Screening for Colon Cancer: CT Colography (”Virtual Colonoscopy”) Compared to Traditional Colonoscopy
Software may construct a three-dimensional CT scan image of the colon, which is termed CT colography, or “virtual” colonoscopy. A report from the University of Wisconsin in the New England Journal of Medicine in October, 2007 compared virtual colonscopy (VC) to standard colonoscopy (SC) screening in approximately 6000 patients who were involved in parallel screening programs. Prior studies of VC have shown that it can detect polyps of greater than or equal to 10 mm 90% of the time. However, these prior reports have urged caution in the adoption of VC for screening. The technique may miss smaller polyps, flat growths, or have diminished sensitivity for lesions in the right colon. The preparation for VC is simpler and perhaps causes less distress. However, at least 10% of VC studies were judged technically inadequate in prior studies, in some cases because of inadequate bowel cleansing. The technique of VC is safe, and because it is non-invasive, the risk of bowel perforation is absent. VC facilities are not widespread, software is potentially expensive, and radiologists are not all trained or experienced in VC interpretation. When VC demonstrates a significant lesion requiring biopsy, then SC is required, which increases the cost (since both studies would be done) in such a patient.
The NEJM report demonstrated that the number of advanced neoplasms (polyps >=10 mm and/or cancers) found was the same for VC (3.2%) or SC (3.4%) screening, in a average risk population of patients. The number of polypectomy procedures was significantly lower in the VC group (561) vs. the SC group (2434). That finding is easily understood, since patients with 1-5 mm lesions found at VC were not further evaluated, and patients with 6-9 mm lesions found at VC were offered either observation or SC. Only lesions >=10 mm at VC were referred for SC and biopsy. Therefore, the referral rate for SC from the VC group was 7.9% (246/3120 patients). 7/3163 patients in the SC group experienced procedure-related bowel perforations.
The potential drawback of VC noted by the authors is the possibility that failure to remove smaller lesions (<10 mm) might result in interval cancer development and progression, before the next planned VC follow-up. The data does support that polyp size is a reasonable surrogate for histologically-advanced (pre-malignant or malignant) lesions, the ones that really should be removed. In the study, lesions <10 mm were histologically advanced in only 20 polyps (15/6283 patients, prevalence 0.2%). In the <=5 mm category of polyps (diminuative), only 4 polyps were histologically advanced.
This study shows the promise of VC as a screening tool for average risk populations of patients. The technique may help “filter” patients who require SC screening, which may improve resource-utilization and reduce costs. The acceptance of VC be the general population might be superior as well. The study supports that VC can be used by experienced centers to achieve screening outcomes that are equivalent to SC. The risk of progression of small lesions that would be observed by serial VC (without biopsy) appears to be very small in this study. Patients who may consider VC as an alternative to SC should do so with caution, investigating the experience of the center, and with attention to following a protocol for re-evaluations such as the one described in this study.
Other methods of initial screening (to target patients who might require SC interventions) are under investigation, such as protein- or gene-based stool sample screening. The use of these methods to identify patients who have cancer-related protein or gene products in the stool might serve as another non-invasive tool to identify patients who need SC. However, these methods are not yet available or sufficiently validated. Therefore, the prudent advice for patients is to continue with regular SC screening methods at intervals outlined in available accepted guidelines.
