigmoidoscopy. A colonoscopy or sigmoidoscopy may also be done if you have signs or symptoms of colon cancer. These include:

  • A change in bowel habits, such as diarrhea or constipation

  • Blood in the stool

  • Stools that are narrow and thin

  • Belly pain or gas pain

  • Weight loss

  • Fatigue

If a biopsy shows colorectal cancer, other tests will be done to see how advanced the cancer is and whether it has spread to other parts of the body. These tests include blood tests and imaging studies such an X-ray, CT scan, MRI, ultrasound or PET scan. Once your doctor has the needed information from these tests, your cancer can be staged. Colorectal cancer stages range from 1 to 4, with stage 4 being the most advanced cancer.

Treatment of colorectal cancer depends on the stage of your cancer, the location, your age, your overall health and your thoughts about the risks and benefits of your treatment options. Surgery is the most common treatment, but other treatments are also important. They include chemotherapy, radiation therapy, targeted therapy and immunotherapy.