***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    Input: Context How often does colonoscopy miss adenomas ? Contribution During a multicenter screening trial , experienced colonoscopists performed same-day optical ( OC ) and virtual colonoscopy ( VC ) on 1233 asymptomatic adults .  Optical colonoscopy performed without knowledge of the VC findings missed 55 of 511 polyps ; 21 of these polyps were adenomas measuring 6 mm or greater . Adenomas missed by OC were usually on the proximal side of a fold or near the anal verge . Virtual colonoscopy missed 14 % of the adenomas that measured 6 mm or greater that were de-tected by OC . Implication s Neither OC nor VC is a perfect test : Each misses 10 % to 14 % of adenomas that measure 6 mm or greater . The Editors Optical colonoscopy ( OC ) is widely accepted as the gold st and ard for detecting colorectal neoplasia ( 1 , 2 ) . However , even in the most experienced h and s , this skilled examination is underst and ably not infallible . Retrospective analysis has suggested that the  OC miss rate for adenomas 10 mm or greater is approximately 10 % ( 3 ) . More recently , prospect i ve back-to-back or t and em colonoscopy studies have reported miss rates for 10-mm adenomas ranging from 0 % to 6 % ( 4 , 5 ) . However , in addition to evaluating relatively small population s of patients with a high prevalence of polyps , a notable weakness common to these studies was that they used OC as its own reference st and ard . In a large , prospect i ve , multicenter trial that was primarily intended to evaluate the performance of virtual colonoscopy ( VC ) in asymptomatic adults ( 6 ) , we also had a unique opportunity to evaluate the adenoma miss rate on OC  by segmentally unblinding the results from same-day VC . By using a reference st and ard other than OC itself for comparison , we could uncover lesions that may be systematic ally missed on repeated colonoscopies . These data not only provide novel insight into  OC miss rates but also indicate the relative blind spots where more attention could be focused . Methods Study Design The institutional review boards at all 3 participating medical centers approved the study protocol for same-day VC and OC , and all patients provided written informed consent . We recruited asymptomatic adults who were referred for colorectal cancer screening . Exclusion criteria were a positive stool guiaic test result or iron deficiency anemia within the past 6 months ; rectal bleeding , hematochezia , or unintentional weight loss of more than 10 pounds within the past 12 months ; OC within the past 10 years or barium enema within the past 5 years ; personal history of adenomatous polyps , colorectal cancer , or inflammatory bowel disease ; and family history of familial adenomatous polyposis or nonpolyposis cancer syndromes . Between May 2002 and June 2003 , 1253 asymptomatic adults enrolled in the study . Eight patients were excluded because of failure to reach the cecum at OC , 6 patients were excluded because of inadequate colonic preparation , and another 6 patients were excluded because of computed tomography ( CT ) system failure . The final study group comprised 1233 asymptomatic adults ( 728 men and 505 women ; mean age , 57.8 years ) who successfully completed same-day VC and OC . Study participants underwent colonic preparation with oral intake of 90 mL of phospho-soda and 10 mg of bisacodyl . To opacify residual colonic fluid and stool for VC examination , patients also consumed dilute oral contrast as previously described ( 7 ) . Our CT protocol and VC technique have also been detailed previously ( 6 ) . To briefly summarize , we obtained supine and prone CT acquisitions on multidetector scanners after patient-controlled rectal insufflation of room air . One of 6 trained radiologists interpreted VC studies by using a commercially available CT colonography system ( Viatronix V3D-Colon , version 1.2 , Viatronix , Inc. , Stony Brook , New York ) . We used the 3-dimensional endoluminal fly-through view primarily for detecting polyps and 2-dimensional images for confirmation and problem solving . We measured polyps on the 3-dimensional view and recorded them by segment ( cecum , ascending colon , hepatic flexure , transverse colon , splenic flexure , descending colon , sigmoid colon , or rectum ) . We defined the proximal colon as including the cecum to the splenic flexure . We prospect ively rated diagnostic confidence for each detected lesion on a 3-point scale ( most certain , intermediate , and least certain ) . One of 17 experienced colonoscopists performed OC immediately after VC interpretation by using st and ard commercial video colonoscopes ( Olympus , Inc. , Melville , New York ) . The colonoscope was advanced to the cecum and then sequentially withdrawn into more distal segments for polyp detection . The colonoscopist measured polyps by using a calibrated linear probe , which is more accurate than either visual or biopsy forceps estimation ( 8) . Our polyp-matching algorithm requires VC and OC agreement according to size ( within a 50 % margin of error ) and location ( within the same or adjacent segment ) . After the colonoscopist evaluated a given segment , a study nurse unblinded the VC results for the previous segment . For any suspected polyp seen on VC that measured 5 mm or greater but was not seen on the initial blinded OC , the colonoscopist closely reexamined that segment and could review the VC images for guidance . We sent all retrieved polyps for histologic examination . For all cases in which a colorectal neoplasm measuring 6 mm or greater was found on second-look OC , we retrospectively review ed both the VC and OC images . We recorded polyp characteristics , such as size , morphologic characteristic ( sessile , pedunculated , or flat ) , and location on VC . If the polyp was situated on a colonic fold on VC , we further subcategorized it as being located on the back ( proximal ) side , front ( distal ) side , or edge of the fold . We analyzed both supine and prone VC sets for all cases . The primary reason that diminutive polyps measuring 5 mm at VC were included for potential unblinding at OC was that , given the relative error in polyp measurement , such polyps found on second-look OC might , in fact , measure 6 mm or greater . This allows for more accurate assessment of the OC miss rate at the 6-mm threshold . We did not include unblinded polyps that measured 5 mm or less on both VC and OC examinations in the final analysis . All study participants completed a detailed question naire on their personal and family medical history . For the purpose s of this study , particular attention was given to the question about previous abdominal or pelvic surgery , since adhesions could conceivably result in a more difficult colonoscopic examination . Statistical Analysis Prospect i ve OC performance was compared against the enhanced reference st and ard of second-look OC after segmental unblinding of VC results . We estimated exact binomial 95 % CIs for OC miss rates . We used the chi-square test to compare the frequency of previous abdominal surgery among patients with and without polyps missed at OC and also to compare the OC miss rates among the 3 medical centers . We calculated the 95 % CIs by using Stata software , version 7.0 for Windows ( Stata Corp. , College Station , Texas ) , and performed the chi-square tests by using SAS software , version 8.0 for Windows ( SAS Institute , Inc. , Cary , North Carolina ) . Role of the Funding Source The funding source had no role in the collection , analysis , or interpretation of the data or in the decision to su bmi t the manuscript for publication . Results The performance characteristics of VC from this prospect i ve , multicenter screening trial , using OC as the reference st and ard , have been previously reported ( 6 ) . Our technique of segmental unblinding also allows for a separate assessment of OC by using the blinded VC results for comparison , which is the focus of this study . We identified 1310 polyps at OC in the 1233 asymptomatic adults ; 511 ( 39.0 % ) of these polyps measured 5 mm or greater ( Figure 1).Of these 511 polyps , 55 ( 10.8 % ) were found only on second-look OC after segmental unblinding of VC results . Twenty-four ( 43.6 % ) of the 55 unblinded lesions were nonadenomatous , including 16 hyperplastic polyps . Of the 31 missed neoplasms , 10 adenomas that measured only 5 mm were excluded from further analysis because of their diminutive size ( 9 ) . Including unblinded lesions , 554 adenomas were detected on OC in this screening sample ; 210 of these measured 6 mm or greater and 51 measured 10 mm or greater . Figure 1 . Polyp flowchart . In 20 patients ( 17 men and 3 women ; mean age , 58.2 years ) , 21 adenomas measuring 6 mm or greater ( range , 6 mm to 17 mm ; mean , 8.1 mm ) were found on OC only after the VC results were unblinded , which represent the lesions of primary interest for this study ( Table ) . The corresponding adenoma miss rate on prospect i ve OC examination was 10.0 % ( 95 % CI , 6.3 % to 14.9 % ) ( 21 of 210 adenomas ) at a 6-mm cutoff . The 20 patients with missed adenomas that measured 6 mm or greater represented only 1.6 % of the study sample ( 20 of 1233 patients ) but 11.9 % of patients with adenomas 6 mm or greater ( 20 of 168 patients ) . At 8-mm and 10-mm thresholds , the OC adenoma miss rates by polyp were 10.5 % ( CI , 5.2 % to 18.5 % ) ( 10 of 95 adenomas ) and 11.8 % ( CI , 4.4 % to 23.9 % ) ( 6 of 51 adenomas ) , respectively . The 10 patients with missed adenomas 8 mm or greater represented 12.2 % ( 10 of 82 patients ) of all patients with neoplasms of this size or greater ; the 6 patients with missed adenomas 8 mm or greater represented 12.5 % of all patients with neoplasms 10 mm or greater . Table . Characteristics of Neoplasms Missed at Prospect i ve Colonoscopic Evaluation Seventeen ( 81.0 % ) of the 21 unblinded neoplasms 6 mm or greater were tubular adenomas , 3 ( 14.3 % ) were tubulovillous adenomas , and 1 ( 4.8 % ) was an adenocarcinoma . Seven ( 33.3 % ) of the 21 unblinded polyps were classified as advanced lesions ( that is , size 10 mm or high- grade dysplasia , prominent villous component , or focus of malignancy ) . There were 15 sessile lesions , 4 pedunculated lesions , and 2 flat BACKGROUND : Colorectal cancer is the second leading cause of death from cancer in Western countries . Early detection by colorectal cancer screening can effectively cut its mortality rate . CT colonography represents a promising , minimally invasive alternative to conventional methods of colorectal carcinoma screening . AIMS : The purpose of this prospect i ve single institutional study was to compare the abilities of routine clinical CT colonography and conventional colonoscopy to detect colorectal neoplasms using second-look colonoscopy to clarify discrepant results . PATIENTS AND METHODS : CT colonography was performed in 100 symptomatic patients using contrast enhanced multidetector CT followed by conventional colonoscopy  on the same day . If results were discrepant , a second-look colonoscopy was performed after unblinding . CT colonographic findings were compared with those of conventional colonoscopy . RESULTS : Conventional colonoscopy found 122 colorectal neoplasms in 49 patients . The overall sensitivity of CT colonography at detecting patients with at least one polyp 6 mm or larger was 76 % and its specificity was 88 % . Its by-patient sensitivity for polyps 10 mm or larger was 95 % and its specificity was 98 % . By-polyp sensitivities were 71 % for polyps 10 mm or larger , and 61 % for polyps 6 mm or larger . A second-look colonoscopy was performed in 19 patients and two initial false-positive findings of CT colonography were reclassified as true-positive . For conventional colonoscopy , this produced a by-polyp sensitivity of 94 % for detection of lesions 6 mm and larger . CONCLUSIONS : CT colonography had both a high by-patient sensitivity and specificity for detection of clinical ly important colorectal neoplasms 10 mm or larger . This suggests that CT colonography has the potential to become a valuable clinical screening method for colorectal neoplasms CONTEXT Conventional colonoscopy is the best available method for detection of colorectal cancer ; however , it is invasive and not without risk . Computed tomographic colonography ( CTC ) , also known as virtual colonoscopy , has been reported to be reasonably accurate in the diagnosis of colorectal neoplasia in studies performed at expert centers . OBJECTIVE To assess the accuracy of CTC in a large number of participants across multiple centers . DESIGN , SETTING , AND PARTICIPANTS A nonr and omized , evaluator-blinded , noninferiority study design of 615 participants aged 50 years or older who were referred for routine , clinical ly indicated 
    Output:
    
CT colonography is highly sensitive for colorectal cancer , especially when both cathartic and tagging agents are combined in the bowel preparation .
Given the relatively low prevalence of colorectal cancer , primary CT colonography may be more suitable than OC for initial investigation of suspected colorectal cancer , assuming reasonable specificity .