Computed tomographic (CT) colonography is a noninvasive method to evaluate the colon and has received considerable attention in the last decade as a colon-imaging tool. The technique has also been proposed as a potential primary colon cancer–screening method in the United States. The accuracy of the technique for the detection of large lesions seems to be high, perhaps in the range of colonoscopy. Overall, the field is rapidly evolving. Available data suggest that CT colonography, although a viable colon cancer screening modality in the United States, is not ready for widespread implementation, largely because of the lack of standards for training and reading and the limited number of skilled readers.
Computed tomographic (CT) colonography was initially proposed as a method to evaluate the colon for polyps and has grown and expanded remarkably over the previous decade. The growth of the new technology and research has been remarkable. There are many specific issues relevant to the test and its performance, which include the following: (1) test accuracy, (2) patient experience, (3) extracolonic lesions, and (4) colon preparation. There are also many new issues related to training. Also, whether the test is ready for widespread implementation is of critical importance.
The major attraction of CT colonography is that it is largely noninvasive (although there are growing concerns about radiation exposure, discussed later) and relatively simple to perform. Perhaps most importantly, because many patients are currently not being offered a colon cancer screening test, CT colonography offers an alternative, and data suggest that some patients who would not otherwise agree to screening may agree to undergo CT colonography.
Accuracy
The issue of CT colonography accuracy (ie, sensitivity to detect polyps and cancers) has evolved. Early studies examining highly selected populations and small cohorts, typically at high risk for colorectal cancer, revealed relatively high sensitivities and provided the foundation for the development of further studies. Later studies revealed wide variability in accuracy. Many larger single-center studies demonstrated extremely high sensitivities, but several well-publicized multicenter trials reported variable results ( Table 1 ). Two more recent multicenter studies incorporated elements that suggest that if generalizable, the technology and methodology is capable of detecting a high proportion of lesions. Although there are caveats, the field has advanced to the point that it should be expected that the sensitivity for detection of large (>1 cm) polyps is in the neighborhood of 90% and that this is a level that approaches that of colonoscopy. The caveats include questions about generalizability. For example, 2 of the studies seem to be more generalizable than the others. Also, for example, readers in some of these studies were highly trained, some studies used specialized preparative approaches, and some studies used specialized software platforms.
| References | Sensitivity (%) | |||
|---|---|---|---|---|
| Per Patient | Per Polyp | |||
| 6–9 mm | ≥10 mm | 6–9 mm | ≥10 mm | |
| Pickhardt et al, 2003 a | (n = 168) 89% | (n = 48) 94% | (n = 210) 86% | (n = 51) 92% |
| Cotton et al, 2004 | (n = 76) 30% | (n = 42) 55% | (n = 119) 23% | (n = 55) 52% |
| Rockey et al, 2005 | (n = 116) 51% | (n = 63) 59% | (n = 154) 47% | (n = 76) 53% |
| Johnson et al, 2008 a | (n = 210) 78% | (n = 109) 90% | (n = 270) 70% | (n = 76) 84% |
| Regge et al, 2009 a | NA | (n = 131) | (n = 173) | (n = 174) |
a These studies reported sensitivity specifically for adenomas rather than lesions, and in addition, reported sensitivity for adenomas (including cancers) greater than 6 mm rather than from 6 to 9 mm in size.
The Intermediate- and Small-Sized Polyp
A critical issue continues to surround the sensitivity of CT colonography for smaller lesions. The available literature is relatively clear on this point. The sensitivity of CT colonography is not as good as that of colonoscopy for the detection of lesions smaller than 1 cm (see Table 1 ). There is even greater controversy about the clinical significance of such small-sized polyp. Some investigators have argued that polyps that have sizes in the range of 6 to 9 mm have little potential for intrinsic neoplasia, whereas others have argued that these lesions pose a greater potential for neoplasia. The evidence suggests low malignant potential for these lesions, although perhaps the larger question relates to their longer-term malignant potential. Smaller polyps, which have an even lower risk of malignancy, are also controversial. A consensus radiology working group has suggested that lesions that have sizes in the range of 6 to 9 mm could be followed. This group has further recommended a scheme that does not recommend that lesion sizes less than 6 mm be reported at all, whereas other groups recommend reporting of all polyps.
A recent report suggested that should the consensus radiology working group’s recommendations for lesions sized 6 to 9 mm be used in a real-world patient population, up to a third of the patients with intermediate-sized lesions would have polypectomy delayed. With so much controversy, it seems that the best approach is to report lesions and to develop natural history data that allow patients and physicians to make more informed decisions about how to manage these lesions.
Bowel Preparation
A critical issue with all current colon-imaging modalities, including CT colonography as well as colonoscopy, is the need for bowel preparation. A fairly robust body of literature has explored the possibility of modified colon preparation. Although some reports have suggested that a high sensitivity for large lesions is achievable, the bulk of the evidence suggests that the “minimally prepared” technique is likely not ready for widespread use. Nonetheless, this area is extremely attractive because elimination of the need for a purgative preparation could greatly enhance screening participation.
Patient Experience and Acceptability
Several studies have specifically examined patient experience with and patient preference for colon-imaging procedures. Patient acceptance and tolerance are obviously complicated and are concerned with preconceived expectations, patient education, physician bias, and local practice patterns. Conclusions from published studies are mixed. Some studies have demonstrated that patients prefer CT colonography, whereas others indicate that patients think that colonoscopy is preferable. Regardless, CT colonography is an important area because the way that the test is perceived and tolerated by patients clearly plays an important role in its use for follow-up examinations.
Safety
CT colonography is likely to be very safe. Air insufflation has been associated with a small risk of perforation; the reported risk in large series of patients seems to be somewhere in the range of 1 in 2000. It seems that perforations occurring in the context of CT colonography are most likely to occur in patients with underlying colon abnormalities, particularly in those with inflammatory bowel disease (ulcerations or other defects in the colon mucosa may tear if stressed). The other situation in which the risk may be elevated is after polypectomy; thus CT colonography should be delayed after polypectomy has been performed.
Perhaps the most controversial area involving putative risk is that of radiation. Collective exposure to ionizing radiation in Western populations has clearly risen in recent years, and the footprint is clear. This exposure has been driven largely by medical imaging, and it is arguable that CT scanning has become all too common in the population in the United States. A routine CT scan of the abdomen administers a radiation dose of approximately 15 mSv. The dose for CT colonography is less. However, estimating the risk to the individual is difficult and controversial. Some estimates suggest that the risk of developing cancer is substantial, whereas others suggest that the risk is low. It is likely that the risk varies markedly and is greater for women and younger patients.
Reduction in radiation dosage seems to be readily feasible, with studies demonstrating that colorectal lesions can be accurately identified using low-dose protocols. Moving forward, it is clear that a major area of investigation will be lowering radiation doses; some current modeling studies have raised the possibility that CT colonography could be performed with essentially negligible radiation doses.
Extracolonic Lesions
CT colonography detects many extracolonic lesions, which have varying degrees of importance (calcifications, gallstones, hernias, bone lesions, abdominal aortic aneurysms, benign and malignant tumors). In one recent study, unsuspected extracolonic cancers were identified in 36 of 10,286 patients, including 11 patients with renal cell carcinoma, 8 with lung cancer, 6 with non-Hodgkin lymphoma, and 11 with other miscellaneous cancers. However, most lesions identified by CT colonography are not clinically important. Although it is clear that CT colonography is able to detect many lesions, it is also clear that the widespread investigation of these lesions leads to additional costs. The magnitude of cost is controversial.
Safety
CT colonography is likely to be very safe. Air insufflation has been associated with a small risk of perforation; the reported risk in large series of patients seems to be somewhere in the range of 1 in 2000. It seems that perforations occurring in the context of CT colonography are most likely to occur in patients with underlying colon abnormalities, particularly in those with inflammatory bowel disease (ulcerations or other defects in the colon mucosa may tear if stressed). The other situation in which the risk may be elevated is after polypectomy; thus CT colonography should be delayed after polypectomy has been performed.
Perhaps the most controversial area involving putative risk is that of radiation. Collective exposure to ionizing radiation in Western populations has clearly risen in recent years, and the footprint is clear. This exposure has been driven largely by medical imaging, and it is arguable that CT scanning has become all too common in the population in the United States. A routine CT scan of the abdomen administers a radiation dose of approximately 15 mSv. The dose for CT colonography is less. However, estimating the risk to the individual is difficult and controversial. Some estimates suggest that the risk of developing cancer is substantial, whereas others suggest that the risk is low. It is likely that the risk varies markedly and is greater for women and younger patients.
Reduction in radiation dosage seems to be readily feasible, with studies demonstrating that colorectal lesions can be accurately identified using low-dose protocols. Moving forward, it is clear that a major area of investigation will be lowering radiation doses; some current modeling studies have raised the possibility that CT colonography could be performed with essentially negligible radiation doses.
Extracolonic Lesions
CT colonography detects many extracolonic lesions, which have varying degrees of importance (calcifications, gallstones, hernias, bone lesions, abdominal aortic aneurysms, benign and malignant tumors). In one recent study, unsuspected extracolonic cancers were identified in 36 of 10,286 patients, including 11 patients with renal cell carcinoma, 8 with lung cancer, 6 with non-Hodgkin lymphoma, and 11 with other miscellaneous cancers. However, most lesions identified by CT colonography are not clinically important. Although it is clear that CT colonography is able to detect many lesions, it is also clear that the widespread investigation of these lesions leads to additional costs. The magnitude of cost is controversial.
New technology
New technology has advanced rapidly in the field of CT colonography. It seems that the technical aspects of CT scanners have now reached a level at which the examination can be performed rapidly and with relatively low radiation doses. New image display techniques are evolving, and multiple different software platforms are currently available. These techniques, for the most part, share more in common than differences. The most disruptive, and likely the most interesting, new technology is computer-aided diagnosis (CAD). CAD is attractive because it may enhance sensitivity and also reduce reader time. However, a consistent theme with CAD is that its use also seems to reduce specificity. This area in general is controversial, and further study is clearly needed.
Training
There is now sufficient evidence to indicate that practically any type of learner, including those with little to no experience in the area of CT colonography, can learn how to perform CT colonography. Further, these readers can learn to read examinations accurately. However, it is also clear that specific training is required to perform CT colonography.
Germane to the discussion about the specifics of training, abundant literature now indicates that, albeit variable, there is a learning curve associated with the process of gaining proficiency in reading the CT colonographic examination. In one study that included teams of a radiologist and gastroenterologist, it was found that accuracy increased after reading 25 cases. In another study, sensitivity improved after reading 50 cases and seemed ideal after interpreting 75 cases. The case threshold of 75 seemed to be important in another study. In a study with nonradiologists (medical students and radiology technologists) after training using a teaching file of 50 cases followed by blind interpretation of 50 cases, the nonradiologists performed similarly to the radiologists and showed improvement in proportion to case volume up to 100 cases.
An extremely controversial issue surrounds the amount of training exactly required to become proficient. The American College of Radiology has recommended an extremely aggressive training experience for those who are not already experienced in the performance and interpretation of CT of the abdomen and pelvis. Their recommendations are as follows:
- 1.
Completion of an Accreditation Council for Graduate Medical Education–approved training program in the respective specialty in which they practice, plus 200 credit hours of Category I Continuing Medical Education (CME) in the performance and interpretation of abdominal-pelvic CT.
- 2.
Supervision, interpretation, and reporting of 500 CT cases, at least 100 of which must be abdominal-pelvic CT during the past 36 months in a supervised situation.
- 3.
Education regarding patient preparation, bowel insufflation, and CT image acquisition.
- 4.
Formal hands-on interactive training using dedicated CT colonography software, including the interpretation, reporting, and/or supervised review of at least 75 endoscopically confirmed CT colonography cases using primary 2-dimensional (D) and/or primary 3D search with routine problem-solving techniques.
While the last 2 recommendations seems to be highly reasonable, the first 2 recommendations do not and indeed do not seem to be based on available literature, which suggests that the learning curve for CT colonography is in the range of 50 to 100 cases.
In contrast, the American Gastroenterological Association has proposed the following training specifically for gastroenterologists because CT colonographic interpretation should address cognitive skills required to perform all aspects of the CT colonographic examination ( Box 1 ). These recommendations include initial training as well as ongoing training, are rigorous, and readily allow gastroenterologists to read CT colonography examinations. An area of common ground between the 2 groups may exist in that it may be acceptable for a reader with less experience than a board-certified radiologist with expertise in cross-sectional imaging to read the intracolonic component of the CT colonography examination, whereas it may be most appropriate for a more-experienced reader to read the extracolonic portion.
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