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PET Scans

Staging Colorectal Cancer


Updated June 10, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Doctors use many different tests to learn how extensive your colon cancer is -- known as staging and grading the cancer. You may have already been "poked and prodded" with blood tests, filmed with radiographic exams or undergone surgery for histological (tissue) exams and biopsies. Unlike screening exams, which primarily find cancer in the colon, these staging exams help your doctor check the rest of your body for cancerous cells that may have metastasized, or spread, outside of the colon.

One of the most comprehensive exams for this purpose is the positron emission tomography, or PET scan. Although PET scans are also used to diagnose organ-specific diseases, such as problems in the heart or brain, they are frequently used to find cancer (metastasis or recurrence) at the cellular level. PET scans are often used in conjunction with computed tomography (CT) scans to locate cancers in the body.

How It Works

Prior to the exam, a tiny amount of fluorodeoxyglucose (FDG), which is a radioactive sugar (radiotracer), is injected into your vein. Approximately one hour following the injection, the sugar has traveled throughout your bloodstream and into your tissues. Cancerous cells soak up this sugar (more so than healthy tissues), virtually lighting them up during your scan. The PET scanner detects the energy from these radiotracers and a computer turns this information into three dimensional pictures, or cross-sections, of your body.


Your doctor will give you preparation instructions when he or she schedules the examination. Your doctor or nurse may:

  • Tell you to dress comfortably
  • Ask what medications you take
  • Ask what over-the-counter pills, supplements or herbal preparations you take
  • Tell you to stop eating and drinking four to six hours prior to your test

During the PET Scan

Upon arrival at the outpatient center or hospital, you may be asked to change into a hospital gown. A nurse or technician will start an intravenous catheter (IV) in your forearm or the vein inside your elbow, and inject the FDG tracer. You will return to a waiting room for up to an hour while the tracer circulates throughout your body (bring something to read or another way to occupy yourself).

The PET scanner is a tubular machine equipped with a hard, flat table. You will be asked to lie flat on the table, and you will enter the machine for scanning, which can take up to an hour. During this time, you will have to lie very still. You will be able to communicate any distress with the technician through speakers -- let him or her know if you are not feeling well.

What Will My Test Show?

Whole-body PET imaging will show any areas of increased metabolism (cells soaking up the sugar radiotracer) throughout your body. Cancer cells, areas of inflammation, and even infection will show as areas of increased metabolism. This information helps your doctor assimilate the best treatment plan for you, as well as deciding whether or not more tests are necessary.

You will not be "radioactive" after the test. The trace amount of radioactive sugars injected into your body are naturally flushed out and are not known to cause any lasting harm. You can accelerate this process by drinking plenty of water on the day following your test.

You will not receive any results immediately upon the conclusion of the exam. The radiology tech or nurse performing the exam is not trained to read the PET results -- a radiologist or nuclear medicine physician must read and compose the test report. You can usually expect test results within two to three days.

Special Considerations

Certain people should not have this test, or they should discuss their concerns with their doctor prior to the exam. Talk to your doctor if you are:

  • Pregnant or unsure if you are pregnant
  • Nursing
  • Claustrophobic
  • Unable to lie flat
  • Unable to lie still
  • Allergic to contrast or injections
  • Diabetic

Your doctor will determine how often you need PET scans, if repeat exams are warranted. He or she may also suggest other screening exams, including fine needle biopsies or magnetic resonance imaging (MRI), to provide a thorough assessment of your health.


American Cancer Society. (2006). American Cancer Society's Complete Guide to Colorectal Cancer. Clifton Fields, NE: American Cancer Society.

American Cancer Society. (n.d.). How is Colorectal Cancer Diagnosed? Accessed April 11, 2012.

American College of Radiology. (n.d.). PET/CT (Positron Emission Tomography). Accessed April 10, 2012.

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