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Original Research |
1 All authors: Mayo Clinic, Mayo Medical School, Mary Clinic E-2, 200 First St. SW, Rochester, MN 55905.
Received January 7, 2005;
accepted after revision March 3, 2005.
Address correspondence to R. L. MacCarty.
Abstract
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SUBJECTS AND METHODS. Seventy-five consecutive patients who had been previously screened for polyps underwent same-day colonoscopy and CTC. Many of the patients had personal histories of previous polypectomies and were undergoing surveillance colonoscopy. The scans were interpreted prospectively by an experienced radiologist. Polyps missed prospectively on CTC were analyzed retrospectively by three experienced radiologists and categorized as perception errors (visible in retrospect), technical errors (e.g., obscured by feces or fluid), or occult (invisible).
RESULTS. Thirty polyps 5 mm or larger were found at colonoscopy, 18 of which were missed prospectively on CTC. Of the 18 missed polyps, 12 could not be identified in retrospect, even though they were located in clean, dry, well-distended colonic segments. These were classified as occult. Ten of the 12 occult polyps showed flat morphology on review of colonoscopy video recordings. Of the remaining six missed polyps, two were classified as perception errors, two as technical errors, and two as a combination of technical and perception error.
CONCLUSION. In this population, colonographically occult polyps were common and accounted for more detection failures than perception errors and technical errors combined. The high prevalence of occult polyps may be explained by the fact that previous screening may have led to removal of easy-to-see polyps, creating a study population with a higher percentage of hard-to-see polyps.
Keywords: colonography • colorectal cancer • CT • polyps
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Differences in patient populations may also be important and have received little attention to date in published articles. To our knowledge, no previous reports have addressed the potential relationship of previous screening to reduced polyp detection. Our purpose was to determine the prevalence of colonographically occult polyps in a population previously screened for colorectal polyps. Our hypothesis was that previous screening and attendant polypectomy would create a population with a relatively high prevalence of occult polyps on CTC and, based on our previous experience, would account for more detection failures than perception and technical errors.
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Patient regimen included a clear liquid diet beginning the day before the study, 4 L of polyethylene glycol (GoLYTELY, Braintree Laboratories), and two 5-mg bisacodyl tablets. One milligram of glucagon was injected subcutaneously 10 min before screening in an attempt to reduce colonic spasm and maximize distention.
All scans were performed with an 8-MDCT scanner (LightSpeed Ultra, GE Healthcare), using either 1.25 mm or 2.50 mm collimation, 13.5 mm/sec table speed, 120 kVp, and 80 mAs. Prone and supine acquisitions were obtained after gentle rectal insufflation of CO2 by hand to patient tolerance. Distention was verified on CT scout images before each acquisition.
Same-day colonoscopy was performed by an experienced staff gastroenterologist or colorectal surgeon blinded to the results of CTC. Polyps seen at colonoscopy were localized to one of eight segments (rectum, sigmoid, descending colon, splenic flexure, transverse colon, hepatic flexure, ascending colon, and cecum) for correlation with CTC. Polyp size was taken from the pathology report whenever the polyp was removed in one piece or from the endoscopist's report whenever the polyp was removed in pieces. Only polyps measuring 5 mm or greater were considered for analysis.
The scans were interpreted prospectively by a board-certified abdominal radiologist with experience interpreting more than 1,000 CTC examinations, who was blinded to the results of conventional colonoscopy, using 2D axial review and 3D endoluminal viewing for problem solving. Scans were interpreted on a remote Advantage workstation (version 4.1, GE Healthcare) using commercial software (Voxtool 3.0.58, GE Healthcare).
Polyps detected on CTC in the same segment or an adjacent segment as reported on colonoscopy were considered true-positive detections. When polyps were missed at the initial prospective interpretation, the scans were then reinterpreted retrospectively in unblinded fashion by three experienced radiologists using 2D axial review with lung and soft-tissue window settings and 3D endoluminal fly-through.
The missed polyps were then categorized by consensus of the three reviewers as perception errors (visible in retrospect), technical errors (obscured by feces, fluid, poor distention, or artifact), or occult (invisible). Videotapes of the conventional colonoscopy examinations were then reviewed in those patients with occult polyps, taking note of size, shape, and relation to mucosal folds, to attempt to explain why they were invisible on CTC.
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Review of the videotapes of the colonoscopy examinations and pathologic findings revealed that the occult polyps ranged in size from 7 mm to 2.1 cm. In 10 cases, flat configuration was judged to be the reason the polyps were invisible on CTC (Figs. 1A, 1B, and 1C). In the remaining two cases, the polyps were oblong and arose close to mucosal folds (Fig. 1D). Endoscopically, most of the occult polyps showed some variation in color (e.g., paler or pinker) compared with the surrounding normal mucosa, adding to their conspicuity at colonoscopy.
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Flat polyps are known to be problematic on CTC, but their prevalence has been thought to be low. Macari et al. [15] reported no flat polyps in their study of 186 patients and concluded that they appear to be rare in Western populations. Flat polyps do not seem to have been a problem in the 1,233 patients studied by Pickhardt et al. [12, 16], who reported a sensitivity of 93% for polyps 8 mm and greater, comparable with colonoscopy (91.5%). The high prevalence of flat polyps in our study group (10 of the 12 occult polyps, 16 of 30 overall) is difficult to explain and requires looking for differences in the population itself.
Early studies of CTC were, by and large, feasibility studies, testing the new technology in populations known to have a high prevalence of often conspicuous lesions. Sensitivities of CTC in these studies were relatively high. The population studied more recently by Pickhardt et al. [12] was a screening population. Subjects who had been previously screened by barium enema within 5 years of recruitment or by colonoscopy within 10 years were excluded. CTC achieved excellent sensitivity with this study group. By contrast, all the subjects in our study had been previously screened, some multiple times. All 12 of the completely occult polyps arose in four patients who had had previous polypectomies.
In this previously screened and surveillance population of 75 patients, prospective per-polyp sensitivity was a disappointing 40% and per-patient sensitivity was a modest 55%. Similarly modest sensitivities were reported by Johnson et al. [9] in a large group of 703 patients. It is noteworthy that 72.6% of those patients had personal histories of previous polyps or cancers. Details of previous examinations and previous polypectomies are not provided for the population studied by Cotton et al. [13]; however, the patients are described as "not constituting a screening population" and were recruited from a pool of patients scheduled for "clinically indicated colonoscopies." Among the clinical indications was "surveillance after polypectomy." The reported sensitivities in this group were also disappointingly low.
Previous screening could adversely affect sensitivity on follow-up studies in two ways. First, it is likely that many of the easy-to-see polyps would be detected and removed at the initial screening, and fewer of the hard-to-see polyps would be detected and removed. If the population were then studied by CTC, it is likely that the remaining polyps would be harder to detect, on average, than the polyps in the original pool, and the sensitivity of CTC would be less than if the initial screening had not taken place. Second, endoscopic polypectomy may be incomplete. Remnants of polyps are flatter than the original intact lesions, and would, therefore, be more difficult to detect on CTC, especially because CTC lacks the ability to discriminate the color differences between polyps and normal mucosa. If reduced sensitivity on CTC after polypectomy is confirmed in additional larger studies, the use of CTC in surveillance populations will have to be considered cautiously.
Several limitations in the current study need to be addressed. First, the study population is relatively small and the number of occult polyps relatively few. Nonetheless, it seems likely that the adverse effect of previous screening was at work in the much larger study reported by Johnson et al. [9] in which nearly three-fourths of the patients had histories of previous polypectomies, and individual sensitivities for three experienced reviewers for all polyps 5 mm or larger were 30%, 35%, and 63%, respectively. It is also possible that the disappointing results reported by Cotton et al. [13] could be partially explained by the effect of previous screening on their population of 615 patients who were going for "clinically indicated" colonoscopies.
A second limitation of our study is the high proportion of hyperplastic polyps, whose significance can be questioned. Recent evidence, however, suggests that some hyperplastic polyps may be significant [17], especially the so-called "sessile serrated polyps," a subset of hyperplastic polyps that originate more often in the right side of the colon and tend to become larger than the diminutive hyperplastic polyps more commonly found in the distal large bowel. Evidence suggests that these larger sessile serrated polyps may be premalignant. In a study by Goldstein et al. [18], 106 right-sided hyperplastic polyps were identified in 91 patients who subsequently developed carcinomas in the same anatomic region of the colon. It is worth noting that the one patient in our group who had the most occult hyperplastic polyps had a personal history of right-sided colon carcinoma.
There is also no doubt that adenomas can be flat and therefore difficult to detect on CTC, as shown by Fidler et al. [19], who studied 22 flat polyps and found eight that were proven to be adenomatous. In an analysis of 176 missed lesions reported by Gluecker et al. [14], 29 lesions were missed by both reviewers and could not be identified in retrospect. Although technical factors could have been contributory, it seems likely that many of these were inconspicuous because they were flat. Among these 29 lesions presumed to be occult, there were 18 adenomas.
In summary, colonographically occult polyps were common in this study population and accounted for twice as many detection failures as technical and perception errors combined. All occurred in patients with histories of previous polypectomies. Previous screening and subsequent polypectomy may have created a study population with a higher percentage of hard-to-see polyps. The potential adverse effect of previous polypectomy on CTC sensitivity needs further study before CTC can be recommended for postpolypectomy surveillance.
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