Colorectal Cancer (Colon and Rectal Cancer)

How colorectal cancer develops

Most colorectal cancers begin as adenomatous polyps or sessile serrated lesions, small growths on the inner lining. Over years, genetic changes accumulate and some polyps become cancer. The cancer then grows through the bowel wall and can spread to lymph nodes, and later to the liver, lungs and peritoneum. About 70% arise in the colon and 30% in the rectum, which is treated somewhat differently. More detail on polyps is on our colorectal polyps page.

Symptoms

Early cancer often causes no symptoms, which is why screening matters. When symptoms develop:

• rectal bleeding or blood in the stool, bright red or dark
• a change in bowel habits lasting weeks: diarrhea, constipation, narrower stools
• iron-deficiency anemia, causing tiredness, breathlessness and pale skin. Right-sided colon cancers often present this way
• abdominal pain, cramps or bloating
• a feeling of incomplete emptying (tenesmus), especially with rectal cancer
• unexplained weight loss
• a lump in the abdomen

Many of these symptoms are more often due to hemorrhoids, IBS or infection, but they should be checked, particularly if they persist.

Early-onset colorectal cancer

Rates are rising in adults under 50, for reasons not fully understood. Younger people are more likely to be diagnosed late because symptoms are dismissed. Rectal bleeding at any age deserves evaluation.

Risk factors

Factors you cannot change

• Age: most cases are diagnosed after 50, though younger cases are increasing
• Family history: a parent, sibling or child with colorectal cancer or advanced polyps roughly doubles the risk
• Inherited syndromes: Lynch syndrome and familial adenomatous polyposis cause up to about 5% of cases
• Inflammatory bowel disease: long-standing ulcerative colitis or Crohn's colitis
• Personal history of polyps or colorectal cancer
• Race and ethnicity: Black Americans and American Indian and Alaska Native people have higher incidence and death rates

Lifestyle factors

• obesity and physical inactivity
• diets high in red and processed meat and low in fiber
• smoking and heavy alcohol use
• type 2 diabetes

Screening

The US Preventive Services Task Force recommends screening for average-risk adults aged 45 to 75, and individualized decisions from 76 to 85. People with a family history, IBD or genetic syndromes start earlier and are screened more often.

Any positive stool or imaging test must be followed by a colonoscopy. The best test is the one you will actually do.

Diagnosis and staging

• Colonoscopy with biopsy confirms the diagnosis.
• CT scan of the chest, abdomen and pelvis checks for spread.
• MRI of the pelvis for rectal cancer, to plan surgery and radiation.
• CEA blood test, a tumor marker used for follow-up.
• Tumor testing for mismatch repair deficiency (MMR or MSI), KRAS, NRAS, BRAF and HER2, which guides treatment and screens for Lynch syndrome.

Survival figures come from population data and vary with individual factors and newer treatments.

Treatment

Colon cancer

• Surgery (colectomy): removes the affected segment and nearby lymph nodes, often by keyhole or robotic surgery. Very early cancers can be removed during colonoscopy.
• Adjuvant chemotherapy: for stage III and some high-risk stage II cancers, usually FOLFOX or CAPOX for 3 to 6 months.

Rectal cancer

• Total neoadjuvant therapy: chemotherapy and radiation (or chemoradiation) before surgery for many locally advanced cancers.
• Surgery: total mesorectal excision. A temporary or permanent stoma may be needed.
• Immunotherapy for dMMR rectal cancer: a PD-1 inhibitor (dostarlimab) produced complete responses in all patients in an early trial, allowing some to avoid surgery and radiation.
• Watch-and-wait: for some people whose tumor disappears after treatment, with close surveillance.

Advanced (stage IV) cancer

• chemotherapy combined with targeted drugs (bevacizumab, cetuximab or panitumumab for RAS wild-type tumors, BRAF and HER2-targeted drugs)
• immunotherapy (pembrolizumab, nivolumab with ipilimumab) for MSI-high or dMMR tumors
• surgery, ablation or radiation for limited liver metastases or lung metastases, which can be curative in some patients

Living with and after colorectal cancer

• Follow-up: CEA tests and CT scans for several years, and colonoscopy at 1 year after surgery.
• Side effects: changes in bowel habits, neuropathy from oxaliplatin, fatigue, and stoma care. Specialist nurses help.
• Lifestyle: regular physical activity is linked to lower recurrence, along with a healthy weight and not smoking.
• Family: close relatives may need earlier screening. Genetic counseling is offered if Lynch syndrome is suspected.

Prevention

• get screened from age 45
• stay physically active and keep a healthy weight
• eat plenty of fiber, fruit, vegetables and whole grains, and limit red and processed meat
• avoid smoking and limit alcohol
• daily aspirin can reduce risk in Lynch syndrome but is not recommended for everyone, because of bleeding risk

Frequently asked questions

What are the early signs of colon cancer?

Early colon cancer often has no symptoms. Early signs can include rectal bleeding or blood in the stool, a lasting change in bowel habits, iron-deficiency anemia, unexplained tiredness and abdominal discomfort. Any of these lasting more than a few weeks should be checked.

At what age should you get screened for colorectal cancer?

Average-risk adults should start at age 45. People with a parent, sibling or child with colorectal cancer, inflammatory bowel disease or inherited syndromes such as Lynch syndrome often need to start earlier, sometimes at 40 or younger.

Is colorectal cancer curable?

Yes, especially when found early. About 90% of people with cancer confined to the bowel are alive 5 years later, and most are cured. Even some people with limited spread to the liver or lungs can be cured with surgery and chemotherapy.

What is the difference between colon cancer and rectal cancer?

Both are colorectal cancers and share causes and screening. Rectal cancer arises in the last 12 to 15 cm of the large bowel. Because of its location in the pelvis, it is more often treated with radiation and chemotherapy before surgery.

Can a stool test replace colonoscopy?

For average-risk people with no symptoms, yearly FIT or a stool DNA test is an effective screening option. A colonoscopy is needed if the result is positive. People with symptoms, high risk or a history of polyps should have colonoscopy.