New approaches to CRC screening
In addition to the recommended screening methods previously discussed, recent technologic advances have led to the development of new tools that may change the approach to CRC screening in the years to come.
Stool screening for DNA mutations: Our growing understanding of the human genome is changing the face of medicine in new and exciting ways. One of the earliest benefits of this new knowledge has been the development of gene-based markers to assist in the diagnosis (and ultimately the treatment) of various diseases.
DNA is a useful marker because it is not degraded during passage through the GI tract, it is excreted continuously, and it remains stable in the stool. A number of DNA mutations associated with the process of colonic carcinogenesis have been recognized, and methods have been developed to detect the presence of these mutations in excreted cells.
Specific combinations of mutations have been identified as being associated with colon adenomas and cancer. Therefore, tests based on these mutations are highly specific for cancer or precancerous lesions and have very low false-positive rates.
Different combinations of mutations are also being assessed for their ability to detect cancers proximal to the colon. Aerodigestive cancers (e.g., lung, esophageal, stomach, pancreas, colorectal) collectively account for more than half of all malignant deaths. Each of these cancers sheds cells (and hence DNA) into the GI tract, and preliminary studies show that altered DNA from these cancers can also be detected in the stools.
A number of issues must be addressed before this approach can be advocated for everyone. First and foremost, the method must be studied in average-risk populations. The tests must also be further refined.
New markers and testing methods are being defined continuously, and there is a lack of consensus on the optimal combination of markers. Appropriate screening intervals must be defined. There may also be challenges with patient acceptance. The existing model requires that the entire bowel movement be sent for processing, which some may find unacceptable.
Finally, cost must be addressed. Current DNA-based testing panels cost $500-$700 per use.
CT colonography/virtual colonoscopy (VC): VC is a radiographic technique that uses computer programming to combine multiple helical CT scans to create two- or three-dimensional images of the interior of the patient’s colon.
These images can be rotated for different views and even combined for a complete view of the colon in a “fly through” mode, which simulates the appearance of the colon lumen seen during traditional optical colonoscopy.
Results from early studies show accuracy to be comparable with conventional colonoscopy for the detection of cancers and large adenomas, with few false positives. At the present time, most published studies have been performed in populations known to be at increased risk for polyps or cancer.
Impressive results of screening VC in an average-risk population in the academic setting have recently been reported. Whether these results can be replicated in other environments remains to be seen.
VC is sometimes touted as a more comfortable, less invasive procedure than traditional colonoscopy. A majority of patients actually find the precolonoscopy bowel preparation (usually requiring potent cathartics) to be much more uncomfortable than the exam itself. In most settings, VC patients usually require a very similar prep to that necessary for conventional colonoscopy. Air insufflation for colonic distension is also a routine component of VC.
For these reasons, patients who have undergone both conventional colonoscopy and VC usually express minimal preference for one procedure over the other. Abnormalities seen on VC must be assessed with conventional colonoscopy, meaning that, in most instances, the patient must undergo the bowel prep a second time.
No organizations currently include stool screening for DNA mutations or VC among their recommended CRC screening measures. For now, these technologies should be considered exciting emerging tools that show considerable promise but have not yet demonstrated comparable performance or superiority to conventional, recommended screening methods in average-risk populations.
At this point, it should be clear that the correct answer to the patient in the opening scenario is:
(d) “There are a number of different screening tests for colon cancer. Let’s talk about them, then decide what’s right for you.”
A variety of factors now at work have the potential to increase screening for CRC. National public awareness campaigns by the ACS and the CDC, as well as media events such as the televised colonoscopy of Katie Couric, have raised public awareness of the disease as an important and preventable health problem.
Concurrently, the growth of evidence on the value of screening for CRC and on the impact of the various screening modalities has led to unanimous support by all major guidelines-issuing organizations, eliminating the conflicting messages previously aimed at health-care providers and the public. And yet, screening rates remain disturbingly low.
Clinicians can make a huge difference in this arena. While a wide variety of reasons for low CRC screening rates have been identified, one contributing factor turns up in nearly every study of this issue — clinician recommendation. Individuals who have been screened often attribute a recommendation from their health-care provider as the primary reason for having undergone testing. Conversely, individuals who have not been screened usually state that their provider has never discussed this issue with them.
Research on possible new screening methods may ultimately provide additional screening options to address the differing preferences of patients. In the meantime, clinicians should be discussing the benefits of CRC screening with all of their patients aged 50 years to 85 years and helping them decide which of the currently recommended screening methods is best for them.
Right now, the best test for CRC is the one the patient will take and can get.
Durado Brooks, MD, MPH, is director of the division of prostate and colorectal cancers at the American Cancer Society in Atlanta, GA.
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- U.S. Preventive Services Task Force. Screening for colorectal cancer: recommendations and rationale. Ann Intern Med. 2002;137:129-131.
- Pignone M, Rich M, Teutsch SM, et al. Screening for colorectal cancer in adults at average risk: a summary of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med. 2002;137:132-141.
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- Walsh JM, Terdiman JP. Colorectal cancer screening: clinical applications. JAMA. 2003;289:1297-1302.
Document last updated, October 25, 2011.