Diagnosing Colorectal Cancer

Colorectal cancer may be diagnosed during a  screening test, which is done before symptoms occur, or as a result of diagnostic testing, which is performed to determine the cause of symptoms.

Screening guidelines recommend testing starting at age 45 for men and women at average risk. Routine screening can help detect colorectal cancer early or help prevent it altogether.

In many cases, colon polyps and early cancers may not cause any symptoms. When signs and symptoms of colorectal cancer do occur, they may include rectal bleeding or blood in the stool; a change in bowel habits, such as diarrhea or constipation; or narrow or thin stools. You might also experience unexplained fatigue, anemia, or weight loss.

These may be symptoms of colorectal cancer or another gastrointestinal condition. If you develop any of these symptoms, no matter your age, see your doctor promptly for evaluation and diagnosis.

Depending on your symptoms, medical history, and physical examination, our specialists may use a number of tests to help diagnose colorectal cancer or another gastrointestinal condition.

They may include blood tests to check for conditions such as anemia or vitamin deficiency; colonoscopy or sigmoidoscopy to examine the colon or rectum; or imaging tests to view the colon, rectum, and abdomen.

Blood Tests

Depending on your symptoms, medical history, and physical exam, your doctor may order blood tests to find or rule out conditions such as anemia, vitamin deficiency, and others.

In people who have been diagnosed with colorectal cancer, blood tests may also be used to look for carcinoembryonic antigen, or CEA, a protein that is monitored to check the cancer’s response to treatment.

Colonoscopy or Flexible Sigmoidoscopy

Colonoscopy or flexible sigmoidoscopy, as well as other screening tests, may be performed to check for colon polyps and cancer in people age 45 and older who are at average risk and have no symptoms.

Colonoscopy is also used to screen people who have certain risk factors that mean they are at increased risk for the disease. In these people, screening should start at a younger age. Screening is done before symptoms occur, because polyps and early colon cancers often cause no symptoms.

In people who do have symptoms, a colonoscopy or flexible sigmoidoscopy may be used to examine the inside of the colon and rectum to help identify the cause, such as hemorrhoids, inflammation, infection, an ulcer, large polyps, or cancer.

During either procedure, doctors can remove any polyps and biopsy any suspicious tissue. Polyps and other tissue are then sent to a pathologist for examination under a microscope. The pathologist determines if the polyp or tissue is benign, cancerous, or another type of condition. If it is cancerous, he or she can identify the type of cancer.

If a large polyp, tumor, or mass is seen, care may need to be coordinated with surgeons and other specialists.

Surgery to Remove a Polyp

Sometimes surgery may be needed to take out a very large polyp that cannot be removed during a colonoscopy or sigmoidoscopy.

While general anesthesia is usually required for a surgical biopsy, the procedure is often done using a minimally invasive technique—either inserting surgical tools through the anus to remove rectal polyps, called a transanal approach, or making a few small incisions in the abdomen and using laparoscopic or robotic-assisted surgery to remove the section of colon containing the polyp.

With laparoscopy, the surgeon inserts a laparoscope, or a tube with a tiny camera at the tip, through an abdominal incision that allows for excellent visualization of the colon. The doctor then inserts small surgical tools through other incisions to remove the section of colon containing the polyp.

A robotic surgery system consists of tiny surgical instruments mounted on several robotic arms. An additional arm has a camera that generates magnified, high-definition, three-dimensional images on a computer screen, which helps guide the surgeon while he or she removes the section of colon containing the polyp. The surgeon controls these instruments and the camera from a console located in the operating room.

Sometimes, open surgery may be required. In this procedure, an incision is made in the abdomen to remove the section of colon containing the polyp. An open approach may be necessary if the abdomen contains scar tissue from previous surgery.

Further treatment depends on the results of tissue evaluation by a pathologist.

Tissue Evaluation

When a polyp or tumor tissue is removed during a colonoscopy, sigmoidoscopy, or surgery, it is sent to a pathologist for examination. If the tissue is cancerous, the pathologist notes the type of cancer, where it started, and how aggressive it is.

In addition, the pathologist often can provide other important information, such as whether certain genetic mutations, such as KRAS or BRAF, are present in the tissue. This information helps doctors determine the most effective treatment options.

Imaging Tests

Depending on your symptoms, medical history, and physical exam, your doctor may recommend one or more imaging tests to pinpoint the cause of your symptoms and provide a diagnosis.

If colorectal cancer is diagnosed, your doctor may perform certain imaging tests to determine if the cancer has spread. This includes examining the extent of the disease in the wall of the colon or rectum, surrounding lymph nodes—small immune system glands, which are often the first sites to which cancer cells spread—and distant sites, such as the lungs or liver.

Doctors then recommend a treatment plan based on the type, location, and extent of cancer.

CT Scan

Our doctors may order a CT scan, in which cross-sectional, three-dimensional images of the body are created using X-rays and a computer. This test enables them to view the colon, rectum, other areas of the abdomen and pelvis, and possibly the chest to diagnose the condition.

For people diagnosed with cancer, a CT scan may also be used to determine if cancer has spread or responded to treatment.

As part of the test, you may need to drink a liquid contrast agent or be given an injection of a special dye into a vein. This enhances the images, giving the doctors a better view of the inside of the body.

MRI Scan

MRI scans use a magnetic field and radio waves to create computerized, three-dimensional images of structures in the body. A computer translates the information from the MRI scan into detailed images. For some people, a contrast agent injected through a vein may be used to create more detailed images.

MRI scans are usually performed in an enclosed device, and may take longer to complete than a CT scan.

Transrectal Ultrasound

During an ultrasound, sound waves are used to create images on a computer monitor. In a transrectal ultrasound, doctors insert a probe through the anus into the rectum. This test can help your doctor determine the extent to which tumors may have grown into the rectal wall.


If you have received a diagnosis of colorectal cancer, your doctor may use a combination PET and CT scan to determine whether the condition has spread and to assess tumor activity, or how rapidly the cancer is growing. This combined scan provides doctors with a set of computer-generated, three-dimensional images.

The CT portion of the test provides cross-sectional pictures of the body, and the PET scan detects tumor activity. During a PET scan, the doctor injects a small amount of radioactive glucose, or sugar, into a vein. This substance collects in tumor cells, which are detected by the computer during the scan.

This type of imaging may also be used after treatment to help doctors determine whether any cancerous cells remain.