As reviewed in this issue of The ASCO Post, the U.S. Preventive Services Task Force (USPSTF) recently updated its guidelines for colorectal cancer screening1 from 2008 and has now included seven acceptable strategies, including direct-visualization modalities (ie, endoscopy and computed tomography [CT] colonography), stool-based tests, and blood-based tests. The guidelines do not prioritize the different screening methods, largely due to a lack of direct head-to-head trials comparing the effectiveness of each method. Although the increased number of screening methods is meant to increase compliance with colorectal cancer screening, it also creates a challenge for patients and care providers in selecting the specific method to use.
Factoring in Patient Preference
To be clear, the best screening test for colorectal cancer for your patient is the one that is completed, as the guidelines stressed. It is estimated that one-third of people who are eligible for colorectal cancer screening have not undergone any type of screening exam.2 Although the reasons for this are complex, certainly patient preference and access to health care are major factors. Despite differences in effectiveness among the tests, the importance of acknowledging patient preference should not be overlooked.
Some patients may prefer tests that are less invasive, such as stool-based or blood-based tests, even if it means repeating them every year and potentially having a more invasive follow-up test, typically colonoscopy. Others may prefer going straight to colonoscopy, which (along with flexible sigmoidoscopy) offers the ability both to diagnose and remove precancerous lesions. Furthermore, patients and providers may not have access to the full repertoire of tests due to differences in health-resource availability, so a broader selection of methods will likely make colorectal cancer screening accessible to these patients. For these reasons, both patients and providers should welcome the new guidelines, as they offer several acceptable pathways for colorectal cancer screening.
Differences in Sensitivity and Specificity
Still, it is also important to be aware that there are substantial differences in the sensitivity and specificity of these methods for polyp and cancer detection. With regard to the stool-based tests, the fecal immunochemical test has largely replaced the guaiac-based fecal occult blood test because of its higher test accuracy and compliance. (It requires only a single stool specimen and has no dietary restrictions.) Despite this fact, it is worth noting that only guaiac-based fecal occult blood test has been proven to cause a reduction in colorectal cancer mortality. However, it is reasonably assumed that the guaiac-based fecal occult blood test results can be applied to fecal immunochemical testing as well, given that the fecal immunochemical test is more accurate for polyp and cancer detection.
Despite the lack of complete data on the screening methods included in the 2016 USPSTF screening guidelines for colorectal cancer, we should not let perfect be the enemy of very good. The greatest benefit to reducing the morbidity and mortality of colorectal cancer will come from increasing access to screening by any means.— Alexander R. Ende, MD, and William M. Grady, MD
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Fecal immunochemical test-DNA is a screening method added to this year’s USPSTF guideline. Fecal immunochemical test-DNA has a marginally higher sensitivity for polyp and cancer detection than the fecal immunochemical test and high specificity. It is currently suggested to be performed every 3 years, although the optimal interval remains to be determined.3 In addition, fecal immunochemical test-DNA testing is approximately 10 times more expensive than fecal immunochemical test testing and does not appear to be a cost-effective screening method compared with the other methods.
Blood-based testing using an assay based on methylated SEPT9 detection is another method added to this year’s USPSTF guidelines. It has a relatively low sensitivity, and there is no information available to guide appropriate intervals for repeat testing. It is widely believed to be the least accurate method for polyp and cancer detection in the list of methods included in the USPSTF guidelines.
Direct-Visualization Tests
The tests based on direct visualization include endoscopic procedures, namely colonoscopy and flexible sigmoidoscopy, and CT colonography. Colonoscopy is the only direct-visualization test with a demonstrated mortality benefit from a prospective study.4 Furthermore, the optical resolution of colonoscopes has improved significantly over the past few years, and operator-based aspects of colonoscopic screening have also improved, which both enhance the ability of the endoscopist to detect subtle lesions. Thus, it is anticipated that the impact of colonoscopy-based screening methods on cancer-related mortality will be even better in the future.
Flexible sigmoidoscopy is still listed as an acceptable alternative (performed every 5 years on its own or every 10 years when combined with yearly fecal immunochemical testing), although it is rarely performed as a screening test anymore in the United States. No trials have directly compared colonoscopy with flexible sigmoidoscopy.
CT colonography is not as well studied as endoscopic screening methods, and although it has a lower complication rate than colonoscopy, it has a relatively high risk of incidental, extracolonic findings and involves patient exposure to ionizing radiation. Thus, there are important differences between the available methods that need to be considered when selecting the method to be used for screening a specific individual.
Closing Thoughts
Compared with 2008, today the number of screening options provided by the USPSTF has increased substantially, even though the data supporting certain methods, such as the blood-based assay, are not substantial. The increased number of options should increase compliance but also introduces the burden of having to select a specific method.
Perhaps the most important and clinically relevant question in the United States is whether colonoscopy is superior to fecal immunochemical test, as these are the two most common methods used in this country. The data to allow such a comparison are lacking, but help is on the way. The CONFIRM trial, which is currently enrolling patients, should help to answer this question. Still, despite the lack of complete data on the screening methods included in the 2016 USPSTF screening guidelines for colorectal cancer, as the saying goes, we should not let perfect be the enemy of very good. The greatest benefit to reducing the morbidity and mortality of colorectal cancer will come from increasing access to screening by any means. ■
Disclosure: Drs. Ende and Grady reported no potential conflicts of interest.
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