DOI: 10.1148/rg.246045063
RadioGraphics 2004;24:1535-1556
© RSNA, 2004
Differential Diagnosis of Polypoid Lesions Seen at CT Colonography (Virtual Colonoscopy)1
Perry J. Pickhardt, MD
1 From the Department of Radiology, University of Wisconsin Medical School, E3/311 Clinical Science Center, 600 Highland Ave, Madison, WI 53792; and Department of Radiology, Uniformed Services University of the Health Sciences, Bethesda, Md. Recipient of a Cum Laude award for an education exhibit at the 2003 RSNA scientific assembly. Received April 2, 2004; revision requested May 5 and received June 10; accepted June 11. As of April 28, 2004, the author became a medical consultant for Viatronix, Inc. Address correspondence to the author (e-mail: ppickhardt@mail.radiology.wisc.edu).
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Abstract
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Computed tomographic (CT) colonography, also referred to as virtual colonoscopy, holds significant promise for effective large-scale colorectal cancer screening. Two-dimensional (2D) and three-dimensional (3D) displays of the CT data are employed, both of which are critical for proper evaluation. Although many radiologists continue to use the 2D images for polyp detection, more emphasis on the 3D images for primary detection of polyps has yielded the best results for screening detection. The primary target lesion for colorectal screening is the adenomatous polyp, since detection and removal of all larger or advanced lesions could potentially prevent approximately 95% or more of all colon cancers. Frankly invasive adenocarcinoma is rarely encountered in an average-risk asymptomatic screening population, but it is of course another important target. In addition to these clinically significant epithelial neoplasms, however, a host of additional lesions and pseudolesions may be encountered that appear polypoid at CT colonography. A subset of "don’t touch" lesions, which should not be confused with potential neoplasms, can also be recognized at CT colonography. A variety of useful techniques and observations can be used to increase the specificity of CT colonography for distinguishing false polyps from true polyps.
© RSNA, 2004
Index Terms: Cancer screening, 75.30 • Colon, CT, 75.12117, 75.12119 • Colon neoplasms, CT, 75.12117, 75.12119 • Colon neoplasms, diagnosis, 75.30
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LEARNING OBJECTIVES FOR TEST 1
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After reading this article and taking the test, the reader will be able to:
- Provide a differential diagnosis for polypoid lesions seen at CT colonography and demonstrate examples.
- Describe techniques and findings that improve specificity for polypoid lesions seen at CT colonography.
- Discuss the diagnostic advantages and limitations for CT colonography as a colorectal screening tool.
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Introduction
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Computed tomographic (CT) colonography, also referred to as virtual colonoscopy, is a minimally invasive, total colonic examination for colorectal cancer screening. Use of both two-dimensional (2D) and three-dimensional (3D) displays is vital for a complete evaluation. Although either the 2D or 3D displays may be used for initial polyp detection, the best results for screening have emphasized the 3D images for the primary evaluation. In fact, state-of-the-art 3D CT colonography has compared favorably with optical or conventional colonoscopy for the detection of clinically relevant lesions (1). The intended target for colorectal cancer screening is the neoplastic polyp, including both benign (adenomatous) and malignant lesions. Ideally, only lesions with malignant potential need be detected and removed. Unfortunately, diagnostic tests such as CT colonography and optical colonoscopy are not entirely specific, because both techniques rely primarily on detecting morphologic changes (ie, discrete protrusion of a lesion into the colonic lumen). With few exceptions, specific diagnosis of a soft-tissue polyp generally requires histologic examination.
Because polyp size and imaging appearance are imperfect surrogates for histologic analysis, radiologists interested in offering CT colonography for colorectal cancer screening should know the wide array of entities that can manifest as apparent "lumps and bumps" in the colon. Broad categories of polypoid lesions include neoplastic mucosal, nonneoplastic mucosal, submucosal, extrinsic, and anorectal lesions (Table 1). In addition, there are several artifacts and pitfalls unique to CT colonography that can also be mistaken for true pathologic entities, as well as a subset of "don’t touch" lesions for which an imaging-specific diagnosis is generally possible. This article demonstrates the differential diagnosis of colorectal polypoid lesions by using examples from routine CT, screening CT colonography, and optical colonoscopy.
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CT Colonographic Technique
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Our protocol for screening CT colonography begins with colonic cleansing with oral phospho-soda (45–90 mL) and bisacodyl (10 mg) (1). For patients with renal or cardiac insufficiency, magnesium citrate is substituted for phospho-soda. To increase sensitivity and specificity for polyp detection, patients also drink 2% CT barium (250 mL) and water-soluble iodinated contrast material (diatrizoate, 60 mL) for the purposes of stool tagging and fluid opacification (2). Minimum required parameters for multidetector CT include 2.5-mm collimation, reconstruction overlap of 20%, 50 mAs (effective), and 120 kVp. Imaging is performed after patient-controlled insufflation with room air (alternatively, CO2 may be used for colonic distention) with the patient in supine and prone positions. Our preferred method for interpreting CT colonography consists of performing a primary 3D endoluminal "fly-through" examination and correlating the results with findings from 2D images. This method (which we refer to as primary 3D polyp detection or primary 3D approach, as compared with using only 2D images for the primary interpretation, or primary 2D polyp detection) requires the use of specific yet commercially available software (Viatronix V3D Colon; Viatronix, Stony Brook, NY) (Fig 1). Since the publication of our CT colonography screening trial in the New England Journal of Medicine (1), our time for interpretation has continued to decrease, in part because the speed of the 3D fly-through has since doubled. In my experience, most screening studies can now be easily read in less than 10 minutes.
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Figure 1. Schematic map of the colon generated by CT colonography software. The Viatronix V3D Colon system automatically isolates the air-filled colon and rectum and generates an endoluminal centerline (green line), which helps allow for time-efficient primary 3D evaluation. Red dot indicates a "bookmark" where a polyp was located.
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It is imperative to recognize that diagnostic success with screening CT colonography hinges on the particular techniques employed. Although CT colonography software and interpretation are emphasized herein, there are other important technical considerations such as colonic preparation (including stool tagging and fluid opacification), colonic distention, and multidetector CT scanning parameters. Considerable evidence suggests that primary 2D interpretation (ie, scrolling through the 2D sections for polyp detection) is inadequate for evaluating populations with low prevalence of polyps, such as in the setting of screening (3,4). Although most radiologists continue to employ a primary 2D approach, I believe this approach is mainly a reflection of the software limitations still present with most CT colonography systems and perhaps a reluctance to change systems or philosophy (5).
In comparison, primary 3D polyp detection with the Viatronix V3D system has proved to be accurate in the screening setting in a large, prospective multicenter trial (1). This study also showed that (a) the learning curve for using this system does not appear to be as steep as that for the 2D approach and (b) there is significantly less interobserver variability compared with a 2D approach; these findings indicate the generalizability of this technique to community practices (1,6). Because of these results, the U.S. Food and Drug Administration has recently approved this system for the specific purpose of screening asymptomatic adults (7). Furthermore, although screening CT colonography is still viewed as "investigational" by managed care organizations nationwide, the CT colonography screening studies performed at our center have been covered by third-party payors as of April 2004 (8). This exception for reimbursement is closely linked to the use of our proved methods with the Viatronix V3D system.
In actual practice, the difference between a primary 2D approach versus a primary 3D approach for polyp detection is readily apparent on the Viatronix system. In my opinion, 2D imaging works well for confirming suspicious lesions detected on 3D endoluminal views, but primary 2D detection requires a rather tedious, fatiguing search pattern. Compared with 2D detection, both polyp conspicuity and opportunity for detection are clearly increased on 3D endoluminal views (5). Additional features with the Viatronix system that allow for efficient 3D evaluation include an automated centerline for navigation, ease of manual navigation, ability to jump collapsed segments rapidly, and translucency rendering (Fig 1) (5,9). It is important to emphasize, however, that most CT colonography systems do not yet allow effective, time-efficient primary 3D interpretation.
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Neoplastic Mucosal Lesions
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The majority of colorectal cancers are believed to arise within benign adenomatous polyps that develop slowly over many years, following the so-called adenoma-to-carcinoma sequence (10). Detection and removal of slow-growing adenomas could potentially prevent the development of approximately 95% or more of cancer cases and is the rationale for colorectal screening (11). Tubular adenomas (Fig 2) account for approximately 80%–85% of adenomatous polyps, are usually <10 mm in size, and typically demonstrate only mild dysplasia (12–14). As such, the majority of tubular adenomas detected at screening CT colonography will be well below any rational size threshold set for polypectomy referral. In fact, although tubular adenomas may account for up to 30%–40% of all diminutive colonic lesions (5 mm and less), these tiny polyps have no practical clinical significance and should not affect determination of surveillance intervals (1,14). Tubulovillous adenomas (Figs 3, 4) represent about 10%–15% of all adenomatous lesions (12–14). These neoplasms tend to be larger than tubular adenomas (often 10 mm and greater) and demonstrate higher degrees of dysplasia. As such, tubulovillous adenomas are a more important target for colorectal screening and cancer prevention. True villous adenomas (Fig 5) are uncommon, representing less than 5% of colorectal neoplasms. These lesions are generally large (2–3 cm or more), are frondlike, and have the greatest risk for malignancy.
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Figure 2a. Tubular adenoma. (a) Endoluminal 3D view from CT colonography shows a rounded, 6-mm sessile polyp located on a colonic fold. (b) Digital photograph from same-day optical colonoscopy shows the matching sessile polyp.
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Figure 2b. Tubular adenoma. (a) Endoluminal 3D view from CT colonography shows a rounded, 6-mm sessile polyp located on a colonic fold. (b) Digital photograph from same-day optical colonoscopy shows the matching sessile polyp.
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Figure 3a. Tubulovillous adenoma. (a) Endoluminal 3D view from CT colonography shows a sessile, lobulated 20-mm polyp extending from a colonic fold. (b) Digital photograph from optical colonoscopy shows the same lobulated lesion. Note the adjacent calibrated guidewire, which provides more accurate endoscopic measurement than open biopsy forceps estimation.
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Figure 3b. Tubulovillous adenoma. (a) Endoluminal 3D view from CT colonography shows a sessile, lobulated 20-mm polyp extending from a colonic fold. (b) Digital photograph from optical colonoscopy shows the same lobulated lesion. Note the adjacent calibrated guidewire, which provides more accurate endoscopic measurement than open biopsy forceps estimation.
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Figure 4a. Tubulovillous adenoma. (a) Endoluminal 3D view from CT colonography shows a 10-mm pedunculated polyp with a well-defined stalk. (b) Axial 2D view shows the same pedunculated polyp (arrowhead). Unlike most other pedunculated lesions, which are more easily recognized as such on 3D views, the stalk and polyp in this case happen to be aligned in a standard 2D plane.
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Figure 4b. Tubulovillous adenoma. (a) Endoluminal 3D view from CT colonography shows a 10-mm pedunculated polyp with a well-defined stalk. (b) Axial 2D view shows the same pedunculated polyp (arrowhead). Unlike most other pedunculated lesions, which are more easily recognized as such on 3D views, the stalk and polyp in this case happen to be aligned in a standard 2D plane.
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Figure 5a. Villous adenoma. (a) Endoluminal 3D view from CT colonography shows a 5-cm irregular cecal mass. This papillary appearance is highly suggestive of a villous tumor. (b) Axial 2D image (without electronic cleansing of opacified fluid) shows the same irregular cecal mass (arrowheads). (c) Digital photograph from optical colonoscopy shows the papillary, frondlike nature of the mass to greater advantage. The lesion was not malignant despite its large size.
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Figure 5b. Villous adenoma. (a) Endoluminal 3D view from CT colonography shows a 5-cm irregular cecal mass. This papillary appearance is highly suggestive of a villous tumor. (b) Axial 2D image (without electronic cleansing of opacified fluid) shows the same irregular cecal mass (arrowheads). (c) Digital photograph from optical colonoscopy shows the papillary, frondlike nature of the mass to greater advantage. The lesion was not malignant despite its large size.
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Figure 5c. Villous adenoma. (a) Endoluminal 3D view from CT colonography shows a 5-cm irregular cecal mass. This papillary appearance is highly suggestive of a villous tumor. (b) Axial 2D image (without electronic cleansing of opacified fluid) shows the same irregular cecal mass (arrowheads). (c) Digital photograph from optical colonoscopy shows the papillary, frondlike nature of the mass to greater advantage. The lesion was not malignant despite its large size.
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Morphologically, adenomas can appear sessile, flat, or pedunculated. In our experience, tubular adenomas are almost always sessile (Fig 2) and rarely appear pedunculated, whereas most pedunculated polyps have tubulovillous histologic characteristics (Fig 4). Size measurement for pedunculated polyps should not include the stalk. Flat adenomas (Fig 6) represent a subset of sessile lesions and are the least conspicuous polyp for detection at both CT colonography and colonoscopy. We define flat polyps as shallow, plaquelike, or broad-based lesions with a height less than one-half the width (15). Except for the larger masses, flat polyps generally measure 3 mm or less in height. However, agreement on a precise definition for what constitutes a flat lesion at both CT colonography and optical colonoscopy has been somewhat elusive. Regardless, by using state-of-the-art 3D techniques, our detection rate for flat adenomas 6 mm or greater was not significantly different from that of non-flat adenomas (15). Furthermore, the small but aggressive flat adenomas that have been described in East Asian populations seem to be rare in the typical Western population (15,16).
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Figure 6a. Flat adenoma. (a) Endoluminal 3D view from CT colonography shows a relatively subtle flat lesion (arrowheads) near the anal verge. This adenoma was missed at prospective colonoscopy before the CT colonographic results were revealed. (b) Corresponding axial 2D image helps confirm a flat rectal lesion (arrow).
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Figure 6b. Flat adenoma. (a) Endoluminal 3D view from CT colonography shows a relatively subtle flat lesion (arrowheads) near the anal verge. This adenoma was missed at prospective colonoscopy before the CT colonographic results were revealed. (b) Corresponding axial 2D image helps confirm a flat rectal lesion (arrow).
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The concept of "advanced adenoma" is important, because it represents the key target lesion for colorectal screening; the term advanced adenoma refers to neoplasms measuring 
10 mm and/or demonstrating high-grade dysplasia, a prominent villous component, or focus of malignancy (17). In practice, only a minority of advanced adenomas measures less than 10 mm, making size the primary factor for their determination (18). For larger polyps measuring 2 cm or more, the risk for malignancy is considerably greater, but the majority of these masses will still be benign, particularly when found in the asymptomatic screening population (Fig 5). These facts remind us that the adenoma-to-carcinoma sequence is fortunately prolonged in most cases and only strengthens the rationale for noninvasive surveillance. When CT colonography is performed on a multidetector CT scanner, a primary 3D evaluation is performed, and tagging of retained fecal debris and fluid is used, the sensitivity of CT colonography for detection of advanced adenomas can equal or exceed that of optical colonoscopy (1,19).
The malignant potential of an adenomatous polyp directly correlates with its size, histologic type, and degree of dysplasia. Fortunately, colorectal cancer is encountered in much less than 1% of asymptomatic adults undergoing screening (1). Malignant polyps can appear similar to premalignant advanced adenomas at CT colonography (Figs 7, 8), but they are more likely to manifest with symptoms. Frankly invasive adenocarcinoma typically demonstrates masslike, eccentric, or annular wall thickening (Fig 9). Unlike polypoid lesions, which are more easily detected on 3D endoluminal views, invasive mass lesions are better depicted on 2D images, which allow for mural and extramural evaluation. In the setting of occlusive carcinoma, however, the 3D endoluminal display remains useful for evaluation of proximal synchronous neoplasms (Fig 10).
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Figure 7a. Malignant polyp. (7a) Axial 2D view shows a relatively flat 4-cm mass in the cecum. (7b) Digital photograph from optical colonoscopy shows the same polypoid mass, which was malignant but not yet invasive at histologic evaluation.
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Figure 7b. Malignant polyp. (7a) Axial 2D view shows a relatively flat 4-cm mass in the cecum. (7b) Digital photograph from optical colonoscopy shows the same polypoid mass, which was malignant but not yet invasive at histologic evaluation.
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Figure 9a. Invasive adenocarcinoma. (a) Contrast material-enhanced 2D curved reformatted image with soft-tissue windowing shows an annular-constricting mass with shouldering (arrowheads) involving the sigmoid colon. Cross-sectional 2D views are much more effective than endoluminal displays for depicting invasive mass lesions. (b) Digital photograph from optical colonoscopy shows the proximal aspect of the mass.
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Figure 9b. Invasive adenocarcinoma. (a) Contrast material-enhanced 2D curved reformatted image with soft-tissue windowing shows an annular-constricting mass with shouldering (arrowheads) involving the sigmoid colon. Cross-sectional 2D views are much more effective than endoluminal displays for depicting invasive mass lesions. (b) Digital photograph from optical colonoscopy shows the proximal aspect of the mass.
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Figure 10a. Occlusive adenocarcinoma. (a) Contrast-enhanced 2D coronal CT image with soft-tissue windowing shows an annular sigmoid mass (arrowheads). The endoscope could not be passed beyond the lesion to evaluate the proximal colon. (b) Endoluminal 3D view from CT colonography shows a synchronous 15-mm polyp on a fold in the ascending colon. Although the 2D views are better for evaluating the invasive primary tumor, 3D views remain valuable for detecting proximal synchronous lesions.
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Figure 10b. Occlusive adenocarcinoma. (a) Contrast-enhanced 2D coronal CT image with soft-tissue windowing shows an annular sigmoid mass (arrowheads). The endoscope could not be passed beyond the lesion to evaluate the proximal colon. (b) Endoluminal 3D view from CT colonography shows a synchronous 15-mm polyp on a fold in the ascending colon. Although the 2D views are better for evaluating the invasive primary tumor, 3D views remain valuable for detecting proximal synchronous lesions.
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Screening CT colonography is best suited for evaluation of average-risk adults, since their a priori risk of having a large polyp necessitating optical colonoscopy is relatively low (18,20). In our experience with a large asymptomatic screening population, the prevalence of adenomas measuring 6 mm, 8 mm, and 10 mm or more was 13.6%, 6.7%, and 3.9%, respectively (1). As part of our current CT colonography screening program, we offer same-day colonoscopy for polypectomy for detected lesions measuring 10 mm or more, thus avoiding the need for the patient to repeat colonic cleansing. On average, approximately 5% of asymptomatic adults will have a lesion measuring 10 mm or more detected at CT colonography, reflecting the fact that some cases may represent "false-positive" studies (ie, a corresponding adenoma is not found at colonoscopy). For CT colonography–detected lesions measuring 6–9 mm, the patient can undergo either noninvasive follow-up CT colonography or colonoscopy for polypectomy. To date, almost all patients in our clinical program with CT colonography–detected lesions less than a centimeter have opted for short-term follow-up CT colonography in 2–3 years in lieu of immediate colonoscopy. The role of CT colonography is more limited for patients at high risk for colorectal neoplasia, or those with familial adenomatous polyposis (Fig 11) or hereditary nonpolyposis colorectal cancer syndromes, because these patients are more likely to require a therapeutic procedure.
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Figure 11a. Familial adenomatous polyposis syndrome with adenocarcinoma. (a) Contrast-enhanced 2D axial CT image shows a large irregular soft-tissue mass in the ascending colon (arrowheads). The CT study was performed as a routine examination without colonic preparation. (b) Digital photograph from subsequent optical colonoscopy shows the large mass seen in a, which proved to be malignant at histologic evaluation. (c) Axial CT image obtained caudad to a shows additional smaller polypoid lesions (arrowheads), which might be difficult to distinguish from stool on this CT scan obtained without colonic cleansing. (d) Digital photograph from optical colonoscopy shows multiple polyps in the transverse colon.
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Figure 11b. Familial adenomatous polyposis syndrome with adenocarcinoma. (a) Contrast-enhanced 2D axial CT image shows a large irregular soft-tissue mass in the ascending colon (arrowheads). The CT study was performed as a routine examination without colonic preparation. (b) Digital photograph from subsequent optical colonoscopy shows the large mass seen in a, which proved to be malignant at histologic evaluation. (c) Axial CT image obtained caudad to a shows additional smaller polypoid lesions (arrowheads), which might be difficult to distinguish from stool on this CT scan obtained without colonic cleansing. (d) Digital photograph from optical colonoscopy shows multiple polyps in the transverse colon.
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Figure 11c. Familial adenomatous polyposis syndrome with adenocarcinoma. (a) Contrast-enhanced 2D axial CT image shows a large irregular soft-tissue mass in the ascending colon (arrowheads). The CT study was performed as a routine examination without colonic preparation. (b) Digital photograph from subsequent optical colonoscopy shows the large mass seen in a, which proved to be malignant at histologic evaluation. (c) Axial CT image obtained caudad to a shows additional smaller polypoid lesions (arrowheads), which might be difficult to distinguish from stool on this CT scan obtained without colonic cleansing. (d) Digital photograph from optical colonoscopy shows multiple polyps in the transverse colon.
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Figure 11d. Familial adenomatous polyposis syndrome with adenocarcinoma. (a) Contrast-enhanced 2D axial CT image shows a large irregular soft-tissue mass in the ascending colon (arrowheads). The CT study was performed as a routine examination without colonic preparation. (b) Digital photograph from subsequent optical colonoscopy shows the large mass seen in a, which proved to be malignant at histologic evaluation. (c) Axial CT image obtained caudad to a shows additional smaller polypoid lesions (arrowheads), which might be difficult to distinguish from stool on this CT scan obtained without colonic cleansing. (d) Digital photograph from optical colonoscopy shows multiple polyps in the transverse colon.
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Nonneoplastic Mucosal Lesions
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Unlike the pathologic continuum seen with neoplastic polyps, nonadenomatous mucosal polyps represent a heterogeneous group of unrelated entities. Although in our experience over 80% of such lesions are diminutive and have essentially no malignant potential, they still may account for about 40% of polyps measuring 6 mm or greater in an asymptomatic screening population (21). As such, detection of these larger nonadenomatous lesions at CT colonography could result in polypectomy that ultimately does not benefit the patient. Fortunately, nonadenomatous polyps measuring 10 mm or more are very uncommon, and CT colonography is less sensitive for detecting them compared with adenomas (21).
The hyperplastic polyp represents the most common nonneoplastic colorectal polyp (Figs 12–14). Although fewer than 25% of hyperplastic polyps measure 6 mm or more, they still account for about 75% of all nonadenomatous lesions of potentially significant size in a screening cohort (21). In our experience, many hyperplastic lesions over 10 mm in diameter have an atypical or flat morphology that makes them very difficult to detect at CT colonography (Fig 14), which is perhaps fortuitous (21). The "mucosal" polyp is the second most frequent nonadenomatous lesion and simply represents normal epithelium in a heaped-up, mamillated configuration (Fig 15). Compared with hyperplastic polyps, these lesions seldom represent a diagnostic dilemma at screening CT colonography, since over 90% are diminutive in our experience (21).
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Figure 12a. Hyperplastic polyp. (a) Endoluminal 3D view from CT colonography shows a 7-mm sessile soft-tissue lesion, which is indistinguishable from an adenomatous polyp. (b) Digital photograph from optical colonoscopy shows the same sessile polyp. Reliable distinction from an adenomatous polyp requires histologic analysis.
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Figure 12b. Hyperplastic polyp. (a) Endoluminal 3D view from CT colonography shows a 7-mm sessile soft-tissue lesion, which is indistinguishable from an adenomatous polyp. (b) Digital photograph from optical colonoscopy shows the same sessile polyp. Reliable distinction from an adenomatous polyp requires histologic analysis.
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Figure 13a. Hyperplastic polyp. (a) Endoluminal 3D view shows a large 11-mm sessile polyp. Bulky hyperplastic lesions of this size are relatively rare. (b) Digital photograph from optical colonoscopy shows the same hyperplastic polyp.
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Figure 13b. Hyperplastic polyp. (a) Endoluminal 3D view shows a large 11-mm sessile polyp. Bulky hyperplastic lesions of this size are relatively rare. (b) Digital photograph from optical colonoscopy shows the same hyperplastic polyp.
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Figure 14. Hyperplastic polyp. Digital photograph from optical colonoscopy shows a large irregular, flat 20-mm lesion (arrowheads) that was seen at CT colonography but only in retrospect. Most lesions with this appearance are fortunately hyperplastic in our experience.
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Figure 15a. Mucosal polyp (normal epithelium). (a) Endoluminal 3D view from CT colonography that simulates a retroflexed rectal view at colonoscopy shows a tiny 3-mm polyp (arrow). Note tip of a rectal catheter. Almost all mucosal polyps are diminutive and therefore will not influence management. (b) Digital photograph from optical colonoscopy shows measurement of the diminutive 3-mm lesion with the calibrated wire.
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Figure 15b. Mucosal polyp (normal epithelium). (a) Endoluminal 3D view from CT colonography that simulates a retroflexed rectal view at colonoscopy shows a tiny 3-mm polyp (arrow). Note tip of a rectal catheter. Almost all mucosal polyps are diminutive and therefore will not influence management. (b) Digital photograph from optical colonoscopy shows measurement of the diminutive 3-mm lesion with the calibrated wire.
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The juvenile polyp is classified as hamartomatous and, as the name implies, is most commonly found in patients 1–7 years old. Although most juvenile polyps regress or slough off, they are occasionally seen in asymptomatic adults. As such, they are usually solitary, pedunculated, and located in the rectosigmoid region (Fig 16) (14). Removal of larger juvenile polyps is indicated because of the risk of bleeding or prolapse. Inflammatory polyps are occasionally seen as an isolated finding in adults. They are believed to be formed by local extrusion of mucosa due to peristaltic forces and may demonstrate a pale fibrinous cap at optical colonoscopy (Fig 17). Inflammatory pseudopolyps can be seen in the setting of severe acute inflammatory bowel disease (ulcerative colitis and Crohn disease) and represent islands of inflamed mucosa surrounded by areas of denuded epithelium (22). Unlike routine CT evaluation of the abdomen and pelvis, CT colonography has a very limited role in the assessment of acute inflammatory bowel disease, because of the risk of perforation. Inflammatory pseudopolyps should not be confused with filiform postinflammatory polyps seen in the regenerative phase. Finally, a variety of hamartomatous polyps can be seen in several distinct polyposis syndromes, such as Peutz-Jeghers syndrome (Fig 18), Cowden disease (Fig 19), and Cronkhite-Canada syndrome (Fig 20).
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Figure 16a. Juvenile polyp. (a) Endoluminal 3D view from CT colonography in an asymptomatic 57-year-old man shows a pedunculated 15-mm polyp extending from a fold in the rectosigmoid region. The polyp was a solitary finding in this case. (b) Digital photograph from optical colonoscopy shows the same polyp. (Reprinted, with permission, from reference 21.)
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Figure 16b. Juvenile polyp. (a) Endoluminal 3D view from CT colonography in an asymptomatic 57-year-old man shows a pedunculated 15-mm polyp extending from a fold in the rectosigmoid region. The polyp was a solitary finding in this case. (b) Digital photograph from optical colonoscopy shows the same polyp. (Reprinted, with permission, from reference 21.)
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Figure 17a. Inflammatory polyp. (a) Endoluminal 3D view from CT colonography shows a small sessile polyp adjacent to a fold (arrow). (b) Digital image from optical colonoscopy shows a pale, fibrinous cap to this sessile lesion, which is suggestive of an inflammatory "cap" polyp.
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Figure 17b. Inflammatory polyp. (a) Endoluminal 3D view from CT colonography shows a small sessile polyp adjacent to a fold (arrow). (b) Digital image from optical colonoscopy shows a pale, fibrinous cap to this sessile lesion, which is suggestive of an inflammatory "cap" polyp.
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Figure 20. Cronkhite-Canada polyp. Endoluminal 3D view from CT colonography in an asymptomatic patient shows a sessile polyp (arrowhead) that was believed to have Cronkhite-Canada histologic characteristics. The patient did not have the associated clinical syndrome.
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Submucosal Lesions
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A variety of submucosal lesions can elevate the overlying epithelium and produce a smooth, polypoid appearance. Although most submucosal lesions have a nonspecific soft-tissue appearance at CT colonography, some characteristic entities, such as lipomas and pneumatosis, can be specifically identified at CT. As such, lipomas and pneumatosis both belong to a group of "don’t touch" lesions that can generally be recognized at CT colonography and left alone (Table 2).
Benign lymphoid polyps from hypertrophied follicles are relatively common but are typically diminutive in nature (Fig 21). Colonic lymphoma is rare and typically seen in immunocompromised patients, such as those with acquired immunodeficiency syndrome (AIDS) or who have undergone solid organ transplantation (Fig 22) (23). The colon is the most frequent gastrointestinal site of involvement by submucosal lipomas, which may appear sessile or even become pedunculated over time. The true nature of these benign lesions is generally recognized at both CT colonography (by fat attenuation on 2D images) and optical colonoscopy (Fig 23); thus, they can generally be left alone unless symptomatic (eg, causing intussusception).
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Figure 21a. Lymphoid polyps. (a) Endoluminal 3D view from CT colonography shows multiple small polypoid lesions (arrowheads). These lymphoid aggregates are typically diminutive and therefore do not affect patient management. (b) Digital photograph from optical colonoscopy shows one of these lesions in the foreground.
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Figure 21b. Lymphoid polyps. (a) Endoluminal 3D view from CT colonography shows multiple small polypoid lesions (arrowheads). These lymphoid aggregates are typically diminutive and therefore do not affect patient management. (b) Digital photograph from optical colonoscopy shows one of these lesions in the foreground.
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Figure 22. Colonic posttransplantation lymphoproliferative disorder (PTLD). Contrast-enhanced CT scan of a lung transplant recipient shows large polypoid masses (arrowheads) in the sigmoid colon. PTLD was proved at colonoscopy and subsequent surgical resection.
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Figure 23a. Colonic lipoma. (a) Endoluminal 3D view from CT colonography shows a large polyp extending from a fold. (b) Axial 2D view with soft-tissue windowing shows that the lesion has fat attenuation (arrowhead), a finding diagnostic of a lipoma. (c) Digital photograph from optical colonoscopy shows the same lipoma.
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Figure 23b. Colonic lipoma. (a) Endoluminal 3D view from CT colonography shows a large polyp extending from a fold. (b) Axial 2D view with soft-tissue windowing shows that the lesion has fat attenuation (arrowhead), a finding diagnostic of a lipoma. (c) Digital photograph from optical colonoscopy shows the same lipoma.
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Figure 23c. Colonic lipoma. (a) Endoluminal 3D view from CT colonography shows a large polyp extending from a fold. (b) Axial 2D view with soft-tissue windowing shows that the lesion has fat attenuation (arrowhead), a finding diagnostic of a lipoma. (c) Digital photograph from optical colonoscopy shows the same lipoma.
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Carcinoid tumors more commonly occur in the ileum, but this neuroendocrine tumor can also originate in the colon and rectum (Fig 24). Gastrointestinal stromal tumor more often arises from the stomach or small bowel, but colorectal origin does occur. These mural-based lesions are often large and obvious on cross-sectional 2D images, but they may be subtle when viewed from an endoluminal perspective, such as with 3D CT colonography and optical colonoscopy (Fig 25). A variety of other submucosal neoplasms, such as leiomyoma, granular cell tumor, Kaposi sarcoma, ganglioneuroma, and hematogenous metastases, may be encountered, although less frequently. Colitis cystica profunda is a rare entity characterized by dilated mucus-filled submucosal glands, typically involving the rectum (Fig 26) (24). Pneumatosis cystoides coli can appear polyposis-like from an endoluminal perspective, as seen with optical colonoscopy (Fig 27) or 3D CT colonography (Fig 28). On 2D CT images, however, the air composition of these cysts is readily apparent (Fig 28) (26).
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Figure 25a. Rectal gastrointestinal stromal tumor. (a) Axial 2D view from CT colonography with soft-tissue windowing shows a mass (arrowhead) in the posterior rectum. (b) Endoluminal 3D view shows a broad-based impression (arrowheads) in the rectal lumen, adjacent to the anal verge. Note tip of a rectal catheter. (c) Digital photograph from optical colonoscopy shows the similar broad-based impression (arrowheads), which was initially disregarded until the CT colonographic findings were revealed.
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Figure 25b. Rectal gastrointestinal stromal tumor. (a) Axial 2D view from CT colonography with soft-tissue windowing shows a mass (arrowhead) in the posterior rectum. (b) Endoluminal 3D view shows a broad-based impression (arrowheads) in the rectal lumen, adjacent to the anal verge. Note tip of a rectal catheter. (c) Digital photograph from optical colonoscopy shows the similar broad-based impression (arrowheads), which was initially disregarded until the CT colonographic findings were revealed.
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Figure 25c. Rectal gastrointestinal stromal tumor. (a) Axial 2D view from CT colonography with soft-tissue windowing shows a mass (arrowhead) in the posterior rectum. (b) Endoluminal 3D view shows a broad-based impression (arrowheads) in the rectal lumen, adjacent to the anal verge. Note tip of a rectal catheter. (c) Digital photograph from optical colonoscopy shows the similar broad-based impression (arrowheads), which was initially disregarded until the CT colonographic findings were revealed.
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Figure 26. Colitis cystica profunda. Digital photograph from colonoscopy shows multiple large irregular lesions involving the rectum that represent dilated mucus-filled submucosal glands.
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Abstract
LEARNING OBJECTIVES FOR TEST...
