Understanding Colorectal Cancer

Colorectal cancer is the third most common cancer after skin cancer in both men and women. But it’s highly treatable when caught early.

Important things to know:

  • Most colorectal cancers are caught early, thanks to colonoscopies and other screening tests.
  • Colorectal cancer includes colon cancer and rectal cancer. Together, the colon and rectum make up the large intestine. The two cancers have differences, however, including in how they’re treated.
  • Symptoms often do not appear until later stages, so these cancers are often referred to as “silent killers.”
  • A risk for some forms of colorectal cancer is passed down in families. 

Anatomy of the large intestine

Medical illustration of parts of the colon, including cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum
Together, the colon and rectum form the large intestine. The colon runs about 5 feet long and is divided into five parts. The rectum, about 6 inches long, connects to the anus to expel stool.

What is colorectal cancer?

What are the colon and rectum? Your colon and rectum make up your large intestine or large bowel. This long, hollow organ is responsible for the last part of digestion. The colon is about 5 feet long, and the rectum about 6 inches long. The rectum stores solid waste until you eliminate it through your anus in a bowel movement. 

Polyps: Cancer happens when cells on the inside surface grow out of control, forming abnormal growths (polyps). Polyps are often noncancerous but can become cancerous if left untreated. As the cancer grows, it may invade deeper layers, where it’s harder to treat. Sometimes cancer cells break off and spread (metastasize) to other parts of the body.

Who gets colorectal cancer?

Colorectal cancer is one of the most common cancers in the U.S. An estimated 153,000 new cases of colorectal cancer are expected in the U.S. in 2023.

The median age at diagnosis is 66 years old. But people aged 45 or older face a higher risk of colorectal cancer. See below for more risk factors.

Common risk factors

Risk factors for colorectal cancer include:

  • Family history: Having a close relative, such as a parent or sibling, with a history of colorectal cancer increases your risk. Some hereditary syndromes increase risk (read more below), though families also share lifestyle factors such as diet.
  • Ongoing gastrointestinal problems: Inflammatory bowel disease, which includes ulcerative colitis and Crohn’s disease, is tied to colorectal cancer.
  • Diet: Eating foods low in fiber and high in animal fats such as red meat can increase risk.
  • Weight: Being overweight or obese can raise your risk.
  • Tobacco use: Cigarettes, cigars and chewing tobacco can raise risk.
  • Inactivity: Lack of regular exercise increases risk.
  • Race and ethnicity: Jews of Eastern European descent (Ashkenazi) and Black people face a higher risk.

Inherited risk factors

Colorectal cancer risk is sometimes passed down from parents through inherited genetic changes, or mutations, called hereditary cancer syndromes. Inheriting a genetic change does not automatically mean you will develop cancer, however. OHSU offers expert genetic testing and counseling to help you identify and manage risk.

Hereditary cancer syndromes that increase the risk of colorectal cancer include:

  • Familial adenomatous polyposis
  • Lynch syndrome (also called hereditary nonpolyposis colorectal cancer, or HNPCC)
  • MUTYH-associated polyposis
  • Peutz-Jeghers syndrome

Colorectal cancer survival rates

Researchers gather data from across the U.S. to calculate the percentage of people who survive at least five years after diagnosis compared with the general population. This is called the relative survival rate, and it excludes the risk of dying from something else.

The numbers are averages and cannot predict the course for an individual, however. They are also, by definition, based on treatments available five or more years ago. 

  • About 91% of people with colorectal cancer live at least five years after diagnosis if the cancer is found before it spreads from its original site.
  • About 35% are diagnosed in early stages, before the cancer has spread.
  • For all patients, the five-year relative survival rate is 65.0%.

Symptoms of colorectal cancer

Early stages of colorectal cancer often have no symptoms. As the cancer grows, it may affect the body’s ability to do its job. Symptoms, from mild to severe, may resemble those of other gastrointestinal conditions, including irritable bowel disease. It’s important to see your doctor if symptoms persist. 

Symptoms include:

  • Changes in bowel habits, such as frequent diarrhea or narrow stools.
  • Abdominal discomfort, including cramping, bloating and gas.
  • Blood or mucus in stools.
  • Changes in appetite.
  • Unexpected weight loss.

Colorectal cancer screening

Colorectal cancer is highly treatable when caught early, so screening is important. Tests help us detect polyps and cancer, even if you have no symptoms. 


OHSU research shows that colonoscopy is the best way to find colorectal cancer early. During a colonoscopy, we use a thin, flexible tube with a camera and light at the tip to examine the entire length of the colon and rectum. If polyps are found, they are often removed in the same procedure. 

OHSU screening recommendations for colonoscopy are:

  • Every 10 years starting at age 45, even if you have no risk factors.
  • Earlier and more frequently if you have risk factors. For example, if you have a family history of colorectal cancer, experts recommend a colonoscopy every five years starting at age 40. 

Other screening tests

Some people are not able to have a colonoscopy because of medical problems. These include damaged intestines, such as microscopic tears from inflammatory bowel disease. Your doctor may recommend other tests, such as:

  • Fecal occult blood test: A stool sample is checked under a microscope for signs of blood, which could indicate polyps or cancer. There are two types of this test:
    • The guaiac fecal occult blood test, or gFOBT
    • Fecal immunochemical testing, or FIT
  • Stool DNA test: A stool sample is examined for gene mutations connected to colorectal cancer.
  • Sigmoidoscopy: Using a thin, flexible tube with a camera and light at the tip, we examine the rectum and lower portion of the colon. We use sigmoidoscopy to evaluate symptoms including pain and a change in bowel habits. These could be a sign of other gastrointestinal problems.
  • CT colonography: A CT scan uses X-rays to create 3D images of the colon and rectum to check for polyps or cancer. Air is pumped in to expand the tissues for better images.
  • Blood tests: A blood sample is examined under a microscope to check the number of blood cells, in case a tumor causes internal bleeding. Blood can also contain a “tumor marker” — a protein called carcinoembryonic antigen or CA 19-9 sometimes produced by colorectal tumors. Blood tests alone are not used to diagnose colorectal cancer, though.
  • Other: A capsule colonoscopy sends a tiny wireless camera through the digestive tract. In 2016, the U.S. Food and Drug Administration also approved a blood test called a SEPT9 gene methylation assay to screen for colorectal cancer. We do not recommend either of these options, though, because we think other alternatives to colonoscopy are better.

OHSU screening guidelines

OHSU has a two-tier guideline to help patients and their doctors choose the best tests. For patients with average risk, the guideline recommends starting at age 45.

  • Preferred: Colonoscopy every 10 years or an annual FIT
  • Acceptable alternatives: 
    • CT colonography every five years
    • FIT and stool DNA test every three years, with an option to add sigmoidoscopy
    • Sigmoidoscopy at 5- to 10-year intervals

Types of colorectal cancer

Adenocarcinoma accounts for more than 95% of cases. Adenocarcinoma starts in cells that produce the sticky protective substance (mucus) lining the colon and rectum. 

Other types include:

  • Carcinoid tumors, which develop in the cells that help regulate digestion.
  • Gastrointestinal stromal tumors, which can start anywhere in your intestines. Some form in the wall of the colon or rectum.
  • Lymphoma of the colon or rectum is a rare tumor that forms in infection-fighting white blood cells called lymphocytes. 
  • Sarcomas, which start in the soft tissues of the colon or rectum, including blood vessels and muscles.

Colorectal cancer stages

Doctors at the OHSU Knight Cancer Institute classify colorectal cancer by stages. The stage indicates how advanced the cancer is and whether it's likely to grow slowly or quickly.  We also consider whether the cancer has grown through layers of the colon or rectum and spread to other areas of the body. This helps us plan the most effective treatment. 

Doctors determine the stages of colorectal cancer through:

  • Physical exam
  • Imaging tests, such as MRI (magnetic resonance imaging)
  • Biopsy, in which a tissue sample is looked at under a microscope

Colorectal Cancer, Stage II

Colorectal tumor sizes, which also correspond to cancer stages. Inset box showing inner to outermost layers of the colon including: mucosa, submucosa, muscle layers, and serosa.
Staging helps your care team measure the extent of cancer so they can make the best treatment recommendations.

This is staging for a colorectal adenocarcinoma:

A group of abnormal cells (neoplasm) is present on the inside surface of the colon or rectum, the mucosa.

The tumor has grown through the surface to the next one or two layers of tissue, the submucosa and a muscle layer called the muscularis propria.

  • Stage IIA: The tumor has grown into the outer layers, called the pericolorectal tissues.
  • Stage IIB: The tumor has grown through the wall of the colon or rectum.
  • Stage IIC: The tumor has grown through the wall and directly invaded or attached to nearby organs or structures.
  • Stage IIIA: One of the following:
    • The tumor has invaded an initial layer, the submucosa. Cancer is in four to six nearby lymph nodes.
    • The tumor has grown as far as a second layer, the muscularis propria. Cancer is in one to three nearby lymph nodes or nearby tissues.
  • Stage IIIB: One of the following:
    • The tumor has invaded as far as the muscularis propria. Cancer is in seven or more nearby lymph nodes.
    • The tumor has spread as far as the outer layers. Cancer is in four to six nearby lymph nodes.
    • The tumor has grown into the outer layers or through the wall of the colon or rectum. Cancer is in one to three lymph nodes or nearby tissues.
  • Stage IIIC: One of the following:
    • The tumor has reached the outer layers or grown through the wall of the colon or rectum. Cancer is in seven or more nearby lymph nodes.
    • The tumor has spread through the wall of the colon or rectum. Cancer is in four to six nearby lymph nodes.
    • The tumor has directly invaded or attached to nearby organs or structures. Cancer is in one or more nearby lymph nodes.
  • Stage IVA: Cancer has spread to one distant organ or set of lymph nodes.
  • Stage IVB: Cancer has spread to more than one distant organ or set of lymph nodes.
  • Stage IVC: Cancer has spread to distant parts of the peritoneum, the tissue that lines the abdominal cavity, and may have spread to distant organs or lymph nodes.

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