What is Virtual Colonoscopy ? 

It is a technique for non-invasive examination of the colon. Unlike conventional colonoscopy which involves the insertion of an endoscope into the colon of the patient, in VC, the colon is inflated with carbon dioxide introduced into the rectum via a soft, small rubber catheter. An EBCT scanner then acquires the image of the colon and a high performance computer software programme generates a virtual fly-through 3-dimensional image of the colon, a similar view seen in conventional endoscopy.

                                          
                        EBCT virtual colonography                   Colon cancer "apple core lesion" in transverse seen

Screening and Surveillance for the Early Detection of Colorectal Cancer and Adenomatous Polyps, 2008: A Joint Guideline from the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology^,

CTC
CTC, also referred to as virtual colonoscopy, is a minimally invasive imaging examination of the entire colon and rectum. CTC uses CT to acquire images and advanced 2-dimensional (2D)- and 3-dimensional (3D)-image display techniques for interpretation. Since its introduction in the mid-1990s, there have been rapid advancements in CTC technology. Computer imaging graphics allow for visualization of 3D endoscopic flight paths through the inside of the colon, which are simultaneously viewed with interactive 2D images. The integrated use of the 3D and 2D techniques allows for ease of polyp detection, as well as characterization of lesion density and location. The 2D images also allow for limited evaluation of the extracolonic structures.

Adequate bowel preparation and gaseous distention of the colon are essential to ensure a successful examination. Patients typically undergo full cathartic preparation along with a clear liquid diet the day before the study, similar to the requirements for colonoscopy. Tagging of residual solid stool and fluid with barium and/or iodine oral contrast agents is being increasingly used and validated in large trials. At CT, a small-caliber rectal catheter is inserted into the rectum, followed by automated or manual insufflation of room air or carbon dioxide. Intravenous contrast generally is not given to patients undergoing screening but can be helpful in some patients with more advanced symptoms. Typically, the entire procedure on the CT table takes approximately 10 minutes, with no sedation or recovery time needed.

CTC—Efficacy and Test Performance.

A recent multi-institutional screening trial using more advanced CTC techniques demonstrated favorable performance. Pickhardt et al studied 1,233 asymptomatic adults and introduced the techniques of stool tagging and primary 3D polyp detection^. This trial reported a 94% sensitivity for large adenomas, with a per-patient sensitivity for adenomas 6 mm of 89%.

In 2005, 2 meta-analyses reviewed the cumulative published CTC performance data, including both high-risk and screening cohorts, with one analysis representing 33 studies on 6,393 patients. On a per-patient basis, pooled CTC sensitivity and specificity for large (10 mm) polyps was found to be 85% to 93% and 97%, respectively. Pooled sensitivity and specificity for detection of small polyps (6 to 9 mm) was 70% to 86% and 86% to 93%, respectively. Of note, the pooled CTC sensitivity for invasive CRC was 96%, comparable with the reported sensitivity for OC.

There also are a number of CTC trials currently in progress within the United States and Europe. Initial results from smaller screening trials utilizing 3D polyp detection by Cash et al¹⁷⁰ and Graser et al¹⁷¹ have shown CTC performance characteristics similar to that of Pickhardt et al, providing at least a measure of independent validation for this screening technique. Also of particular interest is the recently completed ACRIN Study 6664: National CT Colonography Trial, which is sponsored and funded by the National Cancer Institute. The primary aim of this trial was to assess CTC performance for large adenomas and advanced neoplasia in a large screening cohort of 2,500 patients across 15 institutions. State-of-the-art techniques included oral contrast tagging, colonic distention with automated carbon dioxide delivery, multidetector row CT (16 slice) with thin collimation, and both 2D and 3D polyp detection on dedicated CTC software systems. Specialized training and achievement of a high level of expertise were required of the radiologists prior to participation in the study. Preliminary findings announced at the 2007 annual meeting of ACRIN on September 28, 2007, were consistent with other recent studies using state-of-the-art techniques.

Beyond validation, a recent study demonstrated the efficacy of CTC to select patients who would benefit from therapeutic polypectomy. Kim et al recently reported comparative results from primary CTC (with selective recommendation for therapeutic colonoscopy) and primary OC screening arms among 3,120 and 3,163 mostly asymptomatic adults, respectively. Although this study did not randomize participants to CTC versus OC, apart from a slightly higher proportion of individuals with a family history in the OC group, the 2 groups were similar. Similar rates of advanced neoplasia were found in each group, with 3.2% in the CTC group and 3.4% in the OC group.¹⁷²

CTC—Benefits, Limitations, and Harms. CTC provides a time-efficient procedure with good accuracy and minimal invasiveness. No sedation or recovery time is required, nor is a chaperone needed to provide transportation after the procedure. Time permitting, patients can return to work on the same day.

CTC requires the same full cathartic bowel preparation and restricted diet as colonoscopy, which may decrease patient adherence. As an "imaging-only," nontherapeutic evaluation of the colon, patients with polyps of significant size will require therapeutic colonoscopy for subsequent polypectomy. Thus, it is possible to offer same-day polypectomy to patients for whom colonoscopy is recommended without the need for additional bowel preparation.

The accuracy of CTC is influenced by lesion size, and the sensitivity and specificity of CTC improves with polyp size. The accuracy of CTC in measuring polyp size is of particular importance since accurate size estimation is critical for appropriate patient management and for minimizing the false-positive rate.  Pickhardt et al showed that specificity (when polyps were matched for size) was 97.4% for lesions 1 cm but declines to 84.5% for all lesions to all lesions 6mm.

There is controversy over the long-term potential harms associated with radiation dose effects from CT examinations.   In a recent position statement issued by the Health Physics Society, the health effects of low-dose radiation exposure (defined as below 50 to 100 mSv—a threshold many times higher than typical CTC levels) were considered to be "either too small to be observed or are nonexistent."  Nevertheless, although this risk may be theoretical, there is a growing concern that more individuals are receiving multiple diagnostic evaluations with ionizing radiation over a lifetime and that for some individuals the doses over a lifetime can reach levels that are sufficiently high to be of concern. It is important to put these issues into context with respect to screening with CTC.

Since CTC is a minimally invasive test, the risk for colonic perforation during screening is extremely low. In the collective experience of the International Working Group on Virtual Colonoscopy, there were no cases of perforation in over 11,000 screening CTC examinations, and out of nearly 22,000 total CTC examinations (screening and diagnostic), there was only one symptomatic perforation, corresponding to a symptomatic perforation rate of 0.005%.

Because CTC produces an image not only of the colon but also the upper and lower abdomen, there is a chance that incidental extracolonic findings will be observed. Although the overall rates of extracolonic findings have been reported to range from 15% to 69%, the incidence of clinically significant extracolonic findings at CTC has ranged from 4.5% to 11% in various patient cohorts. In an asymptomatic screening population, the incidence of unsuspected but potentially important extracolonic findings is approximately 4.5%, but findings of minimal or moderate potential clinical significance, such as cholelithiasis (6%) and nephrolithiasis (8%), are more common.

CTC—Conclusions and Recommendations. In terms of detection of colon cancer and advanced neoplasia, which is the primary goal of screening for CRC and adenomatous polyps, recent data suggest CTC is comparable to OC for the detection of cancer and polyps of significant size when state-of-the-art techniques are applied. In previous assessments of the performance of CTC, the ACS concluded that data were insufficient to recommend screening with CTC for average-risk individuals. Based on the accumulation of evidence since that time, the expert panel concludes that there are sufficient data to include CTC as an acceptable option for CRC screening.