Colorectal Cancer Diagnosis
Colorectal cancer can be identified during a routine screening. Everyone with an average risk for colorectal cancer should start getting regular colorectal screenings starting at age 45.
People who have symptoms of colorectal 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 colorectal 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 colorectal cancer. They can be used for routine screening that everyone should have starting at age 45. They are also used for patients who have colorectal 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 colorectal 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 colorectal 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 colorectal cancer screening tests that can be taken at home. These do not provide a definitive diagnosis for colorectal cancer, but they can indicate that other, more accurate tests should be used. They are typically not offered to patients who have symptoms of colorectal cancer and are referred for a diagnostic test.
There are several types of at-home screening tests, including:
- Fecal DNA test (FDNA), which identifies DNA changes in the cells of a stool sample
- Fecal immunochemical test (FIT), which identifies blood proteins in stool
- Fecal occult blood test (FOBT), which identifies blood in the stool
Additional diagnostic tests
If you have symptoms of colorectal 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 colorectal 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 colorectal 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 colorectal cancer and if it 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 colorectal 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 colorectal cancer.
Learn more about imaging tests.
Colorectal 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.
The sigmoid colon is the part of the large intestine (colon) closest to the rectum. It absorbs water from stool and pushes the stool to the rectum and anus until it’s ready to be expelled when you use the bathroom.
Colorectal cancer is an umbrella term for colon cancer and rectal cancer. Studies have shown that colorectal cancer is more common in the sigmoid colon, which is on the left side, along with the descending colon and rectum. The rectum is also considered the left side even though it’s not part of the colon.
Gastrointestinal medical oncologist Kanwal Raghav, M.D., estimates that about 70% of colorectal cancers are diagnosed on the left side.
“It’s unclear why colon cancer starts in a particular location, although some explanations exist,” says Raghav. “However, left-side colon cancers are more common than right-side colon cancers.”
The colon starts at the cecum, which is on the right side. It goes up to the ascending colon, across the body to the transverse colon and down to the descending colon and sigmoid colon, which are on the left side.
We spoke with Raghav to learn more about the differences between left-side and right-side colon cancer. Here’s what he shared.
It’s common for left-side tumors to cause symptoms.
It's common for people with left-side colorectal cancer to have changes in their bowel habits or blood in their stool. People with right-side tumors often show fewer symptoms.
Left-side tumors and right-side tumors have different genetic characteristics.
Tumors that develop on the left side of the colon have more mutations of tumor-suppressing genes, like APC and TP53. These genes help prevent tumors from growing. Mutations in these genes make you more likely to develop certain cancers, including colon cancer.
Tumors that develop on the right side of the colon are more likely in people who have hereditary cancer syndromes. These include high microsatellite instability (MSI-high) – also referred to as DNA mismatch repair deficiency (dMMR) – and Lynch syndrome.
Left-side tumors are easier to detect during a colonoscopy.
During a colonoscopy, your doctor examines the walls and lining of your colon by inserting a flexible scope into your rectum and around your large intestine. Doctors examine the left side first since it’s closer to where the scope is inserted. Colorectal cancer usually starts as a polyp in the colon or rectum.
Left-side tumors often present as polypoid lesions, which protrude into the lumen of the gastrointestinal tract and are easier to see. Colonoscopies are good at detecting these polyps, which is why getting this routine screening is so important.
Many polyps on the right side of the colon are sessile serrated adenomas. These are flat and harder to detect during a colonoscopy. Because of this, right-side colon cancers are often diagnosed at advanced stages and may be harder to treat.
Colonoscopy prep is important because your colon will need to be completely clean of any stool, so your doctor can easily see both the left and right side of your colon.
The side on which cancer develops helps determine your treatment.
Overall, left-side tumors have a better prognosis than right-side tumors. This is because colon cancer on the left side responds better to chemotherapy and targeted therapies, like anti-EGFR and HER2 drugs.
Right-side tumors tend to have more genetic mutations, which are often associated with more aggressive cancers. Right-side tumors are also harder to detect, so they’re often diagnosed when the disease is more advanced. Recently, immunotherapy has been effective in treating these cancers because the body can recognize the abnormal cells and attack them. Immunotherapy has helped improve the prognosis for patients with stage IV right-side colon cancer.
Your doctor and care team will work together to develop the best treatment plan for you.
Knowledge of left-side and right-side colon cancer leads to treatment advances.
We’ve been treating colorectal cancer for a long time. But only in the last four or five years have we really started to understand the differences between colon cancer that develops on the left side versus the right side. Before then, it wasn’t as commonly acknowledged.
Over the years, we have learned that colon cancer can’t be viewed as one disease. We’re now looking at it as a complex disease with many subtypes and trying to develop targeted and personalized therapies that can be used depending on the specific type people have. Slowly and steadily, we are moving the bar for personalized therapy in colorectal cancer. And I think the location of the cancer – left side versus right side – is a part of that.
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