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T Colonography:ertinent Issues for the Colorectal Surgeon

avid H. Kim, MD, and Perry J. Pickhardt, MD

Computed tomography colonography (CTC) has rapidly evolved into a clinically effectivetool since its initial introduction in 1994. Although not in widespread use for colorectalcancer (CRC) screening in part due to reimbursement issues, there is emerging consensusthat advances in hardware and software technology as well as in specific protocol tech-niques have allowed polyp detection performance characteristics equivalent to opticalcolonoscopy. CTC issues relevant to the colorectal surgeon revolve around both thoseconcerning CRC screening and those regarding focused problem-solving for specificclinical situations. This article will outline the basic components of a CTC examination aswell as the specific parameters in current use in a large-scale screening CTC program. Thepotential roles of CTC in the CRC screening realm, performance characteristics withstate-of-the-art techniques, theorized radiation risks, and the issue regarding extracolonicfindings will be addressed. The utility of standardized lexicon and reporting system will behighlighted. Problem-solving issues pertinent for the colorectal surgeon to be addressedinclude lesion localization for preoperative planning, synchronous lesion assessment, andsubmucosal lesion characterization.Semin Colon Rectal Surg 18:88-95 © 2007 Elsevier Inc. All rights reserved.

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ince its introduction in 1994,1 computed tomographycolonography (CTC) has rapidly advanced from a re-

earch novelty to a potentially clinically effective tool. Con-iderable progress in underlying computer hardware andoftware as well as refinement in technique have allowederformance characteristics for polyp detection (�8 mm inize) comparable to optical colonoscopy (OC).2 At its core,TC represents a computed tomographic (CT) examinationptimized for the detection of colorectal polyps (Fig. 1). Itnvolves patient preparation factors, scanning factors, andnterpretative factors that must be optimized to maximizeensitivity and specificity of the overall method. Issues perti-ent for the colorectal surgeon include those related to CTCerformance in the screening realm as well as with thoseegarding problem-solving in the diagnostic setting.

TC TechniqueTC technique is currently in a state of evolution. Manyiffering materials and protocols are utilized between the

epartment of Radiology, University of Wisconsin Medical School, Madi-son, Wisconsin.

ddress reprint requests to: David H. Kim, MD, Department of Radiology,University of Wisconsin Medical School, E3/311 Clinical Science Center,600 Highland Avenue, Madison, WI 53792-3252. E-mail: dkim@

puwhealth.org.

8 1043-1489/07/$-see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1053/j.scrs.2007.03.002

arious institutions that perform this examination. All proto-ols revolve around the four following standard components:

● Bowel preparation● Bowel distention● Image acquisition● Image interpretation

he following protocol is the current technique employed athe University of Wisconsin (UW) in the performance ofTC. This protocol has been validated with polyp detectionharacteristics equivalent to OC for �8 mm in a large multi-enter trial2 and is utilized clinically in a large-scale colorectalancer CTC screening program.3 In distinction to OC, noedation is required for the examination.

owel Preparationowel preparation includes both purgation and tagginggents, which are administered 1 day before the scheduledxamination. The overall protocol is fairly simple andtraightforward with each agent separated by approximately

hours (Table 1). In our opinion, current technology re-uires bowel cleansing for satisfactory polyp sensitivity. Min-

mal preparation or “prepless” techniques with and withoutlectronic subtraction are promising but remain under inves-igation.4,5 Complete cleansing also allows for same day

olypectomy referral for those patients with a significant

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CT colonography 89

olyp, whereas minimal bowel preparations require a sepa-ate secondary cathartic administration before therapeuticolonoscopy. Sodium phosphate (Fleet Phosphosoda; CBleet Company, Inc., Lynchburg, VA) is the preferred cathar-ic agent over agents such as polyethylene glycol, resulting indrier colon.6 This is advantageous at CTC where the resid-al fluid cannot be suctioned out as during OC. In addition,atient compliance is improved compared with the large vol-me polyethylene glycol preparation.7 Typically, only a sin-le 45 mL dose of sodium phosphate is required.8 Magne-ium citrate (Sunmark, San Francisco, CA) or polyethylenelycol (Golytely; Braintree, Braintree, MA) is substituted inhose individuals with renal or cardiac insufficiency wherearge fluid shifts are to be avoided. Oral tagging agents areiven after the cathartic phase to tag any residual stool parti-les and residual colonic fluid (with dilute barium and di-trozoate, respectively). Tagging of residual stool and fluidids in both increased sensitivity and specificity. Taggingllows easy delineation of stool from true soft-tissue polyps asell as easy detection of a polyp within a pool of residualuid. The oral contrast agents are generally not apparent tohe endoscopist in cases referred for same day polyp removalue to the dilute nature of the 2% solution of barium andlear optical properties of diatrozoate and has not interferedith subsequent same day optical colonoscopy for CTC-pos-

tive referred patients, in our experience.

owel Distensiondequate colorectal distention is a key requirement for opti-al polyp detection. CO2 instillation via an automated deliv-

ry device (PROTOCO2L; E-Z-EM, Westbury, NY) is becom-ng the preferred method of distention (over room air manualulb insufflation), allowing consistently improved distensionnd decrease in number of collapsed segments.9 In addition,ecause CO2 is actively reabsorbed by the colonic mucosa,he patient experiences less bloating and discomfort follow-ng the procedure as opposed to room air instillation, wherehe intraluminal air may be retained for several hours afterhe procedure. A soft small-caliber rectal catheter with a low-ressure retention balloon is first inserted and CO2 is infusedith a set pressure cutoff, typically at 17 to 20 mm Hg.istention adequacy is assessed on CT scout images and se-

ected 2D transverse images before scanning the entire abdo-en. Imaging is then undertaken in both supine and proneositions. An additional optional decubitus series is con-ucted to further evaluate segments that appear collapsed onoth standard series. CO2 should be continually infusinguring scan acquisition. Spasmolytics such as glucagon or

able 1 University of Wisconsin CTC Bowel Preparation

Time Range Oral Prep Instruction

3-5 p.m. 45 ml sodium phosphate6-8 p.m. 250 ml 2% barium

9-11 p.m. 60 ml diatrizoate

uscopan are not administered in our practice. s

mage Acquisitiont a minimum, a multidetector CT scanner with eight chan-els is required. However, a 16-slice scanner is preferredased on the ability to cover the entire abdomen and pelvisith thin-slice collimation in a single reasonable lengthreath-hold. Typical scanner parameters include the follow-

ng: 1.25-mm collimation, 1.375:1 pitch, 1-mm reconstruc-ion interval, 120 kVp, and 35 to 75 mAs with possible in-reased technique for morbidly obese individuals. Newerose reduction techniques such as tube-current modulationeg, smart mA; GE Medical Systems, Waukesha, WI) allowodulation of the current in both the xy-plane and the z-

irection. The noise index is set at the maximum value toinimize dose. As standard, and as mentioned before, scans

n both the supine and the prone positions should be ob-ained with additional decubitus series as needed.

mage Interpretationhe DICOM images are networked to a dedicated 3D work-tation for post-processing and interpretation. At UW, wemploy a biphasic interpretative approach for CTC utilizinghe Viatronix platform (V3D Colon; Viatronix, Stonybrook,Y). Currently, there remains debate as to the optimal modef polyp detection either by 2D search algorithms or by pri-ary 3D approaches. However, studies to date have shown

airly poor polyp sensitivities even for large (�10 mm) le-ions with a primary 2D approach,10-12 while studies with 3Dlgorithms have demonstrated polyp detection capabilitiesomparable to OC.2,13 Primary 3D search algorithms allowasy recognition of polyps protruding from the mucosal sur-ace, while perceptual difficulties exist with 2D axial searchatterns. It may be difficult to detect even a sizable polyp atD where the polyp may be mistaken for an oblique mucosalold. In addition, a 2D approach is subject to reader fatigue,ince approximately 25,000 to 30,000 transverse 2D imagesust be reviewed on average for each advanced adenomaetected in a CTC screening program.Consequently, we believe that the biphasic approach

hould be the preferred manner of interpretation. This ap-roach centers on the 3D endoluminal view for primaryolyp detection. 2D axial images are utilized for limited sec-ndary polyp detection in areas of relative difficulty at 3Dncluding polyps submerged in the residual colonic fluid andreas of relative collapse. Once a potential lesion is detectedn 3D, polyp confirmation and characterization occurshrough the evaluation of the 2D images on both series.

ertinent Issuesfor the Colorectal Surgeon)ssues relevant to the colorectal surgeon can be divided intohose concerning screening CTC and those regarding CTChen used as a problem-solving modality in the diagnostic

etting.

90 D.H. Kim and P.J. Pickhardt

Figure 1 A 60-year-old male who underwent routine CTC screening. (A) Endoluminal 3D image demonstrates a large10-mm pedunculated polyp in the cecum. (B) Translucency function at 3D confirms the typical red soft-tissuesignature of a polyp. (C) Same day optical colonoscopy confirms the cecal polyp near the appendiceal orifice. (D)

Tubulovillous histology noted at pathology following polypectomy.

Figure 3 A 62-year-old male for routine screening. (A) 3D colonic map demonstrates an elongated colon. Note theincreased tortuosity of the right colon. While localization may be difficult at OC, it is easily depicted at CTC in themid-transverse colon. (B) 3D endoluminal view demonstrates a 17-mm pedunculated polyp in this region. (C)Endoscopic view confirms the large lesion. Polypectomy reveals a villous tumor with invasive carcinoma at the

resection margin ultimately requiring an extended right hemicolectomy.

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CT colonography 91

ssues Related to CTC Screeningerformance Characteristics of CTChere is emerging consensus that CTC is comparable to OC

or polyp detection when a state-of-the-art technique is em-loyed.2,13 A large multicenter prospective trial of asymptom-tic average risk individuals (n � 1233) conducted throughhe Department of Defense demonstrated sensitivities for ad-noma detection of 92% for lesions �8 mm.2 Initially, a fewther studies disputed these promising CTC results.10-12

owever, there is a growing appreciation that the poor per-ormance in the latter could be attributed to several factorsncluding the lack of stool/fluid tagging, thick-slice collima-ion during image acquisition, older imaging protocols, and arimary 2D interpretative approach. In our experience ofver 3500 screening CTC patients, effective polyp detectiony CTC is routinely possible as long as each phase and tech-ical detail of the examination (see Technique above) is ad-quately addressed.3

ole of CTC in Colorectal Cancer (CRC) ScreeningTC will likely fulfill several roles within CRC screening in

he coming years, including (1) a primary screening optionlong with OC to improve overall patient compliance withcreening; (2) a possible noninvasive filter to decrease theumber of therapeutic OC procedures for removal of polypsith little or no malignant potential; and (3) a replacement

or barium enema in cases of incomplete OC screening.Colorectal carcinoma remains a major cause of cancerortality in the United States accounting for approximately

5,000 deaths per year.14 Screening for this disease is partic-larly effective when removal of benign precursor lesionsrevent the future development of cancer. Unfortunately,ompliance with screening is poor and it is estimated that upo 40 million individuals 50 years of age or older do notndergo screening with any of the existing modalities.15 It ispparent that the screening population is a heterogenousroup—some prefer one modality while others may prefernother method. CTC would add another option for fulltructural evaluation of the colon and rectum in addition toC. Simultaneous availability of both should help to increaseverall numbers of screened individuals. Data at UW indeeduggest such a situation where the implementation of a ro-ust CTC CRC screening program has not decreased theumber of screening OC studies but instead has nearly dou-led the total number of patients being screened by totalolonic examination (Fig. 2).

Comparison of the parallel CRC screening programs atW also points to potential filtering capabilities of CTC for

eferral to therapeutic OC, which in turn would ultimatelyelp to conserve these limited resources.16 Because the be-ign precursor target resides among a much larger popula-ion of colorectal polyps, many lesions (including hyperplas-ic, mucosal polyps, and small tubular adenomas) areemoved, which have little or no therapeutic benefit for thecreened individuals. Advanced adenomas are the minorityf adenomas that are felt have the potential to progress toarcinoma over many years and represent the optimal target

f colorectal cancer prevention.17,18 The prevalence of such p

esions is low at 3 to 6% within an average risk asymptomaticopulation.19-21 At UW, the CTC screening protocol consistsf offering polypectomy for all polyps 6 mm and larger, withn option of CTC surveillance for small (6-9 mm) lesions. Inontrast to the screening OC program where nearly all iden-ified lesions are removed, diminutive polyps are not re-orted at CTC. Comparison of the two screening programswith �2000 patients per program) demonstrated similarields of advanced neoplasia between the programs despite ave-fold difference in the number of polypectomies per-ormed.22

Finally, CTC is a logical replacement for barium enema inhe completion of incomplete screening OC examinationsiven the significantly better polyp detection capabilitiesdiscussed in greater detail in the Diagnostic Issues section).

TC Reportingchoing the precedent set in breast carcinoma screening withI-RADS (Breast Imaging and Reporting and Data System),onsensus statements regarding lexicon and standardized re-orting or C-RADS (CT Colonography Reporting and Dataystem) have been developed to (1) better assist referringhysicians in appropriate management decisions based onTC findings; (2) allow comparison of results between insti-

utions; and (3) allow evaluations of examination perfor-ance, patient outcome, and cost.23 CTC results have been

ategorized from C0 to C4 with suggested management re-ponses (Table 2) and a lexicon of standardized descriptorsor identified lesions has been developed. Extracolonic find-ngs have also been categorized from E0 to E4 (Table 2).-RADS is in already in use among many academic centersurrently involved in CTC.

adiation Issuesue to the favorable geometries of this examination,

creening CTC is a low-dose technique ranging from 35 to5 mAs per series (with higher doses for obese individu-ls). The inherent high contrast between a soft-tissue

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llows for reliable detection even though higher levels ofmage noise result from low-dose imaging. It is estimatedhat paired scans (supine and prone) may deliver 8 to 12Sv to organs in the direct beam path and likely much

ower (by a factor of 5 to 10) with optimized protocols.24

he true risk to the individual for cancer induction by theethod remains controversial given the uncertaintyhether a linear, no threshold extrapolation exists at these

ower doses. The position statement of the Health Physicsociety contends that epidemiological studies have notemonstrated adverse health effects in small doses of lesshan 100 mSv delivered over many years and consequentlystimates of risk should be limited to individuals receivingore than 50 mSv per year or a lifetime dose of 100 mSv

ver nature background dose. Risk estimates should note applied below these levels and only be qualitative inature.25 Even proponents of the linear nonthresholdodel concede that that the benefit–risk ratio (in terms of

adiation exposure) is potentially large for CTC.24

xtracolonic FindingsTC allows for detection of potential pathologic processesutside the colon. It remains a limited evaluation given thepplied low-dose technique and noncontrast nature of thexamination. However, significant unsuspected conditionsuch as extracolonic malignancy and aortic aneurysms cannd have been incidentally detected. Several small studiesave shown potentially significant finding rates ranging fromto 11%, which ultimately require some type of evalua-

ion.26-29 In our experience, we have found a similar rate ofbout 7% with potentially significant findings, of which 2 to% prove to be relevant after final workup.22 The detection of

ncidental extracolonic processes in the otherwise asymp-omatic healthy screened individual is a double-edged sword.

hile potentially significant findings can be detected, many

able 2 C-RADS*

Category Definition

C0 Inadequate study/awaiting comparisonsC1† NormalC2‡ Intermediate polyp or indeterminate finding

● Polyp(s) 6-9 mm < 3 in number● Indeterminate, cannot exclude polyp > 6

mm in a technically adequate examC3§ Polyp, possibly advanced adenoma

● Polyp > 10 mm● >3 polyps, each 6-9 mm

C4¶ Mass, likely malignant

Adapted from Zalis et al. Radiology 226:3-9, 2005.Routine screening examination in 5 years.Suggested course of action include immediate polypectomy versusReferral for polypectomy.Surgical consultation.Communication with referring physician as per accepted practice g

ndings are of no great significance after complete workup. v

dditional financial costs and undue anxiety to the individ-al are additional concerns. Judicious handling of extraco-

onic findings is a critical issue for CTC screening programs.tandardized reporting resulting from C-RADS may helphere the extracolonic findings are categorized with sug-ested level of additional evaluation.

ssues Related to Diagnostic CTCesion Localization for Presurgical Planningne of the main advantages of CTC over OC is the precise

natomic localization of colorectal lesions.30 The accuracy ofesion localization for OC significantly decreases as the colonlongates and becomes more tortuous. Particularly for right-ided lesions in a long colon, lesions may reside in locationsery different from than reported at endoscopy. It is notnheard of for “hepatic flexure” polyps or masses to be actu-lly located in the proximal sigmoid in a person with a longortuous colon. CTC, however, is very accurate in correctlylacing these lesions, even in extreme situations (Fig. 3). Dueo its underlying cross-sectional nature, CTC is able to visu-lize the colon in relation to the extracolonic environment.onsequently, a lesion is easily localized within a particular

egment given these extracolonic anatomic landmarks. Inddition, distances from specific internal landmarks (eg, dis-ance from the anorectum or ileocecal valve) can be easilyetermined. This, in turn, allows improved presurgical plan-ing for subsequent removal.

valuation for Synchronous Lesionsynchronous lesions are not an uncommon event, with re-orted rates of synchronous cancers of approximately 3%rom large multi-institutional databases.31 Knowledge of theresence of additional large polyps or cancers would obvi-usly be beneficial for the presurgical planning for an indi-

Category Definition

E0 Limited evaluation; severe artifactE1 Normal or anatomic variantE2 Clinically unimportant finding such as a:

● Simple cyst● Gallstone● Vertebral hemangioma

E3 Likely unimportant finding, incompletelycharacterized such as a:● Minimally complex renal cyst

E4� Potentially important finding such as a:● Solid renal mass● Lymphadenopathy● Aortic aneurysm● Pulmonary nodule > 1 cm

ng follow-up in 3 years, subject to individual patient circumstance.

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CT colonography 93

Figure 4 A 53-year-old male with obstructive splenic flexure mass. (A) Circumferential adenocarcinoma seen at CTCunable to be transversed by the colonoscope at prior OC. (B) The splenic flexure mass has a classic appearance of anapple core lesion at 2D (arrows). (C, D) The proximal colon is well distended (C, 3D map; D, endoluminal view) forevaluation of additional lesions despite the nearly occlusive tumor. The patient subsequently underwent a left hemi-

colectomy after no additional polyps or masses detected in the cecum or transverse colon.

Figure 5 Submucosal mass seen at optical colonoscopy. CTC undertaken for further characterization. (B) 3D endolu-minal view clearly depicts the large submucosal mass within the sigmoid colon (a diverticulum is seen in the back-ground). (A) Translucency function demonstrates a diffusely green appearance consistent with fat. (B) Axial 2D image

confirms the presence of a submucosal lipoma obviating the need for biopsy.

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94 D.H. Kim and P.J. Pickhardt

mal colon in the presence of a distal occlusive lesion,articularly in cases where the endoscope is unable to tra-erse a severely constricting carcinoma. Even in these cases,he proximal colon is able to be distended without difficultyy CO2 to allow this evaluation (Fig. 4). Because CO2 isctively reabsorbed by the colonic mucosa, discomfort isinimal after the conclusion of the examination and no sig-ificant abdominal distention post procedure remains.

valuation for Submucosal Lesionss opposed to OC where only mucosal processes are evalu-ted, the cross-sectional nature of CTC allows for evaluationf the deeper layers of the bowel wall and beyond. Thisllows for detection and characterization of submucosal andntramural processes, which may be difficult to characterizet OC. The extent and precise localization of these lesions cane ascertained. In some cases, a definitive diagnosis can beade, such as in the case of lipomas where a pathognomonic

atty attenuation is present (Fig. 5) or in the case of heman-iomas where scattered phleboliths may be identified.

ompletion Evaluation for Incompleteor Contraindicated) ColonoscopyTC allows for evaluation of the proximal colon in caseshere OC is unable to intubate the cecum. It is the natural

eplacement for the barium enema given its improved capa-ilities for polyp detection.32 Same day referral from OC al-

ows for CTC examination without the need for an additionalowel preparation. However, in this situation no contrastagging will have been performed, which in turn may lead toecreased specificity through adherent soft-tissue attenua-ion stool mimicking true polyps.33 CTC is also useful for thevaluation of patients at increased risk for OC, includinghose on anticoagulation therapy or debilitated patients withncreased sedation risk. In the case of patients on warfarin,he medication does not need to be stopped for the examina-ion and reversal of anticoagulation is only needed in theinority that need a future polypectomy. Because CTC doesot require sedation, patients with poor pulmonary reserver other sedation risks can also be safely evaluated.

onclusionTC has rapidly evolved into a clinically effective modality.hen state-of-the-art technique is employed, it has demon-

trated performance characteristics equivalent to OC. Therere several phases for a complete examination which all muste optimized to allow effective detection of polyps. We doot view CTC as a replacement for OC but rather hope that itdds an attractive option for individuals hesitant to under-ake CRC screening, thus increasing overall compliance for aargely preventable disease. CTC screening strategies such asonreporting of diminutive lesions and imaging surveillancef small (6-9 mm) lesions would significantly decrease theumber of “therapeutic” polypectomies that are of limitedenefit. Aside from the screening realm, CTC is useful as aroblem-solving tool and excels in lesion localization for pre-urgical planning, in the evaluation for presence of synchro-

ous lesions, and in the assessment of submucosal processes. 2

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