Introduction

Rectal cancer is a type of cancer that develops in the last several inches of the large intestine, known as the rectum. This part of the digestive system sits between the end of the colon and the anus, and it plays a role in storing waste before it leaves the body. Because rectal cancer can grow slowly over many years, often starting as small clumps of abnormal cells called polyps, knowing when to seek diagnostic testing is crucial.^[1]

Most people should begin screening for rectal cancer starting at age 45, even if they have no symptoms. This recommendation was recently lowered from age 50 because doctors have noticed more younger people being diagnosed with the disease. If you have certain risk factors, such as a family history of rectal or colon cancer, a personal history of inflammatory bowel disease like Crohn’s disease or ulcerative colitis, or inherited genetic conditions, you may need to start screening even earlier.^[3]

You should seek diagnostic testing right away if you notice warning signs like blood in your stool, changes in your bowel habits that last more than a few days, stools that look narrow or shaped like a pencil, persistent abdominal pain, unexplained weight loss, or a feeling that your bowel doesn’t empty completely after you go to the bathroom. These symptoms don’t always mean you have rectal cancer, as they can be caused by other conditions like hemorrhoids, but it’s important to have them checked by a healthcare professional.^[2]

Diagnostic Methods

When you visit your doctor with concerns about rectal cancer or for routine screening, several types of tests can be performed to look for signs of the disease. The diagnostic process typically starts with simpler exams and may progress to more detailed imaging and laboratory tests if needed.

Physical Examination

Your doctor will begin with a physical exam and a review of your health history. This includes asking about your symptoms, family history, lifestyle habits, and any past illnesses. As part of this initial evaluation, your doctor will likely perform a digital rectal exam (DRE). During this exam, the doctor inserts a lubricated, gloved finger into your rectum to feel for lumps, abnormal areas, or anything unusual. While this exam may feel uncomfortable, it is quick and provides important information about the lower part of your rectum.^[4]

Colonoscopy

A colonoscopy is one of the most important tools for diagnosing rectal cancer. This procedure uses a long, flexible tube with a tiny camera on the end, called a colonoscope, to examine the entire length of your colon and rectum. Before the test, you’ll need to empty your bowels completely by drinking a special cleansing solution the day before. During the procedure, you’ll receive medicines to keep you comfortable and relaxed.^[10]

The colonoscope allows your doctor to see the lining of your rectum and colon on a screen. If the doctor finds any polyps or areas that look abnormal, they can remove small tissue samples right away using special cutting tools passed through the colonoscope. This tissue removal is called a biopsy, and it’s the only way to know for certain whether cancer cells are present.^[10]

Office-Based Scoping Procedures

Sometimes a doctor may perform a simpler examination in their office using a proctoscopy or sigmoidoscopy. These procedures use shorter scopes to look at just the rectum or the rectum and the lower part of the colon. They don’t require as much preparation as a full colonoscopy and can be done without sedation in many cases. However, they don’t allow examination of the entire colon, so they may miss problems higher up in the digestive tract.^[6]

Biopsy and Laboratory Testing

When tissue samples are taken during a colonoscopy or other procedure, they are sent to a laboratory where a specialist called a pathologist examines them under a microscope. The pathologist looks for cancer cells and performs special tests to learn more about them. These tests can reveal important details about how the cancer might behave and what treatments might work best. The results help your medical team create a personalized treatment plan.^[10]

Blood Tests

Several blood tests may be ordered as part of your diagnostic workup. A complete blood count (CBC) measures different types of cells in your blood and can reveal anemia, which might result from bleeding in the rectum. Another test called CEA (carcinoembryonic antigen) measures a specific protein that some rectal cancers produce. While CEA levels alone can’t diagnose cancer, they can be useful for monitoring how well treatment is working later on.^[2]

Imaging Tests to Determine Cancer Spread

If cancer is confirmed, your doctor will need to determine whether it has spread beyond the rectum. This process is called staging, and it involves several types of imaging tests. A CT scan (computed tomography scan) uses X-rays and computer technology to create detailed, three-dimensional images of your abdomen and chest. This helps doctors see if cancer has spread to nearby organs or distant parts of the body.^[10]

An MRI (magnetic resonance imaging) scan uses magnets and radio waves instead of X-rays to create detailed pictures. MRI is particularly good at showing soft tissues and is often used to see how deeply the tumor has grown into the rectal wall and whether it has reached nearby structures. This information is critical because the rectum sits in a tight space surrounded by other organs.^[6]

Endorectal ultrasound is another test that may be used for staging. During this procedure, a special ultrasound probe is inserted into the rectum to create detailed images using sound waves. This test is especially good at showing how far the tumor has grown through the layers of the rectal wall and whether nearby lymph nodes are affected.^[6]

A PET scan (positron emission tomography) may also be recommended. This test uses a small amount of radioactive sugar that is injected into your bloodstream. Cancer cells, which use more energy than normal cells, absorb more of this sugar and show up as bright spots on the scan. PET scans are particularly helpful in looking for cancer that has spread to other parts of the body.^[6]

Other Screening Methods

For people who cannot or prefer not to have a colonoscopy, alternative screening tests are available. The fecal immunochemical test (FIT) looks for tiny amounts of blood in the stool that might not be visible to the naked eye. There are also tests that look for abnormal DNA in stool samples. While these tests can help detect cancer, they are not as good at finding polyps as colonoscopy, and if they come back positive, you’ll still need a colonoscopy to investigate further.^[6]

Diagnostics for Clinical Trial Qualification

Clinical trials are research studies that test new ways to prevent, detect, or treat rectal cancer. If you’re considering joining a clinical trial, you’ll need to undergo specific diagnostic tests to determine whether you qualify. These tests ensure that the trial is safe for you and that you have the type or stage of cancer the researchers are studying.

The qualification process typically begins with confirmation of your cancer diagnosis through biopsy results reviewed by pathologists. The tissue samples are examined not only to confirm that cancer is present but also to identify specific characteristics of the cancer cells. Some trials look for particular genetic changes or biomarkers, which are measurable substances in your body that indicate the presence of disease or how it might respond to treatment.^[3]

Staging tests are almost always required for clinical trial participation. Researchers need to know precisely where your cancer is located, how large the tumor is, and whether it has spread to lymph nodes or other organs. This usually involves the same imaging tests used in standard diagnosis, including CT scans, MRI, PET scans, and sometimes endorectal ultrasound. The results help match you with trials studying cancer at your particular stage.^[6]

Blood tests are another standard requirement for clinical trial qualification. In addition to the CBC and CEA tests mentioned earlier, trials may require tests to check your kidney function, liver function, and overall health status. These tests help ensure that you’re healthy enough to tolerate the experimental treatment being studied and that the treatment won’t cause dangerous interactions with other health conditions you might have.^[2]

Some trials looking at new targeted therapies or immunotherapies require genetic testing of your tumor tissue. Scientists examine the cancer cells for specific mutations or changes in genes that might make the tumor more likely to respond to the treatment being studied. For example, tests might look for changes in genes with names like KRAS, NRAS, or BRAF, or they might check whether your tumor has a characteristic called microsatellite instability (MSI) or problems with DNA repair mechanisms called mismatch repair deficiency.^[3]

Before enrolling in a trial, you’ll also need recent imaging to serve as a baseline for comparison. This means the scans taken at the start of the trial will be compared with scans taken during and after treatment to measure whether the experimental therapy is working. The trial may specify how recent these baseline scans need to be, often within a few weeks of enrollment.

Your performance status, which measures how well you can carry out daily activities, is another qualification criterion for many trials. While this isn’t exactly a diagnostic test, your healthcare team will assess your ability to care for yourself, work, and maintain your normal activities. This information helps researchers determine whether you’re strong enough for the trial and helps them analyze results by grouping participants with similar health levels.

Some trials have very specific requirements based on previous treatments you may have received. If you’ve already had surgery, chemotherapy, or radiation therapy for rectal cancer, you may need documentation of exactly what treatments you received and when. Researchers might also require a certain amount of time to pass since your last treatment before you can join a new trial.