Colon cancer


Most colon cancers begin as a growth called a polyp and develop slowly over many years. Screenings, like colonoscopies, give your doctor the opportunity to remove polyps before they become cancerous or to spot cancer at an early stage, when it is easier to cure. 

Seattle Cancer Care Alliance (SCCA) offers comprehensive treatment for colon cancer from a team of experts who specialize in gastrointestinal cancers.

What is colon cancer?

Cancer can begin in either the colon or the rectum, and it may be called colon cancer or rectal cancer, based on where it started. Sometimes these cancers are referred to together as colorectal cancer. 

  • As you eat, your stomach secretes gastric juices that break down your food. The food and gastric juices mix into a thick fluid that empties into your small intestine. 
  • Your small intestine breaks down the food further, absorbs most of the nutrients and joins your colon.
  • Your colon absorbs water and nutrients from your food and stores waste before it exits through your rectum and anus. 
  • Together your colon and rectum form your large intestine. The upper 5 to 6 feet of the large intestine are the colon, and the lower 6 inches are the rectum. 

There are many similarities between colon and rectal cancers, but there are some differences in the ways they are usually treated.

Polyps and colorectal cancer

In most cases, colon and rectal cancers develop slowly over many years. Most of these cancers begin as a growth of tissue called a polyp in the inner lining of the colon or rectum. Usually polyps bulge into the colon or rectum; some are flat.

  • Polyps are common in people over 50. 
  • Most polyps are benign (noncancerous), but some may turn into cancer. 
  • Removing a polyp early may prevent it from turning into cancer.
  • Certain kinds of polyps are more likely to become cancerous, especially adenomatous polyps (also called adenomas). 
  • Adenomas can turn into cancers called adenocarcinomas. 


More than 95 percent of colorectal cancers are adenocarcinomas. The information in this colon cancer section is about this type.

Other less common types of colorectal cancers include: 

  • Gastrointestinal carcinoid tumors
  • Gastrointestinal stromal tumors
  • Lymphomas


Most people don’t have signs or symptoms of colon or rectal cancer early on. That’s why it’s important to have regular colorectal cancer screenings, which can detect cancer at early stages when the cure rate is high. 

Cancer that starts in different areas of the colon or in the rectum may cause different signs and symptoms. If you notice any of the following signs or symptoms, let your doctor know:

  • Weight loss for no known reason
  • An ongoing bloated feeling, cramping or pain in your abdomen 
  • Constant tiredness and weakness 
  • A change in bowel habits that lasts for more than a few days, such as diarrhea, constipation, narrow stools or feeling that your bowel does not empty completely 
  • A feeling that you need to have a bowel movement that doesn’t go away, even after going to the bathroom 
  • Bright red or very dark blood in your stool or bleeding from your rectum 
  • Low level of red blood cells (anemia) for no known reason

Usually these signs or symptoms are caused by other conditions, like infections, hemorrhoids or inflammatory bowel disease, not cancer. Your doctor can help you figure out just what is causing your symptoms.

  • Learn more about early detection of colon cancer
  • Learn about colon cancer prevention for people at high risk.


If you’re having an evaluation for colorectal cancer at SCCA, most likely you’ve already been diagnosed with colorectal cancer. Your referring doctor based your initial diagnosis on screenings, examinations and tests that may have included the following:

  • Medical history, family history and physical exam — Your doctor asks questions about symptoms and risk factors, including family history, and examines you. This may include a digital rectal examination, in which the doctor inserts a lubricated, gloved finger into your rectum to feel for abnormal areas or masses. 
  • Fecal occult blood test — This simple, at-home test checks for blood in your stool. Studies show that doing this test every one to two years in people ages 50 to 80 reduces deaths from colorectal cancer by as much as 30 percent.
  • Fecal immunochemical test (FIT) — This is another simple, at-home test to check for blood in the stool. A version known as FIT-DNA (such as Cologuard) also checks cells from your colorectal lining for biomarkers that may indicate cancer or a precancerous condition. 
  • Flexible sigmoidoscopy — Your doctor looks at your rectum and lower colon using a thin, lighted tube called a sigmoidoscope. This exam can find precancerous and cancerous polyps and tumors. Regular screening with sigmoidoscopy after age 50 can reduce the number of deaths from colorectal cancer.
  • Colonoscopy — Using a thin, lighted tube similar to the sigmoidoscope but longer, your doctor examines the inside of your entire colon for polyps, tumors and abnormal tissue. This is an important step if other methods suggest you may have cancer.
  • Biopsy — A pathologist, a doctor who specializes in evaluating tissue samples, uses a microscope to look at polyps or tumor samples removed from your colon or rectum during your sigmoidoscopy or colonoscopy. The pathologist can see whether the cells are cancer. 

To fully understand your cancer and recommend a treatment plan, your SCCA team will review your referring doctor’s findings and may order one or more additional tests, such as:

  • A repeat colonoscopy — if your previous procedure was incomplete
  • Imaging studies — such as a computed tomography (CT) scan, positron emission tomography (PET) scan, magnetic resonance imaging (MRI) scan, endoscopic ultrasound or bone scan
  • Blood tests — such as to check your levels of blood cells or your liver function


The treatment that your doctor will recommend for colorectal cancer will be based in part on the stage of your cancer. The stage depends on:

  • How far the cancer has spread through the wall of your colon or rectum
  • Whether the cancer has spread to lymph nodes around your colon or rectum
  • Whether the cancer has spread to other parts of your body, such as your liver or lungs

Colon cancers are grouped into stages I through IV, with stage I being the least advanced and stage IV being the most advanced.

What causes colon cancer?

The exact cause of colorectal cancer is not known. However, studies show that certain factors are linked to increased risk. 

SCCA’s Gastrointestinal Cancer Prevention Program offers a personalized approach to risk assessment, screening and prevention for people at high risk for colon cancer and other gastrointestinal cancers. 

Risk factors you can't change
  • Age — Risk rises as you get older and goes up significantly after age 50 (but younger people can get colorectal cancer, too).
  • History of cancer — If you’ve had colorectal cancer already or ovarian or uterine cancer, you are at higher risk.
  • History of polyps — Most are benign, but some may turn into cancer. Adenomatous polyps are most likely to become cancerous. 
  • History of inflammatory bowel disease — This includes ulcerative colitis and Crohn’s colitis (or Crohn’s disease). 
  • Race and ethnicity — African-Americans are at greater risk of developing colorectal cancer and of dying from the disease than any other racial or ethnic group in the United States. Ashkenazi Jews may have inherited changes in their DNA that increase their risk.
  • Family history of colorectal cancer or polyps — You are at increased risk if a parent, sibling or child had colorectal cancer or polyps before age 60 or two or more relatives had either condition at any age.
  • Familial adenomatous polyposis — This rare, hereditary condition causes hundreds of colorectal polyps. It can appear as early as the teen years and is very likely to lead to cancer.
  • Lynch syndrome — People with this hereditary condition tend to develop cancer at a young age without first having many polyps.
Lifestyle risk factors
  • Smoking — People who have smoked are more likely than nonsmokers to get colorectal cancer and to die from the disease.
  • Diet — Diets high in red meats and processed meats increase risk. Diets high in vegetables and fruits decrease risk. 
  • Exercise — People who aren’t active are at higher risk. Those who exercise regularly are at lower risk. 
  • Obesity — Being very overweight increases risk. 
  • Alcohol — Heavy use of alcohol has been linked to colorectal cancer.
  • Type 2 diabetes — People with type 2 diabetes (which is influenced by lifestyle factors) are more likely to develop colorectal cancer and may do worse after diagnosis.

Using aspirin, nonsteroidal anti-inflammatories or postmenopausal hormones might reduce risk of polyps or colorectal cancer. Talk with your doctor to learn more.