Diagnosing Colorectal Cancer

Diagnostic tests are often performed when an individual exhibits signs and symptoms of colorectal cancer or laboratory studies suggest a cancer may be present. Often a doctor can make a preliminary diagnosis based on the results of a biopsy of a tumor polyp that was removed during colonoscopy. This is especially true if there is a single precancerous or benign polyp and no family history or personal history of cancer. If a polyp or cancer is found however, your doctor will often recommend a series of tests in order to fully consider all factors when making a diagnosis of colorectal cancer.

Diagnostic tests can help you and your medical team make a number of decisions. They can help to confirm the presence of cancer, or rule it out. In cases where cancer is discovered, the tests can help to determine three criteria

the grade of the cancer or how aggressive it may be

the extent of the cancers invasion into the intestinal wall (the stage)

whether the cancer has spread (metastasized)

Biopsies & The definition of poorly, well and moderately differenitated

Polyps that are removed during a colonoscopy are sent to the pathology lab to be be examined and tested.  Pathologists use a grading system to asses the severity of an individual cancer.  This system is based on how closely the tumor cells resemble normal cells.  As normal cells grow and develop they differentiate, which simply means that they become more specialized based on where they are in the body.  Pathologists can view cells under the microscope and discern a difference between the different cells in the body, for example colon cells look distinctly different than liver cells, which look distinctly different than breast cells.  Normal cells in different tissues of the body are considered to be fully differentiated, whereas cancer cells are not fully differentitated. If the cancer cells look very similar to normal cells and are grouped together in an organized fashion, they are called well differentiated. 

Cancer cells fall into one of three categories:

Well Differentiatedcancer cells which appear to be most similar to normal cells
Moderately Differentiatedcancer cells which appear to be somewhat different than normal cells
Poorly Differentiatedcancer cells which appear very different than normal cells

Click on each image below for enlarged versions of the different levels of differentiation:

poorly differentiated
moderately differentiated
well differentiated

Blood Tests

If colon cancer is diagnosed, your medical team will draw your blood to assess various factors which are used to assess the nature of the cancer.

Colorectal cancer may produce high levels of a protein molecule known as carcinoembryonic antigen (CEA). While this test is not reliable for detecting early colorectal cancers, it is a useful tumor marker which can indicated the progress of your treatment. Levels of CEA are measured before treatment in order to provide your medical team with a baseline so that they can track your CEA throughout and following treatment. For surgery, CEA levels may be measured both before and after surgery to evaluate both the success of the surgery. The CEA should decrease when treatment is successful, however in some cases chemotherapy and radiation therapy can cause a temporary rise in CEA due to the death of tumor cells and release of CEA into the blood stream. After surgery, with a curative resection of the cancer the CEA should return to normal in one to four months. Approximately 50% of patients with stage II and III cancers have elevated CEA levels.

Normal CEA Levels:

Nonsmokers:Less than 3 nanograms per milliliter (ng/mL) or 3 micrograms per liter (mcg/L).
Smokers: Less than 5 ng/mL (or 5 mcg/L)


A complete blood count (CBC) is a standard diagnostic test that determines the amount of red and white blood cells in your blood. This information can alert the doctor to whether you have an infection or anemia. Anemia can be a sign of cancer and is often times a side effect of chemotherapy. If you are receiving chemotherapy, your medical team will perform a CBC before each infusion in order to ensure you have an adequate number of white and red blood cells.

Imaging Tests

If colon cancer is being diagnosed, your doctor may order a chest x-ray to see if cancer has spread to the lungs.

Intraoperative ultrasound
The intrapoperative ultrasound is used during surgery to determine whether cancer has spread to the liver. The liver can be directly measured with a special probe that fits onto the end of the ultrasound device, giving doctors a much more accurate picture than one that is taken through the skin with a CT scan.


Liver Ultrasound
Intraoperative ultrasound with special probe fitted for imaging the liver directly

Magnetic resonance imaging (MRI)
This imaging procedure uses radio waves and strong magnets to create images of the interior of the body. MRI scans are helpful in looking for colon cancer that has spread to other parts of the body, particularly, the liver, brain, and spinal cord. An MRI often takes longer than CT scans and you may have to be placed inside a narrow cylinder or tube for the test. An MRI machine makes a loud clicking and thumping noise, which your radiologist will help to ease by providing you headphones with music to block out the noise.

Computed Tomography (CT & Spiral CT Scans)
CT imaging combines special x-ray equipment with sophisticated computers to produce multiple images or pictures of the inside of the body. These cross-sectional images of the area being studied can then be examined on a computer monitor or printed. CT scans of internal organs, bone, soft tissue and blood vessels provide greater clarity and reveal more details than regular x-ray exams. Using specialized equipment and expertise to create and interpret CT scans of the body, radiologists can more easily diagnose problems such as cancers, cardiovascular disease, infectious disease, trauma and musculoskeletal disorders. A CT scan may be used if colorectal cancer has metastasized to other organs. Spiral CT scans provide three-dimensional images at a lower dose of radiation than regular CT scans.

CT scan of the pelvis.


Positron emission tomography (PET)
The PET scan is used along with CT scans to search for metastasized cancer. Before a PET scan, the radiologist will inject a radioactive sugar solution into your vein. Since cancer cells absorb sugar in higher amounts than do normal cells, cancer cells are easily visualized by the radiologist as they take up the sugar. A PET scan will appear mostly dark except for areas where the cancer is absorbing high amounts of the sugar, these areas will appear as Òhot spotsÓ to the radiologist (see images below). PET scans are often combined with a CT scan in a very accurate PET-CT scan, helping your medical team to more accurately visualize where the cancer cells are within your body.


Rectal cancer on FDG PET imaging showing intense glucose metabolism where the cancer is located. FDG is a glucose tracer that indicates tissue metabolic activity in terms of regional glucose uptake.
Colorectal cancer showing metastatic disease to the liver on FDG PET imaging. The heart shows the most FDG uptake (normal). Just below and to the right are two foci of increased FDG uptake showing cancer in the liver.

Rectal cancer on PET/CT imaging showing intense glucose metabolism as well as a detailed anatomical view.

Next: Staging Colorectal Cancer