How is colorectal cancer diagnosed?

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To diagnose colorectal cancer, also called colon cancer or bowel cancer, doctors may do a blood test. They will also do a colonoscopy, which means using a little camera on a thin tube to look inside the bowel, or intestine, and take pictures. Tests may also be done to see if the cancer has spread from the bowel to other parts of the body.

Colorectal cancer is typically diagnosed in a screening colonoscopy. In this video, Eric Changchien, MD, a colorectal surgeon at Riverside Community Hospital, says that sometimes symptoms lead to a diagnosis of colorectal cancer as well.

Dr. Daniel Labow, MD
Surgical Oncologist

A biopsy, in which a doctor removes a small tissue sample from the colon or rectum to test it for cancer, is the most common method used for diagnosis. Aside from biopsies, physicians may also conduct blood tests to confirm a diagnosis or indicate that the cancer has spread to another part of the body.

In addition to these procedures, doctors may also use a variety of imaging tests, including computerized tomography (CT or CAT) scans, ultrasounds, chest x-rays, or positron-emission tomography (PET) scans.

A colonoscopy can also usually determine if a patient has colon cancer. In this video, Daniel Labow, MD, an oncology surgeon at The Mount Sinai Medical Center, describes the symptoms that might show up before a colonoscopy is performed.

Colorectal cancer is often found after symptoms appear, but most people with early colon or rectal cancer don't have symptoms of the disease. Symptoms usually only appear with more advanced disease. This is why getting the recommended screening before any symptoms develop is so important.

If your doctor finds something suspicious during a screening exam, or if you have any of the symptoms of colorectal cancer described below, your doctor probably will recommend exams and tests to find the cause. Colorectal cancer may cause one or more of the symptoms below. If you have any of the following you should see your doctor:

A change in bowel habits, such as:

  • diarrhea
  • constipation
  • narrowing of the stool that lasts more than a few days
  • a feeling that you need to have a bowel movement that is not relieved by doing so
  • rectal bleeding
  • dark stools
  • blood in the stool (often, though, the stool will look normal)
  • cramping or abdominal (stomach area) pain
  • weakness and fatigue
  • unintended weight loss

The third leading cause of cancer deaths, most colorectal cancers arise from lesions on the intestinal lining called adenomatous polyps. The progression from precancerous polyp to full-blown cancer is believed to take 10 years or more, though. Screening allows physicians to remove polyps before they become cancerous and to identify cancer early, when it's most treatable.

Colonoscopy is recommended as a screening test at age 50 for women at average risk and then every decade thereafter. (If you have a family history, inflammatory bowel disease or other factors, you may need more frequent screening.) During a colonoscopy, a gastroenterologist examines the lining of the colon with a lighted scope, looking for cancerous lesions and polyps.

There are other screening methods—fecal occult blood testing, flexible sigmoidoscopy, computed tomography (CT) colonography (virtual colonoscopy) and a stool test for DNA shed by colorectal cancers—but traditional colonoscopy is preferred because it checks the entire colon, finds the most polyps and lesions, and allows the doctor to biopsy or remove polyps and lesions during the procedure. Colonoscopy is particularly important for women, who are more likely to have polyps or lesions deeper in the ascending colon, the area at the farthest reach of the colonoscope. In contrast, sigmoidoscopy examines only the first third of the colon.

Colorectal cancer is diagnosed by biopsy. The most common way today is that a patient has a screening colonoscopy, a polyp is detected and a biopsy is performed. Microscopic analysis reveals the result to be colon cancer. Sometime colorectal caner is found on barium enema or sigmoidoscopy. By the time the patient presents with symptoms, usually the cancer is quite advanced.

Dr. Lata C. Thatai, MD
Hematologist & Oncologist

Either a colonoscopy screening or a diagnostic colonoscopy can allow a doctor to visualize a colon cancer and biopsy it. Watch this video with Lata C. Thatai, MD, from Parkland Medical Center to learn about other ways to diagnose colon cancer.

Colorectal cancer often is diagnosed with colonoscopy. Nearly all cases of colorectal cancer begin with the growth of polyps, or benign growths of tissue, in the intestine. During a colonoscopy, your doctor examines the inside of the rectum and entire colon through a flexible, lighted tube. The doctor may remove polyps and collect samples of tissue or cells for closer examination.

Colon cancer is usually diagnosed through a colonoscopy. Watch John Rioux, MD, of Fawcett Memorial Hospital, discuss the importance of early detection.

Dr. Lisa Ganjhu, DO
Gastroenterologist

Screening is the number-one way colon cancer is diagnosed, but it’s not the only way, according to gastroenterologist Dr. Lisa Ganjhu. Watch the video to learn which early symptoms of colon cancer to watch for.

Most colorectal cancer is found through a colonoscopy. A colonoscopy is a procedure done under sedation. The physician places a thin, tube-like instrument with a light and a lens for viewing inside the rectum and colon. The scope may also have a tool to remove polyps or tissue samples which are checked for cancer in a lab.

The following tests and procedures may be used:

  • Physical exam and history: An exam of the body to check general signs of health, including checking for signs of disease, such as lumps or anything else that seems unusual. A history of the patient's health habits and past illnesses and treatments will also be taken.
  • Fecal occult blood test: A test to check stool (solid waste) for blood that can only be seen with a microscope. Small samples of stool are placed on special cards and returned to the doctor or laboratory for testing.
  • Digital rectal exam: An exam of the rectum. The doctor or nurse inserts a lubricated, gloved finger into the rectum to feel for lumps or anything else that seems unusual.
  • Barium enema: A series of x-rays of the lower gastrointestinal tract. A liquid that contains barium (a silver-white metallic compound) is put into the rectum. The barium coats the lower gastrointestinal tract and x-rays are taken. This procedure is also called a lower GI series.
  • Sigmoidoscopy: A procedure to look inside the rectum and sigmoid (lower) colon for polyps (small pieces of bulging tissue), abnormal areas or cancer. A sigmoidoscope is inserted through the rectum into the sigmoid colon. A sigmoidoscope is a thin, tube-like instrument with a light and a lens for viewing. It may also have a tool to remove polyps or tissue samples, which are checked under a microscope for signs of cancer.
  • Colonoscopy: A procedure to look inside the rectum and colon for polyps, abnormal areas or cancer. A colonoscope is inserted through the rectum into the colon. A colonoscope is a thin, tube-like instrument with a light and a lens for viewing. It may also have a tool to remove polyps or tissue samples, which are checked under a microscope for signs of cancer.
  • Biopsy: The removal of cells or tissues so they can be viewed under a microscope by a pathologist to check for signs of cancer.
  • Virtual colonoscopy: A procedure that uses a series of x-rays called computed tomography to make a series of pictures of the colon. A computer puts the pictures together to create detailed images that may show polyps and anything else that seems unusual on the inside surface of the colon. This test is also called colonography or CT colonography.

This answer is based on source information from the National Cancer Institute.

Doctors will perform a fecal occult blood test to detect any blood in the stool. If blood is found, your physician will most likely perform a colonoscopy or sigmoidoscopy to examine the colon and determine the cause of bleeding.

Both procedures use an endoscope, a flexible tube equipped with a light and camera that is inserted into the rectum and guided into the colon. A colonoscopy is more comprehensive. It can examine the rectum and entire colon, but requires greater preparation on the part of the patient and the use of sedation. Preparations may include a liquid diet several days prior to the procedure, a laxative the night before, and an enema the day of the procedure. Alternatively, a sigmoidoscopy requires less preparation, usually just an enema the day of the procedure, and can be done without sedation. But the procedure is also less comprehensive and can examine only the rectum and the bottom two feet of the colon.

Physicians may also couple a sigmoidoscopy with another procedure known as a double-contrast barium enema. With this, barium is infused into the rectum via an enema tube and an x-ray is taken of the area. The barium helps produce a clearer, more detailed x-ray of the area.

A number of other imaging tests such as ultrasound exams and computed tomography (CT or CAT) scan may be used to visualize organs and see if cancer is present.

Continue Learning about Colon Cancer Diagnosis

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.