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DOI: 10.1148/rg.226025064
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(Radiographics. 2002;22:1525-1531.)
© RSNA, 2002

Plenary Session

CT Screening for Colorectal Cancer1

Judy Yee, MD

1 From the Department of Radiology, UCSF Veterans Affairs Medical Center, 4150 Clement St, San Francisco, CA 94121. From the Plenary Session, Friday Imaging Symposium: Screening for Cancer, at the 2001 RSNA scientific assembly. Received March 25, 2002; revision requested April 19 and received June 28; accepted July 2. Address correspondence to the author (e-mail: judy.yee@radiology.ucsf.edu).

Index Terms: Cancer screening • Colon, CT, 75.12119 • Rectum, CT, 757.12119


   Introduction
Top
 Introduction
Current Colorectal Cancer...
 CT Colonography
 Summary
 References
 
Colorectal cancer is the third most common cancer and the second most common cause of cancer deaths in the United States. It is estimated that 148,300 new cases of colorectal cancer will be diagnosed and 56,600 deaths from colorectal cancer will occur during 2002 (1). This malignancy occurs with equal frequency in men and women. Adenomatous polyps are the known precursors of the majority of colorectal cancers, with the risk of malignancy increasing with increasing polyp size. Detection of these polyps followed by polypectomy has been found to prevent the development of colorectal carcinoma. Despite the screening test options currently available, the majority of people who should undergo screening for colorectal cancer do not do so. CT colonography is a new, potentially powerful screening tool for colorectal cancer that may prove to have better patient compliance.


   Current Colorectal Cancer Screening Strategies
Top
 Introduction
 Current Colorectal Cancer...
CT Colonography
 Summary
 References
 
Four screening tests are routinely used in the United States for the detection of colorectal cancer. Current colorectal cancer screening options include fecal occult blood testing, flexible sigmoidoscopy, air-contrast barium enema examination, and fiberoptic colonoscopy. The American Cancer Society recommendations for colon cancer screening in average-risk adults 50 years of age and older include (a) annual fecal occult blood testing, (b) flexible sigmoidoscopy every 5 years, (c) annual fecal occult blood testing plus flexible sigmoidoscopy every 5 years, (d) double-contrast barium enema examination every 5 years, or (e) colonoscopy every 10 years (2).

Fecal Occult Blood Test
The fecal occult blood test is used to detect blood in the stool and is a guaiac-based test for peroxidase activity. It is safe and inexpensive. However, if it is used as a single screening test, its performance is poor, since most colon cancers bleed intermittently and most adenomatous polyps do not bleed. The sensitivity of the fecal occult blood test as a single test for colorectal cancer is 20%–30% and for a large polyp is 10%–15%. The sensitivity of this test for colorectal cancer increases with repeated screening and ranges between 72% and 78% (3). Many causes of false-positive results exist, such as upper gastrointestinal tract sources of bleeding. There is strong evidence that the fecal occult blood test decreases mortality secondary to colon cancer. Randomized controlled trials and case-control studies have shown that the fecal occult blood test confers a 15%–33% mortality reduction (4–7).

Flexible Sigmoidoscopy
Flexible sigmoidoscopy allows examination of about 60 cm of the colon, and therefore only 40%–65% of lesions are within reach of the sigmoidoscope. Up to 50% of proximal cancers are not associated with a distal index polyp (8). A recent study in 2,885 patients showed that a combination of fecal occult blood testing and flexible sigmoidoscopy failed in the detection of 24% of cases of advanced colorectal neoplasia. However, periodic sigmoidoscopy has been found to reduce colorectal cancer mortality. Three case control studies have shown a 60%–80% mortality reduction, but no results of randomized controlled trials are available (7,9,10).

Double-Contrast Barium Enema Examination
The double-contrast barium enema examination has been found to have sensitivities ranging from 71% to 95% in the detection of colon carcinoma in retrospective studies and is generally considered to be an excellent test for the detection of clinically significant lesions (11–14). However, the sensitivity decreases to as low as 50%–75% in prospective studies (15). A recent evaluation of the performance of the double-contrast barium enema examination compared with that of colonoscopy for colonic surveillance in patients who have already undergone polypectomy showed a low detection rate of 48% for polyps 10 mm diameter and larger, as well as a poor overall detection rate of only 39% for adenomas (16).

Colonoscopy
Fiberoptic colonoscopy is considered the standard for colon evaluation. However, colonoscopy has also been found to miss polyps. In a study of same-day back-to-back colonoscopic procedures, the miss rate was 24% for adenomas overall, 27% for adenomas 5 mm in diameter or smaller, 13% for adenomas 6–9 mm in diameter, and 6% for adenomas 10 mm in diameter or larger (17). The researchers concluded that improvements in colonoscopic technology were needed. Other limitations of colonoscopy include extremely variable patient compliance, the need for multidrug intravenous sedation, and the high cost of the test. In addition, the procedure is time-consuming, is incomplete in up to 10% of cases, and when a lesion is identified, is only 86% accurate in precisely localizing the lesion to a particular colonic segment (18). The prevalence of complications is low—the perforation rate of one in 1,000 increases to one in 500 when polypectomy is performed (3). There is also only indirect evidence that colorectal cancer mortality is reduced by colonoscopy.


   CT Colonography
Top
 Introduction
 Current Colorectal Cancer...
 CT Colonography
Summary
 References
 
First introduced in 1994, computed tomographic (CT) colonography has received widespread attention as a possible screening tool for colorectal polyps and cancer that may help to increase the rate of patient compliance. CT colonography involves the use of helical CT data in combination with advanced graphical software to generate two-dimensional views and three-dimensional endoluminal views of the colon. The endoluminal images, which may be viewed dynamically and interactively, simulate what is seen at conventional colonoscopy.

Adequate colonic preparation and distention are necessary to ensure a maximally diagnostic examination. Residual stool can obscure polyps or cause a false-positive result, and poor distention can limit the detection of polyps (Fig 1). Currently, patients must still undergo a colonic cleansing regimen that starts the day before CT colonography. Various purgatives that are used include polyethylene glycol, sodium phosphate, and magnesium citrate. Once the patient is on the CT table, bowel distention is achieved by means of retrograde insufflation of the colon with either atmospheric air or carbon dioxide via a small rectal tube. One milligram of glucagon may be administered intravenously, although its effectiveness as an antiperistaltic agent for CT colonography is controversial and it adds cost and invasiveness to the procedure (19–21).



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  		Figure 1a.  (a) Axial CT scan of a 58-year-old man shows a small amount of residual fluid with a focal protrusion of heterogeneous-attenuation stool (arrow) into the lumen of the rectum. (b) Three-dimensional endoluminal view shows the polypoid appearance of the fecal material (arrow). Residual stool is one of the most common causes of a false-positive result from CT colonography.

 


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  		Figure 1b.  (a) Axial CT scan of a 58-year-old man shows a small amount of residual fluid with a focal protrusion of heterogeneous-attenuation stool (arrow) into the lumen of the rectum. (b) Three-dimensional endoluminal view shows the polypoid appearance of the fecal material (arrow). Residual stool is one of the most common causes of a false-positive result from CT colonography.

 
Scanning is performed with the patient in two positions, typically supine and prone, and the use of a single breath hold in each position. The use of dual positions has been found to significantly increase the diagnostic ability of CT colonography (22,23) (Fig 2). CT technique should include a thin collimation of 5 mm or smaller, as well as less than 150 mA to reduce radiation dose. A gantry rotation period of less than 1 second is recommended to reduce scanning times. The volumetrically acquired CT data are then transferred to a computer workstation for interpretation. Axial images are magnified, or "zoomed," and used for primary interpretation (Fig 3). These images are interactively correlated with coronal and sagittal multiplanar reformatted views. The three-dimensional endoluminal views are used for problem solving in specific situations such as the distinction of a true polyp from a fold. The three-dimensional view is also used to measure polyp size and to show adjacent lesions (Fig 4). This interpretation method has been found to decrease interpretation times without compromise in performance (24,25).



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  		Figure 2a.  (a) Magnified axial CT scan of a portion of the colon obtained with the patient (a 65-year-old man) in a supine position shows a small polypoid lesion along the lateral wall (arrow). (b) Magnified axial CT scan obtained with the patient in the prone position shows the same small lesion in the identical location (arrow), confirming that it is a true polyp. This finding was confirmed with colonoscopy.

 


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  		Figure 2b.  (a) Magnified axial CT scan of a portion of the colon obtained with the patient (a 65-year-old man) in a supine position shows a small polypoid lesion along the lateral wall (arrow). (b) Magnified axial CT scan obtained with the patient in the prone position shows the same small lesion in the identical location (arrow), confirming that it is a true polyp. This finding was confirmed with colonoscopy.

 


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  		Figure 3a.  (a) Axial CT scan of a 63-year-old man shows a small polyp (arrow) in a portion of the sigmoid colon that is well distended. (b) Three-dimensional endoluminal view readily shows the same small polyp (arrow).

 


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  		Figure 3b.  (a) Axial CT scan of a 63-year-old man shows a small polyp (arrow) in a portion of the sigmoid colon that is well distended. (b) Three-dimensional endoluminal view readily shows the same small polyp (arrow).

 


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  		Figure 4.  Three-dimensional endoluminal view of a 70-year-old man shows two polyps (arrows). The smaller polyp is round, and the larger polyp is ovoid with a more irregular surface contour.

 
Diagnostic Ability
Results of an increasing number of single-center evaluations of the diagnostic ability of CT colonography with colonoscopy as the reference standard are becoming available. Larger and more current studies have shown that CT colonography and conventional colonoscopy have equivalent sensitivity in the detection of the 10-mm-diameter or larger polyps that are considered to be clinically significant. The majority of studies have been performed in high-risk or symptomatic patients. A large prospective multicenter trial of CT colonographic performance in asymptomatic patients is still needed before the method can be used as a routine screening tool for colorectal cancer.

Two different matching schemes help determine the sensitivity of CT colonography. The first is the per-patient sensitivity in which any lesion identified at CT colonography could match any lesion found at colonoscopy. This scheme is the less rigorous of the two but is thought to be more clinically relevant, since in the nonstudy situation, patients with a positive result from CT colonography would undergo colonoscopy. CT colonography has a per-patient sensitivity range of 78%–100% and a specificity range of 90%–96% in the detection of polyps 10 mm in diameter or larger (22,26–28). The per-patient sensitivity for polyps 5–10 mm in diameter ranges between 93% and 94%, and that for polyps 5 mm in diameter or smaller is 82%.

The second matching scheme is the per-polyp sensitivity, which uses location and size criteria to match lesions found at CT colonography with lesions found at colonoscopy. The per-polyp detection sensitivity range is 75%–91% for polyps 10 mm in diameter or larger, 47%–82% for polyps 5–10 mm in diameter, and 55%–59% for those 5 mm in diameter or smaller (Table).


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  		Sensitivity and Specificity of CT Colonography by Matching Scheme with Colonoscopy

 
CT colonography has been found useful in the evaluation of the colon proximal to a lesion that is causing distal obstruction and in the setting of failed colonoscopy. Three published studies document the ability of CT colonography to show the cause of obstruction, as well as to depict additional cancers and polyps in the colon proximal to the distal lesion (29–31) (Fig 5).



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  		Figure 5a.  (a) Axial CT scan of a 62-year-old man shows a large carcinoma (arrows) that obliterates the lumen of the sigmoid colon and prevented a complete colonoscopic evaluation. (b) Three-dimensional endoluminal view shows a large fungating mass that protrudes into the lumen (arrows). The remainder of the colon proximal to this obstructing mass was easily evaluated with CT colonography.

 


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  		Figure 5b.  (a) Axial CT scan of a 62-year-old man shows a large carcinoma (arrows) that obliterates the lumen of the sigmoid colon and prevented a complete colonoscopic evaluation. (b) Three-dimensional endoluminal view shows a large fungating mass that protrudes into the lumen (arrows). The remainder of the colon proximal to this obstructing mass was easily evaluated with CT colonography.

 
Advantages
The advantages of CT colonography are that it presents minimal risk to patients, has a short procedure time (about 10 minutes), can be performed in patients with distal occluding lesions, and can be used to localize lesions more precisely than can colonoscopy. CT colonography also allows diagnosis of extracolonic findings and screening for other clinically important diseases. Moderately significant findings such as gallstones, as well as highly significant findings such as renal cell carcinoma, large abdominal aortic aneurysms, and liver and adrenal masses can be identified (32). The ability to detect lesions outside the colon may be advantageous if future studies prove that the additional expense of working up these extracolonic findings still allows CT colonography to be a cost-effective examination. Otherwise, extensive additional work-up of indeterminate lesions can financially burden the healthcare system and limit the use of this test. Also to be considered are increased patient anxiety if an indeterminate lesion requires further testing and possible increased morbidity if additional procedures are required for potentially benign lesions. Because radiologists are obligated to interpret the entire abdominal and pelvic CT examination, interpretation times may be increased, as may the time spent per case for clinician consultation if an indeterminate or significant lesion is identified.

Limitations
CT colonography is limited in that patients must still undergo bowel cleansing, poor preparation or poor distention will compromise the diagnostic ability of CT, small and flat polyps are not well detected, complete colonic distention may cause discomfort, and the cost is still relatively high. CT colonography is also a "static" imaging examination compared with fluoroscopy, during which the colon may be dynamically viewed over a longer period. Thus, there is a higher likelihood that spasms seen on both supine and prone views obtained during CT colonography may be resolved during fluoroscopy, during which the colon is viewed over the course of several minutes. In addition, the steep learning curve for the interpretation of CT colonography necessitates additional training of radiologists.

Future Directions
An active area of investigation is the development of a "prepless" version of CT colonography for which a purgative and a liquid diet will no longer be required. During meals, patients will ingest barium or iodinated contrast material, which will tag residual stool and fluid in the colon. With the use of computer software, the tagged material could then be electronically removed, leaving the colon essentially cleansed (33,34).

Other areas of investigation include the use of electronically administered carbon dioxide to achieve reliable, optimal colonic distention with less patient discomfort. Multidetector CT protocols need to be optimized as we move to an increasing number of detector rows. Radiation doses must remain at acceptable levels, and low-milliamperage techniques must be used. Conventional and advanced forms of image displays must be optimized to allow maximal rapid interactivity between the various views. Advanced forms of image display under investigation include the various types of map projections and digital unraveling of the colon to produce a "virtual pathology" image. Computer-assisted detection algorithms are also being evaluated to help radiologists improve performance and decrease interpretation times (35,36).

Stool DNA testing is another potential noninvasive technique for colon cancer screening and, along with CT colonography, may be incorporated into colorectal cancer screening algorithms. Colonic neoplasms continuously exfoliate various DNA markers. Targeting multiple DNA mutations found in stool samples may achieve high rates of neoplasm detection with a sensitivity of 91% for cancer and 82% for large adenomas (37). Mutations on K-ras, APC, and p53 genes, as well as mutations on Bat-26 (a microsatellite instability marker) and long DNA have all been found to be markers for colorectal carcinoma.


   Summary
Top
 Introduction
 Current Colorectal Cancer...
 CT Colonography
 Summary
References
 
CT colonographic technique continues to evolve as advances in helical CT and computer technology are made. The performance of CT colonography is dependent on patient preparation, state-of-the-art equipment, and properly trained radiologists who are familiar with interpretation of CT colonographic findings. Although multiple studies have shown that CT colonography has excellent sensitivity and specificity in the detection of large polyps in high-risk or symptomatic patients, we cannot extrapolate this experience to a screening population until confirmatory studies have been performed.


   References
Top
 Introduction
 Current Colorectal Cancer...
 CT Colonography
 Summary
 References
 

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