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Perspective |
1 Department of Radiology, Mayo Clinic Arizona, 13400 E Shea Blvd., Scottsdale, AZ 85259.
Received September 23, 2008; accepted after revision October 1, 2008.
See page 1242 in this issue for the ACR Colon Cancer white paper abstract.
To view the full text of the ACR Colon Cancer Committee white paper, visit
www.arrs.org.
OBJECTIVE. The purpose of this article is to highlight key issues in CT colonography (CTC) for radiologists so they can represent the technology accurately to referring physicians and provide a perspective that will hopefully augment best care for their patients.
CONCLUSION. With publication of the National CT Colonography Trial and the endorsement of CTC for screening by a multisociety task force that included the American Cancer Society, American College of Radiology, and U.S. Multisociety Task Force on Colorectal Cancer, the clinical validation of CTC has been completed, and CTC is now ready for widespread clinical application. Radiologists must be skilled in CTC and knowledgeable about colorectal cancer screening issues.
Keywords: colorectal cancer • colorectal cancer screening • CT colonography
CT colonography (CTC) has been in development for more than a decade, with hundreds of articles now published on its performance and technical capabilities. With the conclusion and publication of the National CT Colonography Trial [1] and endorsement of the technique for screening by a multisociety task force, including the American Cancer Society, American College of Radiology (ACR), and U.S. Multisociety Task Force on Colorectal Cancer [2], the clinical validation of CTC in the prepared colon has been completed. The accompanying white paper on CTC by the ACR Colon Cancer Committee provides an erudite review of clinical indications, performance, acquisition and patient preparation techniques, and implementation [3]. CTC has come of age for mainstream colorectal screening, with growing insurance coverage for a screening indication. The purpose of this perspective is to highlight key issues for radiologists so they can represent the technology accurately to referring physicians and provide a perspective that will hopefully augment best care for their patients.
Colorectal Cancer Screening
Colorectal cancer affects 153,760 patients yearly, with 52,180 deaths [4]. It accounts for the second most common cancer killer because it affects both men and women—yet, ironically, it is preventable in the majority of cases if the precursor adenoma is detected and removed. Symptoms usually indicate advanced malignant disease because adenomas usually are asymptomatic and only discovered by routine screening of asymptomatic individuals. Average-risk individuals (about 80% of the population) should begin screening at age 50 years, whereas those at moderate risk (15% of the population who have a first-degree relative with colon cancer before age 60 years or multiple relatives at any age) should begin at age 40 years. High-risk individuals with genetic syndromes (familial polyposis syndrome and hereditary nonpolyposis colon cancer syndrome) and those with inflammatory bowel disease should be managed individually with endoscopy [5]. Both average- and moderate-risk individuals could be screened with CTC.
The problem is not one of having tests available for screening because multiple tests already exist with varying capability, cost, and invasiveness. The fecal occult blood test (FOBT) has proven effectiveness in detecting cancers and lowering the mortality from colorectal cancer [6–9]. This inexpensive, noninvasive test, however, has a high false-negative rate (cancers only bleed intermittently) and a high false-positive rate. Furthermore, the FOBT does not reliably detect the precursor adenomas because they do not bleed [9–12].
Sigmoidoscopy is a safe and moderately priced procedure with proven mortality reduction [13–16], but at best it only examines the left half of the colon. Detection of left-sided lesions at sigmoidoscopy is an inadequate proxy for right-sided disease and subsequent colonoscopy [17].
Interest and expertise in barium enema screening has been lost in most radiology practices, and even in controlled prospective trials barium enema detects only about half of all large adenomas despite is high safety profile and moderate cost [18].
Colonoscopy has become the reference standard for colorectal imaging, although it is the most expensive and invasive and carries the highest risk (perforation, bleeding, sedation complications) of all available tests. None of the standard tests is therefore optimal in terms of safety, cost, or performance. CTC has been shown to have a high safety profile [19], performance similar to colonoscopy [1, 20], and a cost higher than barium enema but lower than colonoscopy.
Patient acceptance of routine colorectal screening remains a major barrier for all of the previously mentioned procedures, including CTC. In 2000, less than half of U.S. adults 50 years or older had undergone a sigmoidoscopy or colonoscopy within the previous 10 years or had used an FOBT home test kit within the preceding year [21]. The major disincentive for patients considering CTC as a screening option is the laxative purgation (the same as that required for colonoscopy) [22]. Patient education of the advantages of CTC over other colorectal tests may encourage compliance. These advantages include the lack of required sedation and IV line placement for CTC, a quick return to work after the examination, and no need to inconvenience others for transportation to and from the examination. The risk of perforation at CTC is considerably less than at colonoscopy. Furthermore, that the examination only requires two breath-holds on the CT scanner (in the supine and prone positions) with completion of most average examinations in 10 minutes may help reassure hesitant patients. Still, the reality of a full bowel preparation, an enema tip, and full (although brief) colonic insufflation is likely to delay the decision for screening for some. In my experience, patient education regarding bowel preparation and the CTC examination process, with proper setting of expectations and some useful tips (such as chilling the colon lavage before drinking it or adding a powered lemonade mix) can make the experience more tolerable. Once patients understand what to anticipate, the actual experience is usually good—and few will refuse to repeat the examination when recommended.
CTC Performance
The performance of CTC has undergone exhaustive testing. Earlier acceptance of this technique was delayed because of conflicting sensitivity estimates for the detection of large lesions in three large multicenter clinical trials. The study by Pickhardt et al. [19] showed sensitivity similar to colonoscopy, but concerns were raised that community practices might not be able to achieve these results. The National CTC Trial (ACRIN 6664) [23] studied 2,531 individuals across 15 centers composed of both academic and private practices. The findings of this trial were similar to those of Pickhardt et al. and have reassured many groups and medical societies that the technique when properly performed can detect large- and intermediate-size adenomas with sensitivity similar to colonoscopy [1]. An interesting part of this trial was the required training and testing of radiologists. Although some required more training than others, those participating all received a passing score of 90% for easy and moderately difficult to detect lesions [23]. These excellent training scores translated into trial outcomes because the overall sensitivity for the detection of large adenomas in the ACRIN trial was 90% [1]. I suspect that not everyone is wired to interpret these examinations well. Readers are confronted with huge data sets and looking for random and uncommon tiny bumps on the colon surface. Our specialty should not compromise on quality interpretations but instead help practices identify those individuals who are skilled in these types of screening examinations and excuse those who are not. The ACRIN trial also insisted on strict adherence to protocol requirements including stool tagging regimens, mechanical insufflation of the colon, and thin-section and low-dose CT techniques [24]. It is clear that meticulous attention to all aspects of the examination is required to achieve optimal results [3].
Extracolonic Abnormalities and Risks
Extracolonic abnormalities are common in patients of screening age. Fortunately, only a minority of these findings require additional imaging tests if the examinations are interpreted properly, and only a subset of these patients will require treatment [24–28]. As always, the devil is in the details. Radiologists have the ability to detect other important conditions in the course of a colon screening study and thus potentially save additional lives and morbidity. This ability is offset by the knowledge that if every abnormality was noted and followed, CTC would not be cost effective and patients would be exposed to additional unnecessary testing, risk, and cost.
A pragmatic approach to these findings is needed. Radiologists need to recommend follow-up studies for those findings most likely to be of clinical significance. For example, the probable renal or hepatic cyst should be called a benign cyst without follow-up. Clinicians will be grateful if additional testing is minimized, and for those who need addition studies, the recommended optimal follow-up should be included in the report. Radiologists should keep track of the percentage of patients who have significant findings and compare these rates to national norms. Many studies have reported similar findings—combined urgent findings and those requiring additional imaging on follow-up do not exceed 20%.
Unfortunately, the risk associated with the low radiation dose required for CTC has been misunderstood and used to political advantage by some. It is important to understand that the standard dose at CTC represents about half of the dose used for a standard body CT examination. This results in an average dose of approximately 5 mSv. The real risk of this dose is unknown, but the Health Physics Society has stated that doses in this range have risks for the development of radiation-induced cancer that are too small to measure or are nonexistent [29]. Even if a very small risk is assumed from radiation exposure at CT, it must be balanced against the risk of developing colon cancer and the risk of other alternative procedures. The risks of perforation (1:1,000) and death (1:17,000) at colonoscopy are real and can be measured [30], but as a society we have determined that these risks are outweighed by the risk of developing colon cancer (about 1 in 13 without screening) [31]. We have a responsibility to use the lowest radiation dose possible for our patients, but if the facts as presented are understood, then concerns about the technique solely on the basis of radiation risk are unfounded.
Maintaining Quality
Maintaining high-quality interpretations is a responsibility that each
individual, each practice, and our specialty should assume. The ACR has
established a national CTC database within the National Radiology Data
Registry [32]. Selected
process and outcome metrics can be quickly entered online and compared with
national benchmarks. These measures include process metrics related to the CT
technique and the adequacy of patient preparation as well as outcome metrics
related to colon perforation, true-positive and false-positive rates for large
(
1 cm) polyps, and the prevalence of significant extracolonic findings.
Practices seriously interested in providing the best care should be encouraged
to participate in this data registry and manage their practices so benchmark
metrics are achieved.
To serve our patients best, there will need to be a spirit of cooperation between radiologists and gastroenterologists. Guidelines will need to be jointly developed for the proper use of colonography and colonoscopy and for processes to efficiently transfer patients with polyps to colonoscopy. Because a large percentage of the population remains unscreened, there is ample opportunity for both specialties to thrive. Those practices that are able to do this effectively will offer patients a service of high value—and will likely find themselves very busy.
Future of CTC
The future of CTC remains bright because the potential exists to perform this examination without the disincentive of the cathartic bowel preparation. Stool attenuation can be altered with oral agents enabling discrimination of labeled stool from soft-tissue-attenuation bowel wall [33]. Optimally, electronic subtraction of stool would be performed, presenting the radiologist with a virtually cleansed colon to assess. Even without subtraction, results are promising using 2D images. Early results in populations with a high prevalence of polyps are similar to those in the cathartic-prepared colon [34]. Additional confirmatory studies in a screening population are needed. If successful, an entire new era of colorectal cancer screening would be launched—no other full structural examination can be performed without catharsis. Our adolescent technology would have reached adulthood.
Many other questions remain for CTC, including whether it is feasible or appropriate to monitor intermediate-size polyps without polypectomy and, if so, at what intervals. Can new CT reconstruction algorithms further reduce the dose at CTC without compromising image quality? What are the minimum educational requirements and optimal training programs for practitioners? How do gastroenterologists and radiologists work together optimally to provide the best patient care? How can this technology be used to provide care to those underserved by existing colorectal screening methods? Myriad other questions exist for academics to study.
Conclusion
In summary, CTC has completed clinical validation and is now ready for widespread clinical application. Radiologists committed to performing the examination to the highest quality must obtain the education and equipment needed. We must focus our efforts on the best in patient care and dodge the political distractions that will come. We have an obligation to educate referring physicians on the correct use of the technique. Collaborations with gastroenterologists to ensure same-day polypectomy for selected patients will enhance patient care. We must be vigilant that extracolonic findings are properly reported so that only highly significant lesions are recommended for additional follow-up testing. Lastly, we should be committed to ongoing quality measures to improve and to maintain the highest standards of care. Radiology has another exciting opportunity to serve the public and potentially help reduce the incidence of a common cancer.
References
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