How Is Colorectal Cancer Diagnosed?

This content has been reviewed and approved by

Howard Burris, MD
Director, Drug Development
Sarah Cannon Cancer Center
Tennessee Oncology

You might go through a diagnosis because of the results of a colorectal cancer screening examination or because you have some of the symptoms of colorectal cancer. To diagnose colorectal cancer, your doctor will perform a biopsy.

• Biopsy - Your doctor removes a small sample of tissue from the colon and sends it to a laboratory to be checked for cancer. Tissue can be biopsied during a colonoscopy or surgery. Sometimes, biopsies are done using a computed tomography (CT) scan or ultrasound evaluation to guide a needle into a mass through the skin (needle biopsy). A biopsy is the only definitive way to diagnose colorectal cancer.

If the biopsy shows that you have cancer, your doctor will need to know the extent, or stage, of the disease to plan the best treatment for you.

Your doctor might order some of the following tests to find out the stage of your cancer:

• Blood tests - Your doctor checks the level of carcinoembryonic antigen (CEA) and other substances in the blood. Some people who have colorectal cancer have a high CEA level. Your blood might also be tested for anemia, which means not having enough red blood cells, because some people with colorectal cancer also have anemia. The doctor might also have your blood tested for liver function to help figure out whether the cancer has spread to the liver.
• Chest x-ray - X-rays of your chest can show whether the cancer has spread to your lungs.
• Colonoscopy - If you did not have a colonoscopy when you were screened or your cancer was diagnosed, your doctor might perform one to stage your cancer. The colonoscopy helps the doctor find abnormal areas along the entire length of the colon and rectum.
• Computed tomography (CT) scan - An x-ray machine linked to a computer takes a series of detailed pictures of areas inside your body. You may receive an injection of dye before having the CT scan. A CT scan might show whether cancer has spread to the liver, lungs, or other organs.
• Positron emission tomography (PET) scan - Cancer cells use glucose (sugar) more quickly than most other body organs. During a PET scan, the doctor will inject a small amount of glucose that has a radioactive atom into your arm. A special camera in the PET machine can detect the radioactivity. PET scanning is especially useful when your doctor believes that your cancer has spread but does not know where.
• Ultrasound - The doctor inserts an ultrasound probe into your rectum. The probe sends out sound waves that people cannot hear. The waves bounce off your rectum and nearby tissues, and a computer uses the echoes to create a picture. The picture might show how deeply a colorectal tumor has grown or whether the cancer has spread to lymph nodes (tiny organs that help the body fight infections) or other nearby tissues.