The Basics of Colon and Rectal Cancers

Understanding the similarities and the differences.

The American Cancer Society’s estimates for the number of colorectal cancer cases in the United States for 2022 are:

  • 106,180 new cases of colon cancer

  • 44,850 new cases of rectal cancer

(View Article)

Colorectal cancer is the third leading cause of cancer-related deaths in the United States among both men and women. Colon cancer and rectal cancer are often grouped together because they have a lot in common—including some symptoms—but they are different. While the colon and rectum are both part of the large intestine, the colon is approximately five feet long and the rectum is the last five to six inches of the colon that connects to the anus. It is the job of the rectum to act as a storage container and hold the stool until defecation occurs. Cancers that develop in these two parts of the body are called colon and rectal cancers respectively.

The majority of colorectal cancers generally develop over time from precancerous growths called polyps.

These polyps usually grow slowly and do not cause symptoms until they become large or cancerous. Some of the risk factors for colorectal cancer involve a family history of colon or rectal cancer, diet, excess alcohol intake, smoking and inflammatory bowel disease.

Colon cancer can cause both constipation and diarrhea. This may be accompanied by cramping in the stomach. The stool may be streaked or mixed with blood. In rectal cancer, the most common symptom is usually bleeding when going to the bathroom.

Common signs of colorectal cancer can include the following:

  • A change in bowel habits including constipation, diarrhea, pencil thin stools, incomplete evacuation, and bowel incontinence.

  • Blood in or on your stool during a bowel movement. Remember that other health conditions can also cause bleeding including hemorrhoids, anal tears and inflammatory bowel disease including Crohn’s or Ulcerative colitis.

  • Abdominal pain, aches, pelvic cramps, or unexplained bloating that won't go away.

  • Unexplained weight loss.

  • Unexplained vomiting that doesn't go away.

  • Unexplained anemia which is a shortage of red blood cells— the cells that carry oxygen throughout the body. If you are anemic, you may also experience shortness of breath, feel constantly tired and sluggish, so much so that rest does not make you feel better. This anemia is caused by bleeding in bowel movements.

Routine screening is important for early diagnosis.

There are a variety of colorectal polyps, but cancer is thought to arise mainly from adenomas and sessile serrated lesions, which are precancerous polyps. If a polyp is found during a colonoscopy it is usually removed. Polyps removed during colonoscopies are then evaluated under the microscope to determine if they contain cancerous or precancerous cells. Based on the number, size, and type of precancerous polyps found during colonoscopy, your health care team will recommend continued regular surveillance and monitoring. Some colorectal cancers might be present without any signs or symptoms and this makes screening even more important.

Regular screening, beginning at age 45, is the key to preventing colorectal cancer and finding it early. The U.S. Preventive Services Task Force recommends that adults age 45 to 75 be screened for colorectal cancer. Most people should begin screening for colorectal cancer soon after turning 45, then continue getting screened at regular intervals.

You may need to be tested earlier than 45, or more often than other people, if you have:

  • Inflammatory bowel disease such as Crohn’s disease or ulcerative colitis.

  • A personal or family history of colorectal cancer or colorectal polyps.

  • A genetic syndrome such as Lynch syndrome or familial adenomatosis polyposis.

Overall, the lifetime risk of developing colorectal cancer is: about 1 in 23 (4.3%) for men and 1 in 25 (4.0%) for women. (View Article)

About 75% of people who do get colorectal cancer do not get it because of genetics. About 10% to 30% do have a family history of the disease. Some studies have found that having a first-degree relative with colorectal cancer puts you at a risk that is 2-3 times higher than someone without a first-degree relative (mother, father, brother, sister or your child) with colorectal cancer. Your risk can also be higher if you have other people in your family with colorectal cancer, even if they are not first-degree relatives. The age at which any relative is diagnosed is also important. The risk to you is more significant when the relative is diagnosed before age 45.

Screening can be done in a variety of ways although the gold standard or most recommended is colonoscopy.

Stool Tests

  • The guaiac-based fecal occult blood test (gFOBT) uses the chemical guaiac to detect blood in the stool. It is done once a year. For this test, you receive a test kit from your healthcare provider. At home, you use a stick or brush to obtain a small amount of stool. You return the test kit to the doctor or a lab, where the stool samples are checked for the presence of blood.

  • The fecal immunochemical test (FIT) uses antibodies to detect blood in the stool. It is also done once a year in the same way as a gFOBT.

  • The FIT-DNA test (also referred to as the stool DNA test) combines the FIT with a test that detects altered DNA in the stool. For this test, you collect an entire bowel movement and send it to a lab, where it is checked for altered DNA and for the presence of blood. It is done once every three years.

Flexible Sigmoidoscopy

For this test, the doctor puts a short, thin, flexible tube with a light and tiny camera on one end into your rectum. This is a brief outpatient procedure, often performed without sedation. The bowel must be empty for this procedure which is typically done with the help of a laxative and or enema before the test. The doctor checks for polyps or cancer inside the rectum and lower third of the colon. This test is done every 5 years, or every 10 years with a FIT every year.


This is similar to flexible sigmoidoscopy, except the doctor uses a much longer tube to check for polyps or cancer inside the rectum and the entire colon. The bowel must be cleaned-out with the help of a laxative before the procedure begins. A sedative is usually given for this procedure to make it comfortable. A colonoscopy is considered a safe procedure with few risks. During the test, the doctor can find and remove most polyps and some cancers. Colonoscopy also is used as a follow-up test if anything unusual is found during one of the other screening tests. This test is done once every 10 years (for people who do not have an increased risk of colorectal cancer).

CT Colonography (Virtual Colonoscopy)

Computed tomography (CT) colonography, also called a virtual colonoscopy, is a CT scan of the abdomen and pelvis performed after drinking a contrast dye and inflating contrast and air into the rectum. No sedation is needed for this test. Like colonoscopy, the colon must be cleaned out before the examination. If a polyp is found, then a colonoscopy must be performed. This test is recommended every 5 years.

Each test has advantages and disadvantages. Talk to your health care team about the pros and cons of each test, and how often you should be tested based on your risk factors.

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Reason to Hope

In a very small trial (18 patients with Stage 2 or Stage 3 rectal cancer) done by doctors at New York's Memorial Sloan Kettering Cancer Center, patients took a drug called dostarlimab for six months. The trial resulted in every single one of their tumors disappearing. These kinds of results have never been seen in the history of cancer research.  American Psychologist, 2018; DOI: 10.1037/amp0000309