Colorectal Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Evidence of Benefit
Table 3. Randomized Controlled Screening Trials: Fecal Occult Blood Testing continued...
Virtual Colonoscopy (Computed Tomographic [CT] Colonography)
Virtual colonoscopy (also known as CT colonography or CT pneumocolon) refers to the examination of computer-generated images of the colon constructed from data obtained from an abdominal CT examination. These images simulate the effect of a conventional colonoscopy. Patients must take laxatives to clean the colon before the procedure, and the colon is insufflated with air (sometimes carbon dioxide) by insertion of a rectal tube just prior to radiographic examination.
A large, paired-design study was conducted by the American College of Radiology Imaging Network group, with 2,531 average-risk people (prevalence of polyps or cancer greater than or equal to 10 mm = 4%; mean age about 58 years) screened with both CT colonography and optical colonoscopy (OC). The gold standard was the OC, including repeat OC exams for people with lesions found by computed tomographic colonography (CTC) but not by OC. Of 109 people with at least one adenoma or cancer greater than or equal to 10 mm, 98 (90%) were detected by CTC (referring everyone with a CTC lesion of 5 mm or greater). Specificity was 86% and PPV was 23%. There are several concerns from this study, including the following:
- Most, but not all, lesions found by CTC and not by OC were followed up with repeat OC.
- The design itself did not allow for following patients, thus potentially missing lesions that grow rapidly and would only be seen after follow-up.
- Because the centers conducting the screening were primarily academic centers and the radiologists and endoscopists were carefully trained, the generalizability of the findings is not clear.
- Sixteen percent of people had an extracolonic finding that required further evaluation.
Unknowns from the study include the following for either OC or CTC: the number of detected polyps that would have progressed to invasive cancer, and the number of people harmed by the screening process.
Another study reported similar sensitivity and specificity in persons with an increased risk of CRC. In this study, the sensitivity of OC could not be determined because it was done in an unblinded manner. This study suggests that virtual colonoscopy might be an acceptable screening or surveillance test for persons with a high risk of CRC, but this cross-sectional study does not address outcome or frequency of testing in high-risk persons.
Some studies have assessed how well virtual colonoscopy can detect colorectal polyps without a laxative prep. The question is of great importance for implementation because the laxative prep required by both conventional colonoscopy and virtual colonoscopy is considered a great disadvantage by patients. By tagging feces with iodinated contrast material ingested during several days prior to the procedure, investigators in one study were able to detect lesions larger than 8 mm with 95% sensitivity and 92% specificity. The particular tagging material used in this study caused about 10% of patients to become nauseated; however, other materials are being assessed.