**2. Technique**

As advancements in scanner technology and three-dimensional (3D) post-processing helped develop this method to mature into a potential option in screening for colorectal cancer, the fundamentals of the examination remained the same. It is a minimally invasive, CT-based procedure that simulates conventional colonoscopy using 2D and 3D computerized recon‐ structions [10]. CTC utilizes computer virtual-reality techniques to navigate inside a threedimensionalz (3D) patient-specific colon model reconstructed from abdominal CT images, looking for polyps.

usually mobile and polyps are not. The rectal balloon is a potential blind spot for CTC, sometimes masking significant lesions [25, 26]. Also, poor colonic distention can result in

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The primary aim of CTC is the detection of colorectal polyps and carcinomas, however; the precise role of CTC in screening asymptomatic patients is controversial [28]. Studies using patients with known adenomas generally report higher accuracy, while studies employing asymptomatic screening subjects report lower accuracy [28]. Two key areas have held back the widespread application of CTC as a screening test. These key areas are: [1] the variable sensitivity of CTC reported in mass screening programs (see Table) and [2], the expertise re‐ quired to interpret the examination. These two areas are related [29]. Despite these draw‐ backs, the American College of Radiology, has endorsed the use of CTC as a screening tool for colo-rectal cancer stating that the sensitivity and specificity of CTC are high enough and comparable to those of OC [30]. In addition, CTC has received the endorsement of a multisociety task force that included the American Cancer Society and U. S. Multi-society Task

Studies reveal a wide variation in performance measures (sensitivity and specificity) regard‐ ing polyp detection rates, especially for smaller polyps [10]. In an early feasibility study of 44 patients, CTC demonstrated reasonable sensitivity (83%) and specificity (100%) for pol‐ yps larger than 8 mm in size [32]. A second early study performed in 87 patients at high risk for colorectal neoplasia identified 49 patients with a total of 115 polyps and 3 carcinomas [33]. CTC identified all 3 cancers. The sensitivity was 91% for polyps that were 10 mm or more in diameter, 82% [33/40] that were 6 to 9 mm, and 55% [29/53] that were 5 mm or smaller [33]. There were 19 false positive findings of polyps and no false positive findings of cancer. In a larger study of 300 patients CTC demonstrated a sensitivity equal to 90% for polyps 10 mm or larger and 80.1% for polyps at least 5 mm in size [34]. The overall specifici‐

ty for this study was 72.0% [34]. All 8 carcinomas in the study were detected by CTC.

Two later studies assessing the accuracy of CTC had varying results. Pickhardt et al. evalu‐ ated 1,233 asymptomatic patients with CTC and same-day OC [35]. The sensitivity of CTC for adenomatous polyps at least 10 mm in size was 93.8% and 88.7% for polyps at least 6 mm in size, which was comparable to OC. The specificity of CTC for adenomatous polyps at least 10 mm in size was 96.0% and 79.6% for polyps at least 6 mm in size. These encouraging results were followed a year later by less optimistic findings from a study by Cotton et al., that analyzed 600 participants undergoing both CTC and OC [36]. In the Cotton study, 104 of the participants (17.3%) had lesions sized at least 6 mm. The sensitivity of CTC for detect‐ ing 1 or more lesions sized at least 6 mm was only 39.0% and for lesions sized at least 10 mm, it was 55.0% (95% Cl, 39.9% - 70.0%) [36]. The specificity of CTC for detecting partici‐ pants without any lesion sized at least 6 mm was 90.5% and without lesions sized at least 10 mm, 96.0% (95% Cl, 94.3% - 97.6%). CTC missed 2 of 8 cancers [36]. Lack of adequate radiol‐ ogist training to read CTCs may have resulted in the low accuracy found in this study.

a false negative reading [27].

Force on Colorectal Cancer [31].

**3. Screening for colorectal cancer: CTC vs. OC**

CTC examination starts by inflating a cleansed colon with room air or carbon dioxide (CO2) introduced through rectal catheter [14]. With the patient in a prone position, air or CO2 is insufflated under gentle pressure to ensure adequate distention of the bowel. The insuffla‐ tion of gas is usually associated with a mild degree of patient discomfort or pain [15]. Al‐ though not common, vaso-vagal reactions can occur, especially if with small bowel distention occurs [16].

Then abdominal CT slice images are taken in seconds (during a single breath hold) with sub millimeter resolution in both axial and transverse directions resulting in excellent contrast between the colon wall and the lumen. The sliced images are stacked together as a volume image, from which the colon model is constructed. Image segmentation is necessary for the construction of an accurate colon model. Computer graphics are heavily involved to navi‐ gate or fly through inside the 3D colon model. The patient is scanned in both a prone and supine view [17]. Using a second view significantly improves the ability to identify patients with polyps 0.5 cm in diameter or larger [18].

CTC can be performed in patients with prior abdominoperineal resection and sigmoid co‐ lostomy, although increased difficulties with CO2 retention and adequate bowel disten‐ tion exist [19]. The prevalence of transient bacterium after CTC is low therefore it follows that patients with at risk cardiac lesions should not require antibiotic prophylaxis before‐ hand [20].

Most commonly used bowel preparations include sodium phosphates, magnesium citrate and polyethylene glycol (PEG) [21]. Typical oral preparations used for bowel cleansing are: 4 L of PEG solution; 90 ml of phosphosoda; or 300 ml of magnesium citrate. Polyp detection is comparable for all three preparations, although phosphosoda has a significantly higher patient compliance and the least residual stool [22]. Residual fluid coverage negatively af‐ fects the quality of CTC [23].

The use of fecal tagging agents and intravenous contrast is not standardized. CTC experts were surveyed regarding their practice patterns [24]. Thirty-eight percent performed fecal tagging regularly and 81% [21/26] believed intravenous contrast was not necessary [24].

Non-operator dependent false positives and false negatives occur with CTC. For exam‐ ple, inadequately tagged stool can have the same density as a polyp, however the two can sometimes be distinguished by comparing prone and supine views, since stool is usually mobile and polyps are not. The rectal balloon is a potential blind spot for CTC, sometimes masking significant lesions [25, 26]. Also, poor colonic distention can result in a false negative reading [27].
