Rectal Cancer Diagnosis

Rectal cancer can be identified during a routine screening. Everyone with an average risk for rectal cancer should start getting rectal cancer screenings starting at age 45.

People who have symptoms of rectal cancer should be tested regardless of their age. This is considered a diagnostic test.

The following tests may be used as screening and/or diagnostic tests for rectal cancer. They can also show if the cancer has spread and monitor how the disease is responding to treatment.

Endoscopic screening and diagnostic tests

Endoscopic tests are the most effective tests for rectal cancer. They can be used for routine screening that everyone should have starting at age 45. They are also used for patients who have rectal cancer symptoms and need a diagnostic test.

These tests generally are performed under some form of sedation so that you do not feel any discomfort. Endoscopic tests allow your doctor to see the inside of your colon clearly.

Endoscopic tests may include:

• Colonoscopy: A tiny camera on flexible plastic tubing (colonoscope) is inserted into the rectum and advanced through the colon. This gives the doctor a view of the entire colon. Colonoscopies are used for routine screening and to diagnose people with rectal cancer symptoms.
• Sigmoidoscopy: Sigmoidoscopy is similar to colonoscopy but is a shorter examination of the rectum and lower colon. It is used to monitor people with confirmed cases of cancer in the rectum or the last section of the colon. By examining just the rectum and lower colon, doctors can track the disease’s progress and how it is responding to treatment. A sigmoidoscopy combined with a stool-based test can also be used for routine rectal cancer screening.

Biopsy: If doctors remove any polyps during a colonoscopy, they will be examined under a microscope for the presence of cancer cells. The process of removing and examining suspected disease tissue is called a biopsy.

At-home screening tests

There are several types of non-invasive rectal cancer screening tests that can be taken at home. These tests do not provide a definitive diagnosis for rectal cancer, but they can indicate that other, more accurate tests should be used. They are typically not offered to patients who have symptoms of rectal cancer and are referred for a diagnostic test.

There are several types of at-home screening tests:

• Fecal DNA test (FDNA):  This take-home test identifies DNA changes in the cells of a stool sample
• Fecal immunochemical test (FIT): This take-home test identifies blood proteins in stool
• Fecal occult blood test (FOBT): This take-home test identifies blood in the stool

Additional diagnostic tests

If you have symptoms of rectal cancer, or if you have an abnormal screening test result, your doctor may recommend additional tests. These tests may include:

Blood tests: At present, no blood test can definitively diagnose rectal cancer. However, they can provide your doctor with additional information.

• Standard blood tests can provide information about kidney and liver function and blood counts.
• A blood test for the carcinoembryonic antigen (CEA) protein, made by some tumors, can reveal if the tumor is growing, responding to treatment, or has come back after treatment.
• Patients who have been successfully treated for rectal cancer can be tested for circulating tumor DNA (ctDNA). This test can help catch recurring cancer early, before it shows up in an imaging exam.

Imaging tests: Imaging tests can help to provide detailed information about the size or location of rectal cancer and assess if cancer has spread to other parts of the body. Common imaging tests are:

• CT scan: A computed tomography, or CT, scan uses an X-ray machine to take several pictures from different angles, providing a highly detailed image. 
• MRI scan: Magnetic Resonance Imaging, or MRI, uses magnetic fields and radio waves to generate pictures of the body’s soft tissue and organs. MD Anderson offers a specialized type of MRI used specifically to evaluate and help plan care for rectal cancer patients.
• PET/CT (positron emission tomography) scan: PET/CT scans are not routinely part of rectal cancer diagnosis. They are generally used to further evaluate abnormal findings on CT or MRI scans or to monitor patients who have a confirmed case of advanced rectal cancer.

Learn more about imaging exams.

Rectal cancer testing can also include these other tests, which are used less often:

• Virtual colonoscopy or CT (computed tomography) colonoscopy: A focused CT scan of your abdomen and pelvis to create 3D images.
• Endoscopic ultrasound (EUS) or endosonography: A flexible tube with an ultrasound device attached to the tip is inserted through the rectum into the colon. The device sends out ultrasound waves to generate images of the colon and nearby tissue.
• Double contrast barium enema (DCBE): Barium is a chemical that allows the bowel lining to show up on an X-ray. A barium solution is given by enema, and then a series of X-rays are taken.