**8. Extracolonic findings**

ASIR in the prone position. No significant image quality differences were seen between standard-and low-dose images using 40% ASIR. The results of this pilot study show that the radiation dose during CTC can be reduced 50% below currently accepted low-dose techni‐ ques without significantly affecting image quality when ASIR is used [98]. Larger studies are needed to confirm this observation. Despite the increasing use of multi-slice scanners, which are slightly less dose-efficient, the median effective dose remained approximately constant between 1996 and 2004 [96]. Of 83 institutions, 62% used 64-detector row CT and 17

If the current CTC standards for radiation exposure are used for colorectal cancer screening, CTC is still be a viable screening tool, even after taking into account the increased risks of developing future cancers. Using a Monte Carlo simulation, it was found that for every 1 radiation-related cancer caused by CTC screening, 24 – 35 colorectal cancers would be pre‐ vented, implying a favorable risk to benefit ratio in favor of using CTC as a screening tool [100]. This model assumed using CTC every 5 years in patients aged 50 – 80 years old and using an estimated mean effective dose per CTC screening study of 8 mSv for women and 7

An alternative solution to minimize the radiation is to use magnetic resonance imaging (MRI) instead of CT for virtual colonoscopy, i.e., MR colonography (MRC) [101]. However, this MRC alternative solution has several limitation compared to CTC. Currently it is more costly, more sensitive to motion and other artifacts, and has lower spatial resolution but

When asked if they would prefer CTC or OC, patients more often prefer CTC [102]. In one study, 696 asymptomatic patients at high risk for colorectal cancer screening underwent both CTC and OC [103]. Patients were asked using standardized forms about preparation inconvenience and discomfort, examination discomfort and examination preference. Over‐ all, patients preferred CTC to OC (72.3% vs. 5.1%; P <0.001). Reported discomfort however, was similar for CTC and OC (P = 0.63). In another study that evaluated patients with a histo‐ ry of diverticulitis, 74% preferred CTC preferred over OC [71]. Patients found colonoscopy more uncomfortable (p < 0.03), more painful (p < 0.001), and more difficult (p < 0.01) than

Other studies conflict with those mentioned above. Even though CTC is a less invasive alter‐ native than OC, procedural pain is not uncommon. In several studies, the pain associated with CTC was higher than that associated with OC, albeit there is no sedation given for the former test [104]. Using a time-trade off technique, 295 patients reported statistically more pain and discomfort after CTC and showed preference for optical colonoscopy [105]. The

In a well-designed study, 111 patients underwent CTC followed immediately by OC [15]. The preference for either examination was evaluated after completion of both examina‐

pain during CTC however, is usually not so severe as to abort the test [33].

with improvements with technology these disadvantages may be minimized.

(50%) used dose modulation [99].

260 Colonoscopy and Colorectal Cancer Screening - Future Directions

mSv for men.

CTC [71].

**7. Patient preferences**

Extracolonic findings are an important issue for CTC as they increase costs and patient risk by incurring addition tests. The frequency of extracolonic findings in the literature varies considerably as there are no standards for their reporting nor what constitutes a clinically significant extra-colonic finding. Some of the extracolonic lesions found are clinically impor‐ tant although most of them are incidental. In addition to increasing costs of CTC screening programs, they may cause undue worry and anxiety for the patient.

The prevalence of extracolonic findings can be as high as 40% - 75% and are increased with patient age [107-109]. Most extracolonic findings are incidental and not clinically important. The most common extracolonic findings causing further evaluation in one study were lung nodules and indeterminate kidney lesions adding a cost of \$248 dollars per patient enrolled for CTC screening [110].

In general, significant extracolonic findings are found in about 10 - 23% of patients undergo‐ ing CTC [111-113]. Potentially important extracolonic findings were seen in 15.4% (89 of 577) of patients in one study, with a work-up rate of 7.8% (45 of 577)[114]. In another study only 4.4% - 6.0% of patients required follow-up radiologic testing for the extracolonic findings [109]. Another study showed that 10% of 681 patients screened for colon cancer had extraco‐ lonic findings of high clinical importance [115].

Although extracolonic finding add cost to CTC screening programs, they may benefit pa‐ tients by diagnosing other potentially malignant lesions [112]. Unsuspected cancers (colonic and extra-colonic) are found in about 0.5% of screening cases [116]. In a large study, 36/10,286 patients (0.35%) undergoing a screening examination had an unsuspected extraco‐ lonic cancer which included renal cell carcinoma (n = 11), lung cancer (n = 8), non-Hodgkin lymphoma (n = 60, and a variety of other tumors (n = 11)[116). Other studies report a higher rate of 2.7% of extracolonic cancer detection [107].

### **9. Computer-Assisted Diagnosis (CAD)**

An intensive area of research is the development of computer-assisted diagnosis (CAD) al‐ gorithms. CAD can assist radiologists as a second reader to improve accuracy [117, 118]. It has been shown that CAD can aid trained radiologists in the detection of significant pol‐ yps [119].CAD significantly improved polyp detection by 12% in one study, (from 48 to 60%) with only a moderate increase in interpretation time [120]. Another study demon‐ strated that using CAD in second-read mode increased accuracy in 13 of 19 readers 968%); CAD increased sensitivity of finding polyps but decreased specificity slightly [121]. In general, using CAD increases polyp detection but also increases false positives as well [122, 123].

**11. Cost-effectiveness**

fective than FOBT plus flexible sigmoidoscopy[137].

**12. New directions – Noncathartic bowel preparations**

tive than previously thought [139-141].

[144].

With a 6-mm size threshold for polyps, the overall referral rate to optical colonoscopy is about 15% [114]. CTC is usually a less expensive test than OC, however the total costs may not be less if one considers all of the variables such as compliance rates and referral rates for OC after detecting lesions. Using a Markov model, screening by CTC costs \$24,586 per lifeyear saved compared to \$20,930 for OC screening [136]. CTC becomes a more cost-effective test as the compliance rate for screening increases or if the cost for CTC is 54% lower than OC. On the other hand in a recent analysis both CTC and OC were more costly and less ef‐

Virtual Colonoscopy

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http://dx.doi.org/10.5772/52544

A Markov model was used to estimate the cost-effectiveness of CTC screening in an Italian population. In this study, colorectal cancer was reduced by 40.9% and 38.2%, with OC and CTC respectively. As compared to no screening, both CTC and OC were shown to be costsaving with CTC being the less expensive option [138]. Since CTC can accurately detect and simultaneously screen for aortic aneurysms, cardiac atherosclerotic risk factors and osteopo‐ rosis, the benefits of CTC screening in an elderly population may be even more cost-effec‐

One new hope for the future is for patients to undergo CTC without laxatives or the need for a purgative bowel preparation. Patients would only need to ingest fecal tagging agents such as Gastroview or barium, one to two days before the test [142]. A pilot study using a noncathartic bowel preparation (low fiber diet and fecal tagging) had disappointing results demonstrating that the lack of bowel cleansing made the examination subjectively harder to interpret and likely missed significant polyps [143]. A subsequent study however using a noncathartic bowel preparation was performed in a high risk population [144]. Subjects in‐ gested 21.6 g of barium in nine divided doses. This study demonstrated that the sensitivity of CTC using a non-cathartic bowel preparation for polyps greater than 9 mm was over 90%

Limited or non-cathartic bowel preparations may be especially useful in the frail or elderly patient. In a prospective study, 67 elderly patients with reduced functional status under‐ went CTC using a limited bowel preparation consisting of a low-residue diet for 3 days, 1 L of 2% oral diatrizoatemeglumine (Gastrografin) 24 hours before CTC, and 1 L of 2% oral Gastrografin over the 2 hours immediately before CTC [145]. No cathartic preparation was administered. All colonic segments were graded from 1 to 5 for image quality (1, unreada‐ ble; 2, poor; 3, equivocal; 4, good; 5, excellent). Overall image quality was rated good or ex‐ cellent in 84% of the colonic segments. Colonic abnormalities were identified in 12 patients

(18%), including four colonic tumors, two polyps, and seven colonic strictures [145].

Using CAD as a primary reader is feasible but early studies showed less sensitivity than hu‐ man readers [124]. The sensitivity of CAD detected polyps 10 mm or greater was 64% (18/28) in one study [125]. In a later study of 1,186 patients undergoing both CTC and OC on the same day, CAD had a sensitivity of 89.3% (25/28; 95% CI: 71.8%-97.7%) for detecting ad‐ enomatous polyps at least 1 cm in size [126]. The false-positive rate was 2.1% (95% CI: 2.0% - 2.2%). CAD detected both of the carcinomas in the study group. In this study, CAD had a per-patient sensitivity comparable to that of OC for adenomas at least 8 mm in size [126]. Another study found a per-patient sensitivity of 96% was for CAD (in patients with a me‐ dian polyp diameter of 6 mm) using external validation [127]. Several CAD polyp detection systems exist such as Polyp Enhanced Viewing (PEV) and the Summers computer-aided de‐ tection (CAD) system (National Institutes of Health (NIH)). These systems vary and have trade-offs in terms of sensitivity and specificity [128].

#### **10. Safety**

It is difficult to make a head-to-head comparison of the safety of CTC vs. OC since they are different technologies with varying risks. In one study, CTC screening was performed in 3,120 adults and compared to primary OC screening in 3,163 adults. There were seven co‐ lonic perforations in the OC group and none in the CTC group [45]. Colonic perforation has been reported with CTC but its occurrence is rare [129, 130]. Nine perforations out of 17,067 CTC examinations (0.052%) were reported in one study [131]. In another study of 11, 870 CTC studies, the perforation rate was 0.059% [132].

Possible factors that contribute to perforation are presence of an inguinal hernia contain‐ ing colon (n = 4), severe diverticulosis (n = 3), and obstructive carcinoma (n = 1)[132, 133]. In cases of obstructing lesions, gas should be insufflated slowly [133]. Colonic pneumato‐ sis is rarely seen (0.11%) in CTC studies and should not be confused with perforation [134, 135]. Overall, potentially serious adverse events related to CTC occur in less than 0.10% of patients [131].
