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Colon & GI Cancers

Role of Virtual Colonoscopy in Screening for Colorectal Cancer 
  Submitted By: Matthew A Barish, M.D.

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Role of Virtual Colonoscopy in Screening for Colorectal Cancer

Authored by: Tatiana C. Rocha, M.D. and Matthew A. Barish, M.D.
Department of Radiology, Brigham and Women's Hospital
Boston, Massachusetts

Colorectal cancer is the second leading cause of cancer-related death in the United States. Most colorectal cancers begin as benign lesions in the colon called adenomatous polyps. Screening for colorectal cancer with removal of detected polyps is effective in reducing the incidence rate and mortality from this disease. Therefore, current guidelines recommend that people over the age of 50 years (or people with an increased risk of colorectal cancer) should be screened for colorectal cancer at routine intervals. Unfortunately, less than half of the people who should be screened in the United States have had appropriate screening tests. Each of the currently recommended screening tests has advantages and disadvantages. Virtual colonoscopy or CT colonography (CTC) is a new screening test for colorectal cancer. This article discusses this new test and what role it may play in colon cancer screening.

What is colorectal cancer and how does it develops?

Colorectal cancer is the second leading cause of cancer-related death in the United States [1]. Each year in the United States, 155,000 new cases are diagnosed and 65,000 people will die of the disease. The majority of colorectal cancers begin as benign lesions in the colon called adenomatous polyps. Over a period of several years, the cells inside the polyp transform into a malignant lesion known as colorectal carcinoma [2], more commonly known as colon cancer. The time for this transformation to occur in most cases is very long (usually over 10 years). Therefore, the detection and removal of these precursor adenomatous polyps decreases the risk for lethal colorectal cancer significantly. Although polyps are the initial presentation of colorectal cancer, not all types of polyps will become malignant lesions. In fact, most polyps never become cancer. Research has shown that large polyps are at greater risk of becoming cancerous than small polyps. Currently, however, the only way to know for certain whether the polyp can become a cancer is to remove it for pathological analysis.

Why screen for colorectal cancer?

A screening test helps detect cancer at an early stage, before it causes symptoms. Typically, people undergoing a screening test lack any direct symptoms of the disease or cancer in question, though they may have certain risk factors (e.g. family history). A screening test is not a diagnostic test and if a screening test is abnormal, further exams are necessary to make an accurate diagnosis. Many cancers become more difficult to treat as they progress, making the need for early detection especially important. Several studies have shown that screening for colorectal cancer is effective in reducing the incidence rate and mortality from this disease [3-5]. In particular, colon cancer can often be prevented entirely by removing polyps before they become cancerous. Therefore, current guidelines recommend screening of adults who are at average risk for colorectal cancer [6]. Unfortunately, less than one half of the average-risk population of the United States eligible for colorectal cancer screening has had appropriate screening tests [7].

What are the recommended screening methods for colorectal cancer?

Several government agencies, as well as the American Cancer Society (ACS), recommend that people over the age of 50 years or people with an increased risk of colon cancer should be screened for colon cancer at routine intervals [8]. The current methods of screening that are recommended by the ACS are conventional colonoscopy, flexible sigmoidoscopy, double-contrast barium enema, and fecal occult blood testing [6]. Each of these tests has advantages and disadvantages.

Fecal occult blood test: Fecal occult blood testing detects non-visible blood in feces. The test hopes to detect blood released by blood vessels at the surface of colorectal polyps or cancers. The blood vessels in these lesions are often fragile and can be damaged by the passage of stool. The blood detected by this test can be from anywhere in the digestive tract (from mouth to anus) and therefore, if this test is positive, additional testing is needed to determine the source of bleeding. Even foods or drugs that are eaten can cause the test to be incorrectly positive. In addition, this test cannot detect lesions that are not bleeding at the time the test is done. Conservative estimates suggest that a single screening for fecal occult blood may detect as little as 30% of cancers [9, 10].

Double Contrast Barium Enema: The barium enema is a test that detects polyps and cancers using x-rays. Since the entire colon will be examined, cleansing of the entire colon is required. This cleansing usually requires following of a special liquid diet, use of laxatives, and use of enemas. First, a radiologist places a small, thin tube into the rectum, with only 5-8 cm (2-3 inches) inside the patient. A radiologist then fills the colon with barium (a chalky white substance) through the tube. Then the radiologist inflates the colon with air to expand the colon and unfold the surface completely. The barium enema is inexpensive, quick, and has a lower rate of complications than either sigmoidoscopy or colonoscopy. It is capable of seeing more of the colon than sigmoidoscopy, but is not as accurate as colonoscopy. In addition, if the radiologist identifies any large polyps or cancers, colonoscopy will be required to remove the lesion.

Flexible sigmoidoscopy: A sigmoidoscope is a thin, flexible, 2-foot long tube about the thickness of an index finger that contains a camera. A doctor inserts the sigmoidoscope into the rectum and advances the scope into the lower part of the colon. A doctor is then able to view the images on a camera to see inside the colon and can also use tools to biopsy or remove small lesions and polyps. This test may be somewhat uncomfortable, but is rarely painful. In addition, before the doctor can perform this test, the patient must cleanse the colon with laxatives and enemas. Since the sigmoidoscope is only 60 centimeters (2 feet) long, the doctor is unable to see the entire colon with this instrument. Because over half the colon may not be visible, the doctor misses approximately 50% of all cancers and polyps. If a polyp or cancer is found, the doctor will recommend additional testing with colonoscopy to look for polyps or cancer in the rest of the colon.

Colonoscopy: A colonoscope is a longer, more flexible version of a sigmoidoscope. A doctor inserts the device through the rectum and advances the colonoscope through the entire colon. Like sigmoidoscopy, a doctor can view the images made by the camera to see inside the colon and can also use tools to biopsy or remove small lesions and polyps. The doctor will send the removed polyp to a pathologist, who views the lesion under a microscope to see if it contains cancer. Since the doctor can view the entire colon, cleansing of the entire colon is required with a special liquid diet and the use of laxatives and enemas. Conventional colonoscopy is currently the preferred strategy whenever the expertise and resources are available. This is because the barium enema, sigmoidoscopy and fecal occult blood testing are comparatively ineffective for detecting polyps or cancers. Colonoscopy has a greater effectiveness than these other screening techniques but still misses 6-12% of lesions greater than 1 cm and up to 13% of lesions greater than 6 mm [11, 12]. Colonoscopy has other disadvantages as well. Colonoscopy is uncomfortable and sometimes painful, and therefore usually requires the use of intravenous sedation. In addition to carrying some risk of complications, the use of sedation prevents patients from returning to work or pursuing other normal activities for several hours following the procedure. Colonoscopy is an invasive procedure with a small, but real, risk of one to two serious complications per 2,000 procedures.
Although conventional colonoscopy is preferred for screening, it is not an optimal screening tool. An optimal screening tool should be extremely safe, inexpensive, and rapid to perform. The purpose of screening is to identify those individuals more likely to have cancer or polyps. These individuals should then undergo the more definitive, invasive and expensive diagnostic tests (conventional colonoscopy). If a new screening tool were developed, conventional colonoscopy could then be reserved for definitive diagnosis and the removal of polyps detected by that screening test. This is the driving force for developing virtual colonoscopy as a screening test for colorectal cancer.

What is virtual colonoscopy?

Virtual colonoscopy or CT colonography (CTC) is a promising screening test for colorectal cancer. It relies on advances in computer-assisted tomography (CT) scanning equipment to produce high quality pictures of the colon. A radiologist then uses special computer software to examine the images from the inside of the colon and CTC eliminates the need to insert a long tube into the colon or to fill the colon with liquid barium. Currently, however, it is required that the bowel be cleansed before the exam. Research performed at many centers across the country has provided substantial evidence that CTC is better able to detect polyps [Figure 1] than fecal occult blood testing, barium enema, and sigmoidoscopy [13, 14, 15]. There is still controversy regarding the accuracy of CTC as compared to other tests, however there is evidence to suggest that CTC may be useful to detect early disease [12, 16, 17]. It is important to realize that CTC does not need to be equal to conventional colonoscopy since CTC is only a tool to screen to identify people at high risk for polyps or colorectal cancer who should then undergo a definitive conventional colonoscopy. It is expected that a screening test will miss some lesions, as is the case even with colonoscopic screening.

Figure 1: A 12 mm adenomatous polyp in the colon. Visualization of the polyp by virtual colonoscopy in (A). Visualization of the polyp by conventional colonoscopy in (B). Conventional colonoscopy image courtesy of Paula P. Elia, MD and Alvaro G. Freire, MD.

How is virtual colonoscopy performed?

As with conventional colonoscopy, patients must first cleanse the colon of fecal material. This requires the patient to take a laxative and to abstain from eating solid foods. Alternatively, patients may consume a specially formulated low-fiber diet with small amounts of laxative. On the day of the exam, a technologist will ask the patient to lie on the CT scanner table. At the start of the procedure, a technologist inserts a tube into the rectum so that the colon can be filled with gas (either air or carbon dioxide). This tube is very thin (about the thickness of a ball-point pen barrel) and flexible. The tube is much thinner and more flexible than a colonoscope or sigmoidoscope. In addition, unlike a sigmoidoscope or colonoscope, only 5-8 cm (2-3 inches) of the tube is inserted into the rectum. Once the tube is in place, the technologist connects the tube to a device that adds gas into the colon in order to expand the colon and allow the doctor to see the surface of the colon completely unfolded. The technologist then performs a CT scan while the patient lies on their back and then the technologist repeats the scan with the patient lying on their stomach. The time required for each scan is approximately 10-25 seconds. Because sedation is not required, patients are free to leave the CT suite immediately without the need for observation or recovery. Patients can resume normal activities immediately after the procedure and can eat, work or drive immediately afterward. Usually the entire procedure, from arrival to the CT scanner suite to leaving, takes less than 1 hour.

How is CTC used for screening of colorectal cancer?

It is important to understand the relative role of virtual colonoscopy in screening for colorectal cancer.. Virtual colonoscopy is an alternative to, but cannot completely substitute for, conventional colonoscopy. Since virtual colonoscopy can only detect but not remove lesions, patients who have large lesions [Figure 2] detected on virtual colonoscopy will also have to undergo a conventional colonoscopy to biopsy or remove the lesions. However, only a few patients who have a screening CTC will have lesions of sufficient size to warrant colonoscopy [12, 18]. Considering the current published evidence, virtual colonoscopy can be recommended for individuals who, for a number of reasons, cannot or should not undergo conventional colonoscopy. Virtual colonoscopy is also useful for people who are unwilling to undergo conventional colonoscopy. As with any medical examination, people should make the decision to undergo CT colonography in consultation with a physician. Patients should always involve their primary care physicians in the screening process.

Figure 2: A large cancer of the sigmoid colon. Visualization of the cancer by virtual colonoscopy in (A). Visualization of the mass by conventional colonoscopy in (B).

What are the advantages and disadvantages of CTC?

Advantages: Like conventional colonoscopy, virtual colonoscopy allows the physician to inspect the entire colon. This is a big advantage over sigmoidoscopy, which only views the lower half of the colon. Virtual colonoscopy is less invasive, and does not carry the low (1 in 1500), but real, risk of perforation of the colon associated with conventional colonoscopy. It is well tolerated by patients and it does not require sedation. Since sedation is not required, CTC may be much safer in patients with certain medical conditions such as severe lung disease or other breathing problems. In addition, since sedation is not used, there is no recovery time following the procedure.

Disadvantages: Since virtual colonoscopy is performed using a CT scanner, it exposes the individual to a small dose of radiation. The dose of radiation is less than that of a conventional abdominal CT scan, and is less than that of a barium enema. Since virtual colonoscopy can only detect, but not remove, lesions, patients who have lesions above a certain size will need to have a conventional colonoscopy to biopsy or remove these lesions. However, as previously stated, only a minority of patients who have a screening CTC will be found to have lesions of sufficient size to warrant colonoscopy. Virtual colonoscopy also has a lower detection rate for small polyps. As with any procedure, including conventional colonoscopy, there are no guarantees that all clinically significant growths will be detected. It should be remembered than between 10% and 20% of all polyps and up to 5% of colon cancers are missed even on conventional colonoscopy.


Despite all of the available tests for screening, colorectal cancer remains the second leading cause of cancer-related death in the United States. Screening for colorectal cancer is effective in reducing the incidence rate and mortality of this disease by removing polyps before they become cancerous. Unfortunately, less than one half of the average-risk population of the United States eligible for colorectal cancer screening has had appropriate screening tests. The most important recommendation is that all individuals over the age of 50 years (and earlier for patients with a family history of colorectal cancer) should consult with their primary care physician to choose the appropriate colorectal cancer screening test. 


Additional Authors:  
  Tatiana C. Rocha, M.D. Research Fellow 3D and Image Processing Center Department of Radiology Brigham and Women's Hospital 75 Francis Street, Boston, MA 02115 tcrocha@partners.org    

Works Cited:  
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19. Rex DK. Patients with polyps smaller than 1 cm on computed tomographic
colonography should be offered colonoscopy and polypectomy. Am J Gastroenterol. 2005;100(9):1903-5.
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Article Links:  
  • Virtual Colonoscopy Information
  • Brigham & Women's Hospital
  • Colon Cancer Information
  • Medical Textbooks (CollegeBooks.com)
  • Colon Cancer Book Section
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